Claire Goldmann

Wajdi Chaâbani

Claire Hotton

Marianne Impéror-Clerc

Adrien Moncomble

Doru Constantin
email: constantin@unistra.fr

Damien Alloyeau
email: damien.alloyeau@u-paris.fr

Cyrille Hamon
email: cyrille.hamon@universite-paris-saclay.fr

Confinement Effects on the Structure of Entropy-Induced Supercrystals

Keywords: Plasmonic nanoparticles, depletion-induced self-assembly, Liquid cell TEM, self-assembly in confinement, disassembly under electron irradiation, Small Angle X-Ray Scattering

ou non, émanant des établissements d'enseignement et de recherche

Introduction

Silver and gold nanoparticles (NPs) have garnered sustained interest over the past 25 years because of their exceptional interaction with light, originating from localized surface plasmon resonances. Novel materials made by colloidal design can be used to control the flow of light through interparticle plasmonic coupling. [1] One outstanding challenge is to endow these assemblies with a structural response under specific external stimuli, [2] which can actuate a broad range of nanoscale forces. [3] Among these, depletion forces can be used to induce the self-assembly of NPs in a reversible manner. [4] In particular, surfactant micelles induce NP flocculation by depletion-induced attraction. Since this process leads to an overall increase of the entropy of the system by excluded volume interaction, it is alternatively denoted as entropydriven self-assembly. [5] The amplitude of the attraction is proportional to the concentration and radius of the surfactant micelles and to the contact area between two particles. By this process, NPs featuring a high contact area form superstructures that sediment to the bottom of the flask, while the smaller (or rounder) NPs remain in suspension. The process is reversible: the NPs assemble and then redisperse upon dilution. Depletion-induced self-assembly has been used to separate plasmonic NPs of different shapes, [6] or supercrystals (SCs) [7] (when combined with controlled evaporation) but very little for its ability to shape SCs. [8] For instance, Young et al. have shown by small angle X-ray scattering (SAXS) that depletion forces could be tuned in situ by changing the surfactant concentration, the temperature and the ionic strength of the solution, which in turn modify superlattice parameters. [8a] Elsewhere, Yang et al. studied the nucleation and growth of the SCs by in situ SAXS and showed the formation of micron-sized SCs, which packed together during sedimentation to form highly oriented structures. [8c] Liquid-Cell Transmission Electron Microscopy (LCTEM) is an emerging technique for studying the dynamic of individual soft, hard or hybrid nanomaterials in a liquid of controlled composition and temperature. [9] In particular, it has led to breakthroughs in NP synthesis and self-assembly. [9c, 10] Notably, the effects of NP shape, NP-substrate interactions and the structure and charge of the ligand shell on the structure and dynamics of the SCs has been investigated and optimized to better understand the nanoscale forces driving SC formation and to visualize the crystallization processes of large 3D nanocrystal superlattices by LCTEM. [11] Nevertheless, LCTEM observations are not always directly comparable to bulk experiments because of the presence of interfaces, limited mass transport and radiolysis effects. Indeed, NPs are imaged near a surface with a charge state that depends on the illumination conditions and in a confined environment a few hundred nanometer thick, which slows down their diffusion. [12] Another challenge of this technique is understanding the effects of radiolysis on the solution chemistry and soft-material damage. [13] Thus, revealing the differences between bench-scale synthesis and in situ TEM data is essential to better use LCTEM results in nanomaterial and SC fabrication. A way of meeting this challenge is to perform complementary in situ characterization such as SAXS, spectroscopy or, alternatively, computer simulations. [10b, 10c, 14] In this work, we synthesized and assembled gold triangles (AuNTs) and silver nanorods (AgNRs) by depletion-induced self-assembly. We analyzed the structure of the SCs with a multi-scale approach from the bulk solution (SAXS and ex situ scanning electron microscopy (SEM)) to the single NP level (in situ LCTEM). Thus, we revealed how the confinement within the liquid-cell affects the structural properties of SCs and we quantitatively studied their evolution under electron irradiation to evaluate the possible driving forces of these structural reconfigurations.

Results and discussion

Triangular nanoprisms (AuNTs, 75 nm ± 4 nm in edge length and 30 nm ± 5 nm in thickness) and silver nanorods (AgNRs, 216 ± 19.6 nm in length and 27 ± 1.7 nm in width) were prepared by seed-mediated growth from mono-twinned gold seeds and penta-twinned gold bipyramids, respectively (Figure 1 a, e and Figure S1 andS2). [15] The synthesis of AuNTs and AgNRs was performed using cetyltrimethylammonium chloride (CTAC) as colloidal stabilizer and shape directing agent. CTAC was also used as osmotic agent in depletion-induced self-assembly experiments. The suspensions were probed by SAXS (Figure 1 b,f). Colloidal crystallization is observed above a certain threshold CTAC concentration, which varies with the shape of the NPs (115 mM for the AuNTs and 40 mM for the AgNRs). At 10 mM CTAC, the SAXS curve corresponds to the form factor (in blue) of the NPs, indicating well-dispersed suspensions. At higher CTAC concentration, Bragg peaks indicate not only the flocculation but also the crystallization of SCs. For both AuNTs and AgNRs the spectra exhibit Bragg peaks in 1:√3:2:√7:3 progression, characteristic of the in-plane hexagonal arrangement of the NPs. In the case of the AuNTs, the unit cell is composed of two parallel NPs having opposite orientation (rotated by 60°) and shifted along their normal from one layer to another, defining an interlocked hexagonal structure. This organization was confirmed on ex situ scanning electron microscopy (SEM) images (Figure 1c-d and Figure S3). This particular configuration stems from the arrangement between the basal facets and the lateral facets of the beveled AuNTs. [7e] In the case of the AgNRs no lamellar organization is detected, indicating that in solution the nanorods form monolayers without interlayer stacking.

Confirming this analysis, isolated nanorod rafts were observed on ex situ SEM images, with some lamellar organization that can be ascribed to additional ordering by evaporation-induced self-assembly (Figure 1g-h and Figure S3). [16] These two SC structures schematized in Figure 2 show that shape anisotropy can be used to form superstructure with controlled dimensionality by favoring side-to-side over end-to-end packing. We then studied the colloidal crystals in LCTEM in the presence of 115 mM CTAC for the AuNTs and 40 mM CTAC for the AgNRs. The suspension containing assembled AuNTs and AgNRs were separately confined between two SiN membranes with 500 nm spacing layer. At first, a remarkable re-ordering of the NPs is observed in the liquid cell. Indeed, the AgNRs form supramolecular wires (hereafter denoted as strips) with an average size of 2.9 ±1.9 µm in which nanorods are stacked side by side (Figure 3a-b). These strips also tend to stack side by side along the SiN membrane with 5 ± 3 units per stack, but no 3D stacking was observed. SCs made of AuNTs are observed along horizontal or vertical orientations with respect to the membranes.

The in-plane hexagonal arrangement is observed when the AuNTs lay flat on the SiN membrane and the interlocked stacking is seen when the AuNTs stand perpendicularly to the SiN membrane (Figure 3c-d). Contrast analysis of the in-plane hexagonal arrangements shows that the face-to-face stacking of the NPs in the direction perpendicular to the membrane is limited to between 2 and 3 NPs and these SCs in horizontal orientation are mainly composed of NP monolayers and bilayers (Figure S4). However, up to ten layers of AuNTs are directly observed when the face to face stacking goes along the membrane plane (i.e. for AuNTs in vertical orientation) but crystallization perpendicular to the substrate is also very limited (Figure 3d).

Those type of low dimensionality structures were always observed by LCTEM. The reconfiguration of the lattice therefore occurs during the sealing process and occurs within less than 15 minutes that is the delay time between the preparation of the chip and the observation of the samples by LCTEM.

In the following, we compare the size and structure of SCs confined in the liquid-cell and in bulk solution. The median surface area of AgNRs in beakers was measured by ex-situ optical microscopy to be 0.35 µm 2 with first quantile (Q1) of 0.15 µm² and Q3 = 0.77 µm². Similar analysis on the AuNTs revealed a median surface area of 0.32 µm 2 with Q1 = 0.16 µm² and Q3 = 0.61 µm² (Figure S5). This is in reasonable agreement with the extension of the monocrystals measured by in situ SAXS (from the full width at half maximum of the SAXS peaks) to be about 1 to 2 µm for both AuNTs and AgNRs. Furthermore, the edge-to-edge interparticle distances (de-e) were deduced from SAXS measurements to be 23 nm for the AgNRs while the AuNTs were separated by 13 nm in the hexagonal plane. In LCTEM, the SC's surface area was measured to be 0.7 ± 0.8 µm 2 and 0.6 ± 0.4 µm 2 for the AuNTs and the AgNRs, respectively. Also in good agreement with the SAXS analysis, de-e was measured by LCTEM to be 21 nm for the AgNRs (Figure S6) and 13 nm for the AuNTs in the hexagonal plane (Figure S7). These edge-to-edge distances were not affected by the electron dose rate (Figure S6). Note that the out of plane stacking distance of AuNTs was measured to be 15 nm by LCTEM when the AuNTs are observed in vertical orientation (Figure S7). This out of plane distance could not be confirmed by SAXS because the expected 001 peak cannot be measured: it presumably overlaps with the 110 and 200 ones and is very weak because of the high relative size dispersion in thickness of the AuNTs (17%). These large interparticle spacings (more than two surfactant bilayers [17] ) result from the balance between attractive depletion and repulsive electrostatic interactions. [8a] For comparison, in the dried state, the interparticle distance was found to be always below 5 nm by ex situ MEB analysis (Figure S8). Although the SCs size and lattice parameters are of the same order in bulk solution and in the liquid-cell, the volume reduction or the affinity of metal NPs for the SiN membrane favor the formation of 2D SCs with similar structural features of the 3D SCs revealed in bulk experiments (see schematic representations in Figure 2).

In complementary experiments, we tried to modify the volume of the liquid cell and the affinity of the NPs for the SiN membrane to evaluate the respective impact of these two parameters.

Interestingly, SCs with similar structures are observed in liquid cells with spacers of only 150 nm (Figure S9). LCTEM sample preparation typically involves a plasma-cleaning step of the SiN membrane to remove contamination and improve surface wetting. In LCTEM experiments performed with uncleaned liquid-cells, the NPs forms large 3D aggregates that diffuse in the liquid, presumably because of lower affinity of NPs for membranes with reduced hydrophilicity and surface charge (See Video S1). [12b] The high mobility of these large assemblies prevents their structural analysis. Nevertheless, these two experiments indicate that the lower dimensionality of SCs observed in liquid cells compared to the SCs formed in beakers is very likely due to membrane interaction rather than reduced volume effects. Prolonged LCTEM observation led to the disassembly of the nanostructures by progressive NP ejection (Figure 4, Video S2-S3). In the case of AgNRs, NP ejection occurred preferentially from the ends of the strips. Some instances of strips breaking into two parts and of strip reformation during the disassembly process were observed (Figure 4b). Hence, these assemblies resemble "living polymers," evolving via a step-growth mechanism because both monomers and oligomers can attach to the reactive ends of the strips. [18] In the case of AuNTs, NP ejection occurred preferentially at the edge of the lattice, rather than within the SCs, presumably because more bonds (membrane-NPs and NPs-NPs) would need to be broken in the latter case. Fast desorption / readsorption processes are observed at the edge of the SCs (Video S3). When a vacancy appeared within the core of the SCs, the hole rapidly grew within the SC, leading to an increased rate of lattice disassembly. These two experiments were performed at different dose rates: 0.066 e/A 2 s for the AgNRs and 0.592 e/A 2 s for the AuNTs We then investigated the disassembly kinetics. Experiments were performed at different dose rate, for both AgNRs (Figure 5) and AuNTs (Figure S16), and the number of particles N(t) at each time was retrieved via automatic image analysis. We define the time-dependent coverage n(t) = N(t)/N(0), which is represented against time in Figure 5. In all cases, the disassembly kinetics is similar: it starts with a slow decline, followed by a dramatic acceleration (Figure 5).

In order to describe the experimental situation, we need to account for the interactions that hold the SC together (depletion, van der Waals etc.), denoted below by "cohesion" and for those maintaining the SC in contact with the membrane (electrostatics, depletion, etc.), and generically termed "adhesion". Thus, in our model (see the SI for details), a SC consists of a (time-dependent) number of particles N(t), each one interacting with the substrate and with its neighbors via the adhesion and cohesion energies EA and EC, respectively. To preserve the generality of the model, we do not attempt to further separate these essential ingredients into components. Both EA and EC are presumably attractive in the absence of the electron beam (because the SC is stable and remains attached to the substrate), but they might become increasingly repulsive under the beam, for instance as the membrane and/or the particles become similarly charged. The desorption rate can be written as 𝑘 (𝑛) = 𝐴𝑒𝑥𝑝[-𝐵𝑛(𝑡)], with 𝐴 = 𝐾𝑒𝑥𝑝and 𝐵 = . K is a (presumably very large) kinetic pre-factor accounting for the desorption rate in the absence of any interaction and z is the coordination number (for instance, z = 4 for a planar square lattice). The fits for the AgNRs are quite good (see Figure The same treatment can be applied to SC formed by triangles, with very similar qualitative conclusions but with a much lower constant α=0.8±0.12 Å 2 /e -(Figure S17), implying that AuNTs were more difficult to eject. This can be explained by the stronger interaction of the AuNTs with the membrane, most likely due to their higher surface area in contact with the membrane. Note also that the desorption kinetics varied with the two possible arrangements of the AuNTs (vertical or horizontal orientations), which do not disassemble at the same rate. For instance, in the two experiments shown in Figure S18, the AuNTs were either all flat or consisted of a mixture of flat AuNTs and standing AuNTs respectively. Although we used the same dose rate (0.592 e/ Å 2 s), their disassembly kinetics do not overlap (Figure S16), since the initial arrangement of the AuNTs was not equivalent. This experiment shows that standing AuNTs are less stable against desorption than lying AuNTs because this configuration minimizes their interaction with the membrane. Although we worked in the low dose conditions (< 1 e.Å -2 .s -1 ) that are typically used for imaging organic soft matter, [10c] a beam-induced disassembly process is revealed in all experiments. Electron irradiation is known to affect both the liquid and the insulating SiN membrane, in a dose-rate dependent manner, and to modify (i) the ionic strength, (ii) the pH, (iii) the concentration of reactive oxygen species and iv) the surface charge of the SiN membrane. The production of ionic species by radiolysis would result in a decrease of the distance between the NPs, as reported in other studies, [8a, 10c, 14b] but this was not observed in the present work (Figure S6). Moreover, given the quaternary amine functionality of CTAC, assemblies are expected to be robust with regard to pH. The disassembly process could be induced by radiolysis products (mainly hydroxyl radicals), which are known to favor crosslinking or chain scission in organic polymers depending on the dose rate. [19] In the framework of LCTEM experiments, Dissanayake et al. [13c] showed that polymerization of branched polyethylenimine occurs at low cumulative dose, followed by chain scission at higher cumulative dose. Interestingly, this work and others [20] describe metal NPs as hotspots for radiolysis in the liquid-cell. Taken together, a reasonable hypothesis explaining our observations is a disassembly process mediated by the degradation of the CTAC ligands at the surface of the NPs by polymerization and/or chain scission. The latter mechanism would result in the production of insoluble alkane and tertiary amine (through cleavage of quaternary ammonium) and/or a mixture of short amphiphilic ligands and insoluble alkane (through cleavage of quaternary ammonium) and/or a mixture of short amphiphilic ligands and insoluble alkane (through cleavage of the aliphatic chain). This would in turn induce the colloidal destabilization of the NPs and their release in suspension. However, our model suggests that the cohesion of the SCs is independent of the dose rate, which is inconsistent with a disassembly process driven by the beam induced-degradation of CTAC. Another possible explanation relies on an induced positive charge of the SiN membrane building up under electron irradiation due to the generation of secondary electrons. [21] Charging effects at the interface with the SiN membrane have been repeatedly put forward to explain the anomalous diffusive behavior of metal nano-objects in LCTEM that move on the window by sporadic jumps driven by electrostatic repulsion with a dose-rate-dependent frequency. [12] Thus, we infer that the electron beam renders the membrane less adhesive for NPs, because of a charge inversion of the [12c, 14b, 22] consistent with an increase of the NPs/membrane interaction during imaging. In our case, the NPs are positively charged, so the electron beam renders the membrane less adhesive for NPs, leading to NPs ejection from the image area. [10c, 23] Another study observed different self-assembly behavior (attraction or repulsion) by mitigating solvent polarity in order to adjust electrostatic interaction. [24] All these observations can be rationalized by considering modified electrostatic interaction under electron beam irradiation.

Conclusions

AuNTs and AgNRs were assembled into bulk SCs by depletion-induced self-assembly, forming 3D and 2D superlattices respectively. The structural reconfiguration of the SCs is evidenced after loading them in confined cells used for in situ TEM observations and we show that this effect is induced by a strong NP-membrane interaction. Although the symmetry of the material is preserved, the affinity of the NPs for the membrane induces the delamination of the SCs into superlattices of lower dimensionality. Moreover, extended beam irradiation leads to disassembly of the lattices, which is well described by a model accounting for the desorption kinetics. This kinetic analysis suggests that the membrane becomes less adhesive upon electron irradiation, leading to NP ejection from the lattices. This work shows that LCTEM can be used to study entropy-driven self-assembly processes, even if caution should be taken to compare with bulk experiments. This works also shed lights on the reconfigurability of SCs obtained by depletion-induced self-assembly, which can rearrange under confinement. This is relevant for applications of these materials in confined geometries or in interaction with charged interfaces.

Experimental section

Materials. Gold chloride trihydrate (HAuCl4•3H2O ≥99.9%), silver nitrate (AgNO3 >99%), hydrochloric acid (HCl 37%), sodium borohydride (NaBH4 ≥96%), trisodium citrate dihydrate (99%), L-ascorbic acid (AA ≥99%), sodium iodide (NaI 99.9%), cetyltrimethylammonium bromide (CTAB ≥99%), CTAC (25 wt % in H2O), and benzyldimethylhexadecylammonium chloride (BDAC 99%) were purchased from Merck and used without further purification. All solutions were prepared in water 18.2 MΩ.

Characterization of the NPs. UV/Vis/NIR absorption spectra were collected using a Cary 5000 UV-Vis-NIR. All experiments were carried out at room temperature, using disposable polystyrene cuvettes with optical paths of 1 cm. TEM was performed at IMAGIF (I2BC CNRS, Gif s/Yvette, France) using a JEOL JEM-1400 microscope operating at 120 kV with a filament current of about 55 μA.

Concentration measurement. 10 µL of AuBPs were diluted in 2000 µL of water and the absorbance at 400 nm was recorded with a UV-visible spectrometer in order to estimate the Au(0) concentration, [Au] = 37.3 mM. [25] Triangles synthesis and purification. The triangles synthesis was performed following the method described by Scarabelli et. al. [15a] 1) AuNTs synthesis. Seeds. In a 20 mL vial, 50 µL of HAuCl4•3H2O (25 mM) were added to 4.7 mL of CTAC (100 mM). Under fast stirring, 300 µL of NaBH4 were added and the reaction medium was left to rest at room temperature for 2 hours.

AuNTs growth: 2 solutions were prepared freshly and at the same time:

-Solution A: 8 mL of water + 1.6 mL of CTAC (100 mM) + 80 µL of HAuCl4•3H2O (25mM)

+ 15 µL of NaI (10 mM) + 40 µL of ascorbic acid (100 mM).

-Solution B: 20 mL of water + 20 mL of CTAC (100 mM) + 1000 µL of HAuCl4•3H2O (25mM)

+ 300 µL of NaI (10 mM) + 400 µL of ascorbic acid (100 mM).

As fast as possible and under fast stirring, 100 µL of seeds was added in A, and 3.2 mL of this mixture was added in B. The reaction medium was left to stir gently for 2 hours at room temperature. The particles were then centrifuged and washed with CTAC 10 mM.

2) Purification. The AuNTs were added in a solution of 5 mL of CTAC, with a final concentration of CTAC of 130 mM. The solution was left to rest overnight, the precipitate and the supernatant were carefully separated and analyzed by UV-visible-NIR spectroscopy. The supernatant was discarded; the precipitate was washed and stored in CTAC 10 mM.

Rods synthesis and purification. The AgNRs synthesis consists in overgrowing silver on gold bipyramids (BPs). [15b] 1) Preparation of the gold bipyramids. Seeds. In a 50 mL bottle, 2.65 mL of CTAC (25 wt % in H2O) and 400 µL of HAuCl4 25 mM were mixed to 33 mL of water and heated at 30°C for 10 minutes. 4 mL of sodium citrate 50 mM were added and the heating was continued for 30 minutes. Under fast stirring, 1 mL of freshly prepared NaBH4 25 mM was added, the bottle was closed and put in the oven at 40°C for 5 days. Bipyramids synthesis. 500 µL of AgNO3 (100 mM), 10 mL of HAuCl4 (25 mM), and 10 mL of HCl (1 M) were added in 500 mL of CTAB (100 mM). Then, 4 mL of AA (100 mM) was added, followed by 7.5 mL of seeds (15 mL for smaller BPs). After 4 h at 30 °C, the bipyramids were centrifuged twice and purified by depletion during one night at 30 °C in BDAC (350 mM). [26] The supernatant was removed, and the precipitate was redispersed in water and washed twice with 10 mM CTAC. The AuBPs were finally redispersed in 1 mL of 10 mM CTAC. For small BPs, the purification is not possible, the particles were only centrifuged and washed with CTAC 10 mM.

2) Silver overgrowth. 134 µL of BPs were dispersed in a solution of 17.42 mL of water and 2 mL of CTAC 100 mM. The reaction medium was heated to 70°C for 5 minutes and 250 µL of AgNO3 (100 mM, 5 eq) were added, followed by 200 µL of ascorbic acid (500 mM), and heated for 2 hours at 70°C. The particles were centrifuged at an appropriate speed and washed with CTAC 1 mM.

SAXS experiments. The colloidal crystals structures were probed by SAXS at the SWING beamline of the SOLEIL synchrotron (Saint-Aubin, France), using a beam energy of 16 keV and a sample-to-detector distance of 6.481 m. Data treatment (angular averaging and normalization) was done using the Foxtrot software developed at the beamline.

LCTEM. Liquid STEM imaging were simultaneously performed in bright field and dark field modes using an aberration corrected JEOL ARM 200F microscope operating at 200 kV and a liquid-cell TEM holder (Poseidon select, Protochips inc.). A solution of SCs (i.e. colloidal solution with 115 mM of CTAC for the AuNTs and 40 mM CTAC for the AgNRs) was encapsulated in the liquid-cell by using conventional loading process. [27] The Au spacers of the liquid-cells were 150 nm or 500 nm thick. STEM imaging was performed with a probe current of 140 pA measured with a Faraday cup. The electron dose rate of each experiments was automatically calculated in electron.s -1 .Å 2 by the Axon software (Protochips inc.) by dividing the probe current by the irradiated area (i.e. the image area in STEM mode). We used the magnification to tune the dose rate. Image analysis was performed by image segmentation, using the built-in functions of Matlab software.

Figure 1 :

 1 Figure 1: Depletion-induced self-assembly of AuNTs and AgNRs. a) TEM image of AuNTs, b) SAXS data with curves corresponding to two AuNT suspensions loaded with 10 mM CTAC (blue) and 115 mM CTAC (orange). c-e) SEM images of AuNTs assemblies at different magnification. f) TEM image of the AgNRs. g) SAXS data with curves corresponding to two AgNRs suspension loaded with 10 mM CTAC (blue) and 40 mM CTAC (orange). h-i) SEM images of AgNR assemblies at different magnification.

Figure 2 :

 2 Figure 2: Schemes depicting the superlattices obtained by depletion-induced self-assembly in bulk and after loading in the LCTEM chip.

Figure 3 :

 3 Figure 3: Depletion-induced self-assembly of AuNTs and AgNRs in confinement. a-d) STEM images of (a-b) AgNRs assemblies and (c-d) AuNTs assemblies at different magnifications within liquid-cell. Images are recorded in high-angle annular dark field (b) or bright field (a, c, d ) modes. Arrow in (d) point toward a superlattice of standing AuNTs (i.e. in vertical orientation).

  and the disassembly was faster for the AuNTs (40s) compared to the AgNRs (150s). It is worth noting that the AgNRs strips diffuse in the liquid during TEM observations while the SCs made of AuNTs stay attached to the window, which can be explained by the different affinity of Au and Ag NPs for the SiN membrane (see below). More time-lapse STEM images are shown in the SI (Figure S10-S13).

Figure 4 .

 4 Figure 4. Time-resolved disassembly of plasmonic superlattices by LCTEM. a-c) Timelapse STEM-HAADF images showing the disassembly of (a-b) AgNR threads and (c) 2D AuNT hexagonal lattices respectively. Outlines in (b) indicate the dissociation of a thread in two parts, which reform as a single thread a few second later. The dose rate was 0.066 e/A 2 s in (a-b) and 0.592 e/A 2 s in (c).

5 ,Figure 5

 55 Figure 5 (bottom). We can conclude that B (and thus EC) remains constant, while A changes with 𝑑 ̇, due to a change in either K or EA. The dependence of A (and hence of kD) on the dose rate is linear: 𝐴 = α𝑑 ̇, with α=11±1.5 Å 2 /e -.

Figure 5 :

 5 Figure 5: SC survival under electron irradiation. a) Normalized number of particles N(t)/N(0) in AgNR SCs at different dose rates (symbols) and fits with the kinetic model described in the text (lines). b-c) Fit parameters A and B (see text for the definitions).

  membrane, which would lead to the detachment of the positively charged NPs when Columbic repulsion overcomes the local cohesion energy of SCs. Our model showing the dependence of the adhesion energy with the dose rate supports this hypothesis. Moreover, no NPs were redeposited on the area after ejection, further supporting an irreversible change of the surface state of the membrane. There are several studies in the literature showing conflicting behavior of nanoparticles under the electron beam during LCTEM, with observations of attraction to the image area and motion away from the image area. Studies reporting beam induced selfassembly used negatively charged ligands for the NPs,

Acknowledgement

The CNRS is acknowledged for funding and support. This work had benefited from an Investissements d'Avenir grant from Labex PALM (ANR-10-LABX-0039-PALM) and the ANR Project ARTEMIA. The present work had also benefited from the electronic microscopy facility of Imagerie-Gif, (http://www.i2bc. parissaclay.fr), member of IBiSA (http://www.ibisa.net), supported by "France-BioImaging" (ANR10-INBS-04-01), and the Labex "Saclay Plant Science" (ANR-11-IDEX-0003-02). The authors acknowledge SOLEIL for the provision of synchrotron radiation facilities (experiment 20210815) and Thomas Bizien for help using the SWING beamline. The authors acknowledge the French national project METSA for supporting the microscope platform of MPQ laboratory.

Supporting Information Available

The Supporting Information is available free of charge at: Characterization of the NPs, additional SEM images, LCTEM image sequence of the disassembly process are available on-line.


Ziqing Wu
email: ziqing.wu@mines-albi.fr

Raphaël Oger

Matthieu Lauras

Benoit Montreuil

Louis Faugère

Physical Internet Enabled Hyperconnected Circular Supply Chains

Physical Internet (PI) and Circular Supply Chains (CSC) are both promising supply chain paradigms aiming toward the significant improvement of sustainability. PI potentially provides a set of solutions for implementing CSC as both emphasize interconnectivity. This paper presents the Hyperconnected Circular Supply Chain (HCSC) framework, which aims to combine the strengths of both CSC and PI to create a comprehensive set of guidelines for supply chain stakeholders. The proposed HCSC framework encompasses nine core characteristics, addressing aspects such as product design for circularity, on-demand materialization in PI open production fabs, local material and energy recovery, hyperconnected logistics systems for materials and products, sharing economy enablement with PI, circular functionalities exploitation for existing facilities, open and hyperconnected sustainability performance monitoring, and technological and business model innovation.

Introduction

According to the Footprint Data Foundation, humanity's consumption level has exceeded the planet's ability to sustain it by 1.8 times (Global Footprint Network, 2022). The Circular Economy (CE) offers a potential solution to address unsustainability challenges by transitioning from the traditional linear "take-make-dispose" model to a restorative and regenerative framework, with a zero-waste vision (Ellen MacArthur Foundation, 2013;Geissdoerfer et al., 2017). End-of-life objects are not perceived as simply disposable items, but as valuable inputs that can be reintroduced into production processes or repurposed for alternative uses. Within a CE, material and energy loops are decelerated, constricted, and sealed, accomplished through durable design, maintenance, repair, reuse, remanufacturing, refurbishing, and recycling (Geissdoerfer et al., 2017).

Supply Chain Management has historically focused on adding value, maximizing profitability through operational efficiencies, and ensuring customer satisfaction (Stock & Boyer, 2009). However, the growing importance of sustainability-related concerns has prompted the development of new frameworks, such as Circular Supply Chain Management (CSCM), which integrates circular thinking into the traditional supply chain paradigm [START_REF] Batista | In search of a circular supply chain archetype -a content-analysis-based literature review[END_REF]Farooque et al., 2019;Montag, 2022).

While conventional supply chains aim to maximize the supply chain's added value, Circular Supply Chains (CSC) seek to reconcile sustainability and circularity goals (Chopra & Meindl, 2016;Vegter et al., 2020;Montag & Pettau, 2022). CSCM serves as a vital mechanism for implementing CE principles within supply chain processes and practices. CSC research is currently in its nascent phase, offering numerous research opportunities for the practical adoption of its principles (Montag, 2022).

Interconnectivity among various stakeholders is an inherent requirement for CSC. To effectively and efficiently valorize secondary materials and resources, it is essential to facilitate the seamless material, information, and financial exchange among independent companies (Saavedra et al., 2018). The Physical Internet (PI), an alternative supply chain paradigm, is distinguished by its hyperconnectivity (Montreuil, 2011;Montreuil et al., 2012). This feature implies a high degree of interconnection among components and actors across multiple layers, facilitating real-time interactions at any time and place (Montreuil, 2020).

Physical Internet (PI) potentially provides a set of solutions for implementing CSC principles, through hyperconnectivity, horizontal collaboration, and optimal use of supply chain assets [START_REF] Ballot | Physical Internet: First results and next challenges[END_REF]. Despite its potential, no existing study has explored the possible synergy between PI and CSC to the best of our knowledge. The research question of this paper is thus: How can PI inspire and contribute to the implementation of CSC? In response, we introduce a conceptual framework for Hyperconnected Circular Supply Chains (HCSC) to contribute toward bridging the gap.

The remainder of this paper is structured as follows: Section 2 provides an overview and key elements of CSC and PI. In Section 3, we introduce the concept of Hyperconnected Circular Supply Chains (HCSC) and elaborate on its major characteristics. Section 4 concludes and suggests future research directions.

Circular Supply Chains and Physical Internet

As the aim of this paper is to investigate the potential synergy between CSC and PI, it is necessary to provide a concise introduction to these two concepts and their key characteristics. Therefore, the first part of this section is dedicated to outlining the definition of CSC and its archetypal characteristics, as described by Montag (2022); the second part describes the major characteristics of PI.

Circular Supply Chains

Considering the early stage of CSC and CSCM conceptualization, there is currently no consensus on their definitions. [START_REF] Batista | In search of a circular supply chain archetype -a content-analysis-based literature review[END_REF] characterize CSC as the coordination of forward and reverse supply chains, while Geissdoerfer et al. ( 2018) defined CSCM as the configuration and coordination of various organizational functions to achieve CE objectives. Jia et al. (2020) regard CSCM as a notion that complements Closed-Loop Supply Chains.

Among the various definitions of CSCM, the one proposed by Farooque et al. ( 2019) is considered to provide a comprehensive and integrated perspective on the subject (Montag, 2022). Their definition is as follows: "Circular supply chain management is the integration of circular thinking into the management of the supply chain and its surrounding industrial and natural ecosystems. It systematically restores technical materials and regenerates biological materials toward a zero-waste vision through system-wide innovation in business models and supply chain functions from product/service design to end-of-life and waste management, involving all stakeholders in a product/service lifecycle including parts/product manufacturers, service providers, consumers, and users". Montag (2022) has synthesized six archetypal characteristics of CSCM, which include: Rimperatives, restorative and regenerative cycles, sustainability framework, value focus, holistic system thinking, and paradigm shift. R-imperatives encompass a variety of product recovery strategies, such as repair, remanufacturing, refurbishing, and recycling (MahmoumGonbadi et al., 2021). These strategies involve reintegrating end-of-life products into new or alternative usage cycles, retaining their original form, or transforming them to serve different purposes, ultimately extending their life span and reducing waste. Restorative and regenerative cycles entail the circulation of both technological and biological nutrient-based products and materials throughout the economic system (Ellen MacArthur Foundation, 2013). Sustainability framework necessitates the holistic integration of the Triple Bottom Line into CSCM. This implies the simultaneous consideration of environmental, social, and economic aspects.

Value focus emphasizes the importance of leveraging various strategies to maximize resource efficiency. These strategies encompass the power of inner cycles, in which tighter loops yield greater benefits by replacing virgin materials; the power of cycling longer, where products' useful lives are prolonged by undergoing numerous consecutive cycles and remaining within each cycle for an extended period; the power of cascaded use and inbound material/product substitution, which advocates for circulating materials across a variety of product categories; and the power of purer input, necessitating a certain degree of material purity and product/component quality to maximize the value generated through the circular supply chain processes (Ellen MacArthur Foundation, 2013). Holistic system thinking highlights the significance of addressing each stage of the supply chain and comprehending the intricate interconnections among organizations. The paradigm shift entails a considerable transformation in both supply and demand sides to fully embrace CE principles.

Physical Internet

PI, as described by Montreuil (2020), is a "hyperconnected global logistics system enabling seamless open asset sharing and flow consolidation through standardized encapsulation, modularization, protocols and interfaces, to improve the capability, efficiency and sustainability of serving humanity's demand for physical objects". This vision, proposed to tackle the challenges of unsustainable global logistics, is conceptualized through a set of thirteen characteristics, which cover a wide range of logistics and supply chain issues, from product design and realization to transportation services (Montreuil, 2011). The following paragraphs in this section present these thirteen characteristics of PI, along with related recent research updates.

Encapsulate merchandises in world-standard smart green modular containers. One of the key pillars of PI is to use standard and modular PI-containers instead of current heterogeneous good packages, cardboard cases, and pallets (Landschützer et al., 2015). PI-containers shall be designed with several generic characteristics such as easy to handle, store and transport, connected, and environmentally responsible (Montreuil et al., 2015).

Aiming toward universal interconnectivity. The primary goal is to significantly reduce the time and cost associated with load breaking by fostering seamless connections across the supply chain. Achieving this level of interconnectivity necessitates the standardization of loading units, processes, and services, as well as digital interoperability among diverse stakeholders [START_REF] Ballot | Roadmap to the physical internet[END_REF]Pan et al., 2021).

Evolve from material to PI-container handling and storage systems. Logistics nodes in PI embrace automated standardization and interconnected processes [START_REF] Ballot | Roadmap to the physical internet[END_REF]. In particular, Chargui et al. ( 2022) explore the PI's impact on cross-docking platforms, emphasizing their open, automated nature compared to traditional setups.

Exploit smart networked containers embedding smart objects. PI-containers need unique identification, tracing, tracking, and monitoring capabilities, while adhering to data compatibility, interoperability, and confidentiality standards. As active products, they should conduct scheduled reporting, event initiation, goal adjustment and communication, peer interaction, negotiation with equipment, and learning from experiences (Sallez et al., 2016).

Evolve from point-to-point hub-and-spoke transport to distributed multi-segment intermodal transport. With this feature, PI contributes to reduce driving distances and durations, greenhouse gas emissions and social impact of truck driving (Fazili et al., 2017).

Embrace a unified multi-tier conceptual framework. In urban environments, this characteristic can be manifested as appropriate urban pixelization and an interconnected multi-layer logistics web linking meshed networks (Montreuil et al., 2018).

Activate and exploit an open global supply web. Supply network nodes should be accessible to most actors, enabling on-demand service capacity usage and effective handling of stochastic client demand [START_REF] Ballot | Roadmap to the physical internet[END_REF]. Nodes can be classified into four categories: production fabs, deployment centers, logistics hubs and customer interfaces (Montreuil, 2020). Production fabs, conceived under the hyperconnected mobile production framework, incorporate distributed, outsourced, on-demand, modular, additive, mobile, containerized, and hyperconnected production strategies (Fergani et al., 2020). Deployment centers facilitate the prepositioning of objects to efficiently fulfill demand, while logistics hubs streamline processes such as consolidation, cross-docking, sorting, swapping, and transshipping. Customer interfaces encompass smart lockers, retail stores, and other points of customer interaction.

Design products fitting containers with minimal space waste. Product design should consider logistics, ensuring compatibility with specific container sizes and easy completion near the point of use. This entails creating modular products for quick assembly, disassembly, and adaptation to local conditions. This idea can be considered as an integration of design tools, including design for assembly, disassembly, and logistics (Chiu & Okudan, 2010).

Minimize physical moves and storages by digitally transmitting knowledge and materializing objects as locally as possible. This can be achieved by standardized dematerialization specifications, readily accessible hyperconnected mobile production facilities, and robust legal safeguards to protect intellectual property and ensure product authenticity. 3D printing is a promising technology for the implementation of this feature (Ryan et al., 2017).

Deploy open performance monitoring and capability certifications. PI emphasizes the need for live, globally accessible performance tracking, which promotes informed decision-making and continuous improvement. Transparency is essential for building trust and collaboration among logistics stakeholders. Encouraging improvement through benchmarking based on publicly available performance records fosters continuous innovation within the industry.

Prioritize webbed reliability and resilience of networks. In PI, the network utilization adapts to reduce adverse effects by effectively combining transport and storage resources, altering modes and routes as needed. This ensures consistently high network performance and a guaranteed service level [START_REF] Ballot | Roadmap to the physical internet[END_REF]Kulkarni et al., 2022).

Stimulate business model innovation. PI requires a paradigm shift in the logistics industry, prompting stakeholders to adapt and seize opportunities. By promoting diverse business models and collaboration among manufacturers, distributors, and retailers, PI fosters innovation, efficiency, and sustainability in the logistics ecosystem.

Enable open infrastructural innovation. PI encourages the enhancement of infrastructure capacity by leveraging standardizations, rationalizations and automations (Montreuil, 2011). Such advancements are expected in vehicles, carriers, facilities and their interconnections.

Hyperconnected Circular Supply Chains

In this section, we present the Hyperconnected Circular Supply Chain (HCSC) framework, depicted in Figure 1. The framework is composed of nine key characteristics that contribute to the improvement of product design, manufacturing, logistics operations, ultimately promoting circularity and interconnectivity within supply chains. These characteristics are derived from merging CSC and PI features discussed in Section 2. The connections between HCSC and PI features, as well as HCSC and CSC features, are visually illustrated in Figure 2 through a qualitative Sankey diagram. HCSC features are displayed in the center, and the flows connecting CSC and PI features represent their relative contributions to the development of HCSC features. The nine characteristics of HCSC will be elaborated upon in the subsequent parts of this section.

Design products for circularity

In the HCSC framework, products are specifically designed to ensure they can be easily disassembled, repaired, ungraded and recycled. Emphasis is placed on modular design, where products are composed of self-contained units or modules (Nowak et al., 2018). Product subassemblies and modules should be tailored to specific PI-container sizes to fully capitalize on the efficient logistics operations enabled by PI.

Repairing and upgrading of products are significantly simplified for end-users due to the ease of exchanging modules with new, refurbished, or upgraded ones. The rapid delivery of these modules, facilitated by the efficient logistics operations of PI, further contributes to the accessibility of repairing and upgrading processes, thus retain values. Moreover, the cost of repairing or upgrading an existing product is expected to be considerably lower than purchasing a new one, making it a more sustainable and economically viable option for consumers. Additionally, recovery processes for products are smoother as a result of their easy disassembly. This allows for the seamless reintegration of modules and units into another usage cycle following appropriate treatment.

Materialize objects on-demand in PI open production fabs

Products are occasionally discarded due to the unavailability of spare parts or the prohibitive cost of sourcing or producing the necessary components. PI open production fabs can address this issue by manufacturing spare parts on demand and close to the point of use. By digitally transmitting drawings and production specifications for required spare parts to PI open production fabs, localized production can be achieved, enabling rapid delivery to users. Additive manufacturing is one of the key technologies that empowers localized on-demand manufacturing (Foshammer et al., 2022).

Materializing objects on demand can also help address the complexity arising from product differentiation strategies employed by manufacturers. These strategies are intended to attract customers but can create significant additional workload for product recovery processes. Customized refurbishment or upgrading solutions often need to be developed for each model within a product category. The HCSC framework offers a potential solution by proposing a single standard core for refurbishment or upgrading, with model compatibility addressed by the customized production capabilities of open production fabs, operating on demand. This HCSC feature helps extend product lifetimes and preserve value by simplifying repairs, refurbishments, and upgrades. It is enabled by the PI's open global supply web and guided by the principle of minimizing physical moves and storages.

Recover materials and energy as locally as possible

The zero-waste vision of CSC highlights the potential for outputs from one actor to serve as inputs for others. "Waste" can be converted into valuable resources through recycling (materials recovery) and thermal processing (energy recovery) (Themelis & Bourtsalas, 2019). Building recycling centers and waste-to-energy power plants within or near urban areas can contribute to localized resource utilization and minimizing transportation needs. This feature exemplifies the PI principle of minimizing physical moves and storages while simultaneously leveraging and promoting universal interconnectivity and network resilience. As actors interconnect through resources and energy, their dependence on remote suppliers diminishes, leading to a more efficient supply chain with increased resilience. Moreover, it supports the implementation of R-imperatives and fosters restorative and regenerative cycles.

Deliver materials and products with hyperconnected logistics system

The enhanced logistics efficiency provided by PI can significantly transform the way that objects are realized, used, and recovered. In HCSC, multiple options are provided for users to return end-of-life products, such as home pick-up, retail store, recycling bins, or smart lockers (Bukhari et al., 2018). The hyperconnected logistics system ensures a smooth circulation of materials and products. This feature is empowered by several characteristics of PI, including universal interconnectivity, PI-container handling and storage systems, distributed multi-segment intermodal transport, a unified multi-tier conceptual framework and an open global supply web. It can facilitate the cycling of materials and potentially drive a paradigm shift.

Enable sharing economy with

The sharing economy is characterized by consumers providing temporary access to their underutilized physical assets ("idle capacity") to one another, often in exchange for financial compensation (Böcker & Meelen, 2017). This concept can also be found in the Service Web of PI, which aims to enhance the accessibility and usage of assets and goods through interconnected open pooling (Darvish et al., 2014). For instance, it is conceivable that infrequently used tools, such as hand tools, gardening tools, or power tools, can be easily transported via hyperconnected logistics network. Customers can access these tools without incurring high ownership costs, using them as needed. The production of fewer tools overall leads to reduced energy and resource consumption. This feature aligns with the value focus and sustainability aspects of CSC. By using objects more efficiently, value is retained. Economic, environmental and social benefits are expected, as both providers and users can receive monetary rewards, efficient use of goods can conserve resources otherwise needed for production, and beneficial human interactions can be fostered.

Exploit new circular functionalities of existing facilities

Both CSC and PI recognize the presence of untapped value in existing assets, and exploring alternative uses for these assets can contribute to enhanced efficiency and sustainability. CSC emphasizes the importance of R-imperatives, wherein repurposing materials or components is a common practice. PI seeks to optimize production, logistics and transportation assets utilization. For example, public transportation vehicles and infrastructure are employed for parcel delivery (Crainic & Montreuil, 2016), thereby reducing the need for additional trucks in urban areas.

In HCSC framework, the potential of existing customer interfaces and other infrastructure will be maximally exploited to smoothly interconnect circular processes and make CE practices more accessible to end users. For instance, retail stores can integrate repair, refurbishment, and recycling functions for certain products. This feature necessitates innovation in both business models and infrastructure to unlock additional value for existing facilities. By generating revenue, reducing environmental impact through the avoidance of new constructions, and creating job opportunities, it contributes to the triple bottom line of sustainability.

Deploy open and hyperconnected sustainability performance monitoring

In the HCSC framework, the implementation of performance assessment and capability certification systems necessitates the inclusion of sustainability and CE metrics, in addition to traditional measures such as speed, service level, and reliability. These new metrics can be categorized into four dimensions: circularity, economy, environment, and society (Vegter et al., 2020;Montag & Pettau, 2022). By employing an open and hyperconnected performance tracking system and incorporating holistic system thinking, continuous improvement and innovation are encouraged across all stakeholders within a supply chain ecosystem.

Embrace technology innovation

Transitioning to a CSC requires an innovation-rich process that includes restructuring and adjustment, with both technical and non-technical aspects needing consideration. Technological innovation can enhance the eco-design of products, services, and promote sustainable consumption (Kasmi et al., 2022). This feature capitalizes on the PI's ability to facilitate open infrastructural innovation, contributing to a greater variety of material and energy recovery options and improving the circulation of materials and energy.

Stimulate business model innovation

Similar to PI, HCSC fosters the evolution of business models. As CSC hinges on cooperative efforts among multiple actors and system-wide integration, innovative organizational and commercial approaches are needed to enable stakeholder cooperation and potentially trigger a paradigm shift. The ReSOLVE framework, featuring regenerate, share, optimize, loop, virtualize, and exchange as key business actions, was introduced by the Ellen MacArthur Foundation (2015) as a tool to generate circular initiatives.

Conclusion and Perspectives

The Hyperconnected Circular Supply Chain (HCSC) framework proposed in this paper aims to offer a comprehensive set of guidelines for implementing Circular Supply Chains (CSC) by harnessing the synergy between CSC and Physical Internet (PI). This framework comprises nine core characteristics that encompass product design, manufacturing, waste management, logistics, performance monitoring, and innovation. It is designed to be serving as a blueprint for various supply chain stakeholders to follow in order to optimize resource utilization, reduce waste, and minimize the environmental impact of their operations while maintaining high levels of customer satisfaction. Transitioning towards HCSC necessitates embracing change, proactively adapting products and processes, fostering collaboration, and promoting open communication on best practices.

Future research avenues include further refining and expanding of the HCSC framework to incorporate additional characteristics; exploring the framework's full potential through case studies across various industries; investigating its implementation potentiality at different scales through simulations, digital twins, field pilots, and living labs; quantifying, analyzing and learning from a multi-criteria and holistic perspective.

Figure 1 :

 1 Figure 1: Core characteristics of Hyperconnected Circular Supply Chains

• Böcker, L., & Meelen, T. (2017). Sharing for people, planet or profit? Analysing motivations for intended sharing economy participation. Environmental Innovation and Societal Transitions,23,[28][29][30][31][32][33][34][35][36][37][38][39] https://doi.org/10.1016/j.eist.2016.09.004 • Bukhari, M. A., Carrasco-Gallego, R., & Ponce-Cueto, E. (2018). Developing a national programme for textiles and clothing recovery. Waste Management & Research: The Journal for a Sustainable Circular Economy,36(4), 321-331. https://doi.org/10.1177/0734242X18759190 • Chargui, T., Ladier, A.-L., Bekrar, A., Pan, S., & Trentesaux, D. (2022) S., Trentesaux, D., McFarlane, D., Montreuil, B., Ballot, E., & Huang, G. Q. (2021)


Ugo Jacovella
email: ugo.jacovella@universite-paris-saclay.fr

Corentin Rossi

Claire Romanzin

Christian Alcaraz

Roland Thissen

C Ro- Manzin

Monitoring the Light-induced Isomerisation of the Prototypical Polycyclic Aromatic Hydrocarbons C 10 H + 8 through Ion-Molecule Reactions

Keywords: Gas-phase reactions, Ion-neutral reactions, Isomerisation, Polycyclic aromatic hydrocarbon cations, Structure elucidation

Structural rearrangements in ions are essential for understanding the composition and evolution of energetic and chemically active environments. This study explores the interconversion routes for simple polycyclic aromatic hydrocarbons, namely naphthalene and azulene radical cations (C 10 H + 8 ), by combining mass spectrometry and vacuum ultraviolet tunable synchrotron radiation through the chemical monitoring technique. Products of ion-molecule reactions are used to probe C 10 H + 8

structures that are formed as a function of their internal energies. Isomerisation from azulene radical cation towards naphthalene radical cation in a timescale faster than 80 µs was monitored, whereas no reverse isomerisation was observed in the same time window. When energising C 10 H + 8 with more than 6 eV, the reactivity of C 10 H + 8 unveils the formation of a new isomeric group with a contrasted reactivity compared with naphthalene and azulene cations. We tentatively assigned these structures to phenylvinylacetylene cations.

Introduction

Polycyclic aromatic hydrocarbons (PAHs) and their derivatives represent a major class of organic molecules that have enthralled chemists since their discovery due to their peculiar aromaticity nature. [1] They have attracted attention from various chemical communities as they are essential in the understanding of the chemical network at play in the interstellar medium [2][3][4] and in hydrocarbon combustion processes. [5][6][7][8] Furthermore, anthropogenic activities lead to the release of a large amount of PAHs in our atmosphere and soils. As many PAHs have been shown to be toxic, mutagenic and/or carcinogenic to humans as well as microorganisms, causing severe health and environmental issues, [9][10][11][12] understanding their formation, transformation, and destruction mechanisms is thus of upmost importance.

Charged and predominantly positively charged PAHs play a determining role in the chemical evolution in environments exposed to energetic processes such as flames, [START_REF] Homann | Symposium (International) on Combustion[END_REF][START_REF] Fialkov | [END_REF] plasmas, [15,16] and interstellar medium. [17,18] In these environments most of the small-size PAHs exist in the gas phase. The absorption of ultraviolet (UV) radiation can drive several chemical transformations such as fragmentation and/or isomerisation that can modify the chemical properties of the newly formed species. The amount of internal energy deposited into the PAH systems can stir the isomeric distribution population, which in turn affects the available dissociation pathways and the chemical properties of the systems. The unimolecular reactions leading to fragmentation processes have been experimentally and theoretically studied extensively for the prototypical PAH system, e.g., naphthalene cation (Naph + ) and its close-lying isomer azulene cation (Azu + ) [19][20][21][22][23][24][25][26][27][28][29] but exclusively theoretically for the isomerisation reactions that interconvert them. [26,28,30] The C 10 H + 8 gas-phase ion chemistry resulting from absorption of UV photons has proven to be very complex, owing to the richness of low-lying isomers creating a complicated potential energy surface (a simplified version can be found in Figure 1), further frustrated by kinetic considerations as many isomerisation and fragmentation routes are competing on a very extended potential energy surface. [26,28] Mass spectrometry (MS) studies have evidenced that, for both Naph + and Azu + , the major dissociation pathways correspond the loss of H and C 2 H 2 . [19,27] Additionally, at high photon energy (20.58 eV) the fragments observed in the mass spectra starting from both species are similar, [19] which indicates that common transient species must be reached prior to dissociation. However, MS is only sensitive to the mass-to-charge ratio so that without using activation methods, structural information cannot be retrieved.

Experimentally, two studies have investigated the C 8 H + 6 structures resulting from the loss of a C 2 H 2 from

Naph + . [23,29] Schroeter et al. [23] utilised a large panel of complementary mass spectrometric techniques. 8 potential energy surface including several interconversion routes and the main C 2 H 2 loss channels. PE + , PA + , BCB + , PVA + , Naph + , and Azu + stand for the radical cation of pentalene, phenylacetylene, benzocyclobutadiene, phenylvinylacetylene, naphthalene, and azulene, respectively. The energies are taken from the work of Mebel and coworkers. [26] The italic values indicate transition state (TS) energies and the shaded grey area represents the energy range covered by the lowest to the highest TS of the six most probable Naph + ↔ Azu + pathways. species obtained from the loss of acetylene upon dissociative ionisation of Naph, they have been able to rule out phenylacetylene radical cation (PA + ), acyclic structures, and an eight-membered ring structure. They proposed the formation of benzocyclobutadiene radical cation (BCB + ) to be the most favoured one, as previously suggested theoretically by Lifshitz and coworkers. [31] However, the presence of other isomers could not be firmly excluded and rightly so because later Bouwman et al. [29] showed spectroscopically the presence of another structure, the pentalene radical cation (PE + ). and Mayer, [28] that combines density-functional theory (DFT) calculations with Rice-Ramsperger-Kassel-Marcus (RRKM) theory, to conclude that the main C 8 H + 6 isomer from C 2 H 2 -loss at low internal energies of the naphthalene cation is PE + , whereas the contributions of PA + and BCB + grow as the internal energy of Naph + rises. Mebel and coworkers [26] had computed the C 10 H + 8 potential energy surface at higher level of theory using composite method (G3(MP2,CC)//B3LYP) and used RRKM theory to study the photodissociation of Azu + , indicating that the main isomer from C 2 H 2 -loss at low internal energies of

Azu + is also PE + .
We present hereafter results using the chemical monitoring technique consisting in tracking reactivity changes as a function of photoionisation energy. The increase in photon energy produces ions with higher internal energy content and eventually drives unimolecular isomerisation reactions, where the newly formed isomer might exhibit different reactivity with the neutral target. Therefore, changes in reactivity can be used as proxies for the formation of new isomers. We report the structures of C 10 H + 8 species ≈ 80 µs after photoionisation of Azu and Naph -corresponding to a microcanonical rate coefficient of ≈ 10 4 s -1 .

This technique was recently successfully used to disentangle the structural diversity of C 6 H + 4 formed upon dissociative ionisation of PA, Azu, and Naph. [32] Experimental section

Reactive ions (C 10 H + 8 ) were produced by photoionisation of Azu and Naph in the 7 to 20 eV photon energy range using the DESIRS undulator-based vacuum ultraviolet (VUV) beamline [33] at the SOLEIL synchrotron facility in Saint-Aubin (France). Higher order light was removed from the beam by using a gas filter either filled with Kr, Ar or Ne depending on the energy range (8-14, 8-15.5 and 13.5-20 eV respectively).

The reaction products are monitored using a tandem mass spectrometer known as CERISES, [34] which is composed of two octopoles placed between two quadrupole mass filters. The first quadrupole mass filter is used to mass select the desired reactant ions, the first octopole is used to guide these ions to the reaction cell while controlling their collision energy, the second octopole enables to radially confine the ionic products of reactions and to guide them prior to mass analysis with the second quadrupole. The measurements of the quantity of parent and product ions and the absolute pressure of the neutral reactant in the reaction cell enable to obtain the absolute reaction cross sections. Therefore the cross section can be obtained as a function of the collision energy by varying the potential applied along the first octopole and/or the internal energy of the parent ions by varying the photon energy used for the photoionisation process. In the present study, sensitivity to collision energy was recorded for all products at few photon energies, in order to identify the reaction mechanism, either adduct formation versus charge transfer (rapid decrease of cross sections proportional to velocity versus close to constant cross section in the 0-20 eV collision energy range). When scanning the photon energy, the collision energies were set between 0.07 and 0.15 eV in the centre-of-mass reference frame, depending on the system under investigation. Tracking of the isomerisation process relies on the observation of reactivity changes as a function of the photon energy. Four different neutral targets have been used as the difference in reactivity between isomeric ions can also depend on the chemical nature of the neutral reactants. The four neutral reactants were the two C 3 H 4 isomers propyne and allene, methanol (MeOH), and 1,2,4,5-tetramethylbenzene (TMB). They were introduced in the 4 cm long reaction cell at a maximum pressure of 70 nbar, ensuring single collision regime.

Results and discussions

Thermodynamical considerations

The ionisation energies (IEs) are important thermodynamical data, especially in the realm of charge transfer (CT) reaction mechanisms. Indeed, a sine qua non condition for a CT reaction is that the IE of the neutral partner is lower than the IE of the neutral counterpart of the ionic partner. In addition, the cross section for CT reactions are normally governed by Franck-Condon factors and energy resonance but mostly in a simple approach the CT cross section increases with the decrease of the IE difference of the two reactive species.

All IEs of the molecules used in this study are summarised in Table 1.

Appearance energies (AEs) of ionic fragments result from the convolution between thermodynamical energies and kinetic processes, which are important trackers of the unimolecular chemistry happening in the Species Naph [35] Azu [36] Allene [37] Propyne [38] TMB [39] MeOH [40] IEs (eV) The internal energy of the parent ions is tuned by scanning the photon energy used for the photoionisation process. Two important considerations should be noted: i) for each photon energy, the ions will be generated with a distribution of internal energy as the excess of energy can be distributed between the ion and the photoelectron. The use of photoelectron-photoion coincidence experiment enables to produce ions with known internal energy defined by the difference between the photon and the ionisation energy (example of such experiment on Naph + can be found in Ref. 27). The CERISES ion source can be tuned in this mode of ion production, [41] however, the acquisition time becomes very long and would not have permitted to carry out the present study. ii) A high density of accessible states and/or long Franck-Condon progressions are mandatory to obtain a quasi-continuum of states and thus a gradual change of the internal energy distribution of the produced ions. The He I photoelectron spectra of Naph [42] and Azu [43] show that this is the case for both systems. 2.

Isomerisation from ionised Naph

We assume that near the first ionisation threshold of Naph (8.14 eV), the structure of C 10 H + 8 formed corresponds to Naph + . This assumption enables to conclude that Naph + undergoes strong CT reaction with TMB (panel b) of Figure 2), as expected, but presents a weak chemical reactivity -no reactivity with allene and propyne (panels c) and d) of Figure 2) and two weak reaction channels from the reaction with MeOH (panel e) of Figure 2). The cross section of the CT reaction decreases as the internal energy of Naph + increases and at ≈ 12.8 eV a dissociative CT channel (CH 3 loss) kicks in. The dissociative CT reaction can serve as a Naph + thermometer, which will be useful in the following sections.

The high-energy region exhibits a richer chemistry that involves bond forming/breaking reactions. All together with the fact that the CT cross section starts to plateau at the same energy where chemistry arises indicating the formation of a new population of C 10 H + 8 isomers. In other words, these observations imply that beyond a photoionisation energy of ≈ 14.4 eV no more internal energy can be deposited into Naph + without driving isomerisation and/or dissociation reactions. Although we do not have unambiguous means of elucidating the structures of the isomer(s) formed above 14.4 eV, their higher chemical reactivity could indicate ring-opened structures such as alkylated benzene species. Solano and Mayer [28] have separated C 10 H + 8 isomers based on kinetic barriers and RRKM rate constants, they concluded that C 10 H + 8 population rapidly equilibrates to a mixture of E-and Z-phenylvinylacetylene (PVA) cations (structure shown in Figure 1).

The formation rate of Azu + (second lowest energy on the C 10 H + 8 potential energy surface) from Naph + is much slower than the rate to form E-and Z-PVA + . The cross section spectrum of the reaction of Naph + with TMB does not present evidence of isomerisation from Naph + to Azu + . Indeed, as the IE of Azu is lower than the IE of TMB no CT reaction from Azu + to TMB is expected. The cross section of the CT between Naph + and TMB is regularly decreasing up to 14.4 eV so that no significant abrupt drop is observed as one would expect if Azu + was significantly formed. This observation is in agreement with the slow RRKM rate calculated by Solano and Mayer, [28] however, a signal depletion is always more challenging to observe than a growth on background-free baseline so we cannot rule out that a small part of Naph + isomerises to Azu + . From the spectroscopic observation of PE + , Bouwman et al. [29] inferred a formation mechanism of PE + involving an isomerisation from Naph + to Azu + prior to dissociation. They generated ions directly inside an ion trap by two-photon ionisation process at 193 nm, creating an excitation of 12.85 eV.

This excitation energy is ≈ 0.4 eV below our C 8 H + 6 AE from Naph, suggesting that their observable time window is longer than ours so that they can explore the Azu + part of the potential energy surface from Naph + . According to the RRKM calculations of Mebel and coworkers [26] and assuming that we have a microcanonical rate constant of 10 4 s -1 , the isomerisation from Naph + to Azu + should occur between 14.4 and 15.1 eV. Photon energy of 14.4 eV coincides with the apparition of an other group of isomer(s), probably PVA + . [28] Solano and Mayer [28] calculated that PVA + should be predominant at low internal energy but that formation of Azu + should compete more and more with PVA + as the photon energies increase.

However, we did not clearly observe the formation of Azu + , this isomerisation reaction might be occurring but it can be obscured by the reactivity of PVA + and additionally the absence of a specific reaction channel associated with Azu + further complicates its detection. 3.

Isomerisation from ionised Azu

Similarly to the Naph + case, we can make the assumption that the low-energy region corresponds to the formation of only Azu + . No CT occurs with TMB as anticipated based on the IE differences. No chemical reactivity is observed with the two C 3 H 4 isomers and neither with a functionalised reactant partner such as MeOH. The second energy region exhibits the same chemistry as Naph + and undergoes CT with TMB so that we can unambiguously infer that isomerisation from Azu + to Naph + is taking place in region ii).

The maximum CT cross sections reached starting from Naph and Azu are 280 and 60 Å2 , respectively, but no quantitative discussion on the isomerisation yield can be made due to the unknown internal energy distribution of the C 10 H + 8 ions. The dissociative CT channel opens up only at 11.8 eV when starting from

Naph but is present at the isomerisation threshold from Azu, and its cross section parallels the CT cross section to reach a plateau for Azu + while it had an anti-parallel behaviour in Naph + . This indicates that a constant internal energy population of Naph + is produced after isomerisation and produces Naph + with a large amount of internal energy. Interestingly, the cross sections of the dissociative CT reach a plateau between 4 and 5 Å2 for both Naph + and Azu + , which can be understood either as Naph + ions are formed with more internal energy by isomerisation than by direct photoionisation or that the temperature distribution of Naph + formed from Azu + is narrowly distributed around hot temperatures. The CT and thus the isomerisation begins 4.2 eV above the ionisation threshold of Azu. Mebel and coworkers [26] have identified six groups of possible interconversion pathways between Naph + and Azu + , which are all thermodynamically accessible within 4.2 eV of energy. The extrapolation of the RRKM calculations that they performed predicts that at such internal energies the two most probable rearrangements occur through either bicyclobutane or norcaradiene mechanisms. Furthermore, they predicted that it would take about 0.37 µs at 12.4 eV for Azu + to isomerise to Naph + , which is two order of magnitudes faster than our time limit of detectability thus comforting our observations.

The high-energy region reveals the same chemistry as the high-energy one in Naph + . At 20 eV, the cross sections are identical within the experimental uncertainties, indicating that the C 10 H + 8 isomeric population must be very similar starting from Naph or Azu, which rationalises the finding that the photoionisation mass spectra of the two isomers are nearly identical at 20.58 eV. [19] In the case of Naph, the apparition of the "highly" reactive C 10 H + 8 population arrives 6.3 eV above the IE, which is quite similar to the Azu case -6 eV above the IE. The similar amount of internal energy needed for the systems to reach the reactive C 10 H + 8 isomers further indicates that at high energies Naph + and Azu + explore similar parts of the C 10 H + 8 potential energy surface leading to akin isomeric populations.

Conclusion

This work provides evidence for several C 10 H + 8 isomerisation reactions upon photoionisation from azulene and naphtahlene by monitoring different structure-specific ion-molecule reaction channels as a function of 11 photoionisation energies. We could not observe the interconversion from Naph + to Azu + in our observable time window (< 80 µs), whereas the interconversion from Azu + to Naph + was observed through CT reaction with TMB and bonds forming/breaking chemical reactions with MeOH. Azu + and Naph + exhibit very similar chemical properties at high energy, i.e., ≈> 6 eV of internal energy. The increase in reactivity, the large amount of energy needed to form these species, and the previous RRKM studies point toward structures where one ring has been opened such as PVA + isomers.

Overall Azu + and Naph + showed a very limited reactivity except for CT reaction. In an astrophysical context, as nearly no reactivity below the Lyman α (13.6 eV) is observed, it appears that Azu + and Naph + will mainly play a role in the chemical evolution of the interstellar medium through CT reactions or through their reactive ionic photo-fragments.

We have presented direct evidence of 5-to 6-membered ring and 7-to 6-membered ring conversion with the isomerisation from Azu + to Naph + . We unveiled the apparition of a third family of isomers that enhances the reactivity of the C 10 H + 8 species that might play an important piece in energised environments.

In a more global picture, these results on the prototypical PAHs that are Naph + and Azu + emphasise the role that may be played by isomers that are not necessarily the most stable ones.

Figure 1 :

 1 Figure 1: Schematic representation of the C 10 H +8 potential energy surface including several interconversion routes and the main C 2 H 2 loss channels. PE + , PA + , BCB + , PVA + , Naph + , and Azu + stand for the radical cation of pentalene, phenylacetylene, benzocyclobutadiene, phenylvinylacetylene, naphthalene, and azulene, respectively. The energies are taken from the work of Mebel and coworkers.[26] The italic values indicate transition state (TS) energies and the shaded grey area represents the energy range covered by the lowest to the highest TS of the six most probable Naph + ↔ Azu + pathways.

Figure 2

 2 Figure 2 displays in panel a) the photoion yield of C 10 H + 8 from Naph over the 7.8 to 20 eV photon energy range and the reactivity of Naph + with TMB, Propyne, Allene, and MeOH in panel b), c), d), and e), respectively. Two regions can be distinguished: i) the low-energy region [8.1 (IE) -14.4 eV] and ii) high-energy region [> 14.4 eV]. The reactivity of Naph + is summarised in Table2.

Figure 2 :

 2 Figure 2: Panel a) shows the ion yield obtained for C 10 H + 8 using Naph as a precursor. Panels b), c), d), and e) display the reaction cross sections of the corresponding C 10 H + 8 ions with TMB, propyne, allene, and MeOH, respectively, as a function of the photon energy. The collision energy in the centre-of-mass reference frame is indicated as E CM . Empty symbols or duller ones indicate that a different gas was used to filter out higher order light.

Figure 3 8 fromFigure 3 :

 383 Figure 3 shows in panel a) the photoion yield of C 10 H + 8 from Azu over the 7.1 to 20 eV photon energy range and the reactivity of Azu + with TMB, Propyne, Allene, and MeOH in panel b), c), d), and e), respectively. Three regions can be distinguished: i) the low-energy region [7.4 (IE) -11.6 eV], ii) middle-

Table 1 :

 1 Adiabatic ionisation energies of the species used in this study. IEs are truncated to a relevant resolution for the present study.

  AEs are obtained by selecting with the first quadrupole the parent ions (C 10 H + 8 ) and selecting the fragment ions in the second quadrupole. The AEs of the C 8 H + 6 "metastable" ions resulting from unimolecular dissociation of Naph and Azu are found to be 13.2 and 12.6 eV, respectively, considering the observable time window accessible here. It corresponds to the integrated fragmentation signal for parent ion exiting the first quadrupole mass filter (≈ 80 µs) and its travel time towards the beginning of the second quadrupole mass filter (≈ 400 µs).

	8.14	7.41	9.69	10.37	8.07	10.85
	system. The "metastable"

Table 2 :

 2 Reaction products and corresponding cross section for C 10 H + 8 from naphthalene reacting with TMB, Propyne, Allene, and MeOH.

	Reactant	Product	m/z	Mechanism	Cross section σ ( Å2 ) hν < 14.4 eV hν > 14.4 eV
	TMB TMB Propyne Allene MeOH MeOH MeOH C 9 H + C 10 H + 14 C 9 H + 10 C 12 H + 8 C 12 H + 8 C 10 H 9 O + C 10 H 8 O + 11 /C 14] 134 Charge transfer [280-210] 119 [0-5] Dissociative charge transfer (loss of CH 3 ) 152 0 Adduct formation followed by loss of CH 4 152 0 Adduct formation followed by loss of CH 4 145 0 Adduct formation followed by loss of CH 3 144 0 Adduct formation followed by loss of CH 4 MeOH C 7 H + 4 88 Adduct formation followed by loss of 72 amu [0.02-0.14]	210 5 0.2 0.15 0.15 0.2 0.09 0.1

8 H 7 O + 119 Adduct formation followed by loss of C 2 HO/C 3 H 5 [0.35-0.

Table 3 :

 3 Reaction products and corresponding cross section for C 10 H + 8 from azulene reacting with TMB, Propyne, Allene, and MeOH.

	Reactant	Product	m/z	Mechanism	Cross section σ ( Å2 ) hν < 11.6 eV 11.6 < hν < 13.4 eV hν > 13.4 eV
	TMB TMB Propyne Allene MeOH MeOH MeOH C 9 H + C 10 H + 14 C 9 H + 10 C 12 H + 8 C 12 H + 8 C 10 H 9 O + C 10 H 8 O + 11 /C 8 H 7 O + 119 Adduct formation followed by loss of C 2 HO/C 3 H 5 134 Charge transfer 119 Dissociative charge transfer (loss of CH 3 ) 152 Adduct formation followed by loss of CH 4 152 Adduct formation followed by loss of CH 4 145 Adduct formation followed by loss of CH 3 144 Adduct formation followed by loss of CH 4 MeOH C 7 H + 4 88 Adduct formation followed by loss of 72 amu	0 0 0 0 0 0 0 0	[0-60] [0-4] [0-0.25] [0-0.2] [0-0.1] [0-1] [0-0.15] [0-0.55]	60 4 0.3 0.25 [0.1-0.4] 1 0.15 0.5

Acknowledgements U.J. is grateful to the SOLEIL facility for providing beamtime under project 20201123. C.A. acknowledge the synchrotron SOLEIL for the support to the associated CERISES setup of ICP since 2008 and subsistence expenses during beamtime periods. The ICP group acknowledges support from the LabEx PALM (ANR-10-LABX-0039-PALM, Project ERACOP).

Author Contributions

Conflicts of interest

There are no conflicts to declare.


Corentin Rossi

Christian Alcaraz

Roland Thissen

Ugo Jacovella
email: ugo.jacovella@universite-paris-saclay.fr

Keywords: Gas-phase reactions, Ion-neutral reactions, Isomerization, Carbocations, Structure elucidation

published or not. The documents may come

Tunable photoionization chemical monitoring (TPI-CM)-A means to probe molecular ion structures and monitor unimolecular processes through bimolecular ion-molecule reactions: Past, present, and future

Tunable photoionization chemical monitoring (TPI-CM) -a means

to probe molecular ion structures and monitor unimolecular processes through bimolecular ion-molecule reactions: Past, present, and future

Corentin Rossi 1 , Christian Alcaraz 2,3 , Roland Thissen 2,3 , and Ugo Jacovella The way in which molecules can arrange themselves is at the root of organic chemistry and elucidating the structures present in isomeric mixtures remains a major challenge nowadays. A tantalizing question for chemists is how molecules transform from one structural configuration to another one. This review introduces in details a technique -tunable photoionization chemical monitoring -that couples tandem mass spectrometry and photoionization enabling to answer the two aforementioned questions for molecular ions.

It is based on tracking reactivity changes in bimolecular ion-molecule reactions as a function of the internal energy of the ions. This is illustrated with i) the structural elucidation of ortho-benzyne distonic cation within a C 6 H + 4 population and ii) the tracking of the isomerization from azulene radical cation as it gets gradually energized to naphthalene cation. In both cases, charge transfer reactions have been primarily used because of their universality. Finally, a state-of-the-art of the TPI-CM technique using the CERISES mass spectrometer is given, indicating current limitations as well as prospects of improvement.

Introduction

Spectroscopy is a powerful tool providing a characteristic fingerprint of molecules highly sensitive to their structure. Although such signature is isomer-specific, yet, if more than one isomer is present in a mixture, the spectral signatures of the different isomers may overlap in the observed spectrum, preventing unambiguous spectral assignments. Furthermore, generating appreciable amount of desired molecular ions has proved to be challenging in some cases. A good example is the case of the tropylium cation (c-C 7 H + 7 ). Indeed, it took nearly 90 years since the prediction of its existence by Hückel [1] to obtain the first gas-phase spectra in the infrared [2,3] or ultraviolet spectral domain. [4] The rearrangement between the tropylium ion and its close-lying isomer, the benzylium ion (C 6 H 5 CH + 2 ), is a fundamental reaction in the field of physical organic chemistry which has been studied for over half a century and still enthralls chemists, however, to study this reaction a structural probe is needed. The pioneering work of Olah and Dunbar [5,6] followed by similar work using isotopic labeling by Ausloos [7] used the reactivity differences of benzylium and tropylium towards neutral toluene to disentangle benzylium from tropylium. Indeed, benzylium ions exhibit strong methylene-transfer reactions to form xylenium ions

(C 8 H + 9
), whereas tropylium is non-reactive. Although the difference in reactivity in itself is a good structural probe, no information on the rearrangement pathways can be retrieved. A more evolved approach has been introduced and named "chemical monitoring" that consists in keeping the mass spectrometric protocol identical but varying only one parameter at a time, typically, the internal energy of the ions or the collision energy. This concept was first demonstrated by Schröder and co-workers on the unimolecular keto/enol tautomerization of the acetamide cation. [8] The internal energy of the ions was tuned using monochromatized synchrotron radiation. The tandem mass spectrometer used is known as CERISES [9,10] , which is detailed in the next section. Schröder and co-workers used the chemical monitoring technique to investigate the tropylium/benzylium system, combining ion-trap mass spectrometry (IT-MS) with an electrospray ionization (ESI) source. [11] The internal energy of the ions was tuned by changing the temperature of the transfer capillary at the exit of the ESI source. They concluded that the kinetics data obtained were sufficient for kinetic modeling but the IT-MS apparatus was not suitable to retrieve absolute rate constants. The measurements of absolute reaction cross section is one of the preeminent features of the CERISES instrument.

In this mini review, we first present the specifications of the experimental setup -CERISES -as well as its different operational modes. This section is followed by detailed explanations of the tunable photoionization chemical monitoring (TPI-CM) technique, and more specifically using CERISES at the SOLEIL synchrotron. deposited into the system -exemplified with naphthalene/azulene cations interconversion. [13] The last part is a conclusion on what has been done and gives a perspective on what could be done.

Experimental setup

The CERISES (Collisions Et Réactions d'Ions Selectionnés par Electron de Seuil) experimental setup is a tandem mass spectrometer dedicated to ion-molecule reactions and the measurement of their absolute reaction cross sections. Nowadays, CERISES is an associated setup on the DESIRS undulator-based vacuum ultraviolet (VUV) beamline at the SOLEIL synchrotron facility in Saint-Aubin (France). [14] CERISES is schematically represented in Figure 1. The ions are generated from a neutral precursor that is leaked into the photoionization chamber where it is irradiated by the VUV radiation. Higher order light are removed from the beam by using a gas filter filled with the suitable rare gas. The photoionized precursors and potential ionic fragments are extracted toward the first quadrupole. After being mass selected, parent ions are guided The control of the internal energy of the ions can be done to some extent by varying the wavelength of the photoionizing radiation. The neutral precursors must have accessible quantum states with reasonable

Franck-Condon factors in a quasi-continuum fashion in order to linearly and gradually increase the amount of internal energy deposited into the molecular system as the energy of the radiation increases. These

properties are usually fulfilled for large molecular species but are harder to satisfy for small molecules.

Furthermore, direct photoionization does not produce only ions with energy corresponding to the difference between the photon energy and the ionization energy (IE) but rather produces a population of ions with a distributed internal energy. To overcome these limitations, CERISES can be operated in combination with the Threshold PhotoElectron PhotoIon Coincidence (TPEPICO) technique, referred to as the coincidence mode. In this operation mode, photoelectrons are analyzed and discriminated on the basis of their energy and only threshold electrons lift a deflecting potential on the source lens that allows for the extraction of the associated photoion. The cost of the coincidence mode is a low count rate (at most 500 parent ions per second) which results in a very long acquisition time and prevents its used for exploratory studies where many different ion-molecule systems need to be explored, but very suitable for in-depth specific reaction.

Principles of the tunable photoionization chemical monitoring (TPI-CM) method

The chemical monitoring technique enables to unveil ionic structures based on structure-specific chemical reactions. The combination of this technique with tunable photoionizing radiation allows to elucidate the structural evolution of the generated ionic species as a function of the internal energy deposited in the molecular system. In other words, it can be used to monitor unimolecular reactions such as dissociative ionization [12] or isomerization [8,13] by tracking changes in reactivity with increasing photon energy. The principle of the tunable photoionization chemical monitoring (TPI-CM) technique is schematically represented in Figure 2.

We consider here the simple case of a neutral molecule (1) that can only be ionized into two possible stable cationic species -isomer 1 + and 2 + , with significantly different appearance energies (AEs) -hν 1 and hν 2 , respectively. Alternatively, we could suppose another simple case, where a larger precursor P can only dissociate into similar ionic fragments (1 + and 2 + ). For simplicity, we assume the same energies hν 1 and hν 2 to form 1 + and 2 + from 1 and P. A set of two different neutral partners is chosen in such a way that each one can only react with a specific ionic isomer.

In case 1, only the cationic counter (1 + ) of the neutral precursor 1 -or the fragment (1 + ) from dissociative ionization of P -reacts with the neutral target (green triangles). Different reaction mechanisms can be considered -long range type such as charge transfer (CT) reaction or via the formation of an adduct followed or not by elimination. At the photon energy corresponding to the AE of 2 + , the reaction cross section decreases as population of 1 + decreases. It reflects that the isomerization barrier from 1 + to 2 + has been overcome (2 + has become accessible through dissociative ionization).

In the second case, schematized on the bottom part of Figure 2, only 2 + reacts with this second neutral reactant (yellow squares). Once 1 (P) is irradiated with energies ≥ hν 2 , the isomerization channel from 1 + to 2 + (the dissociative ionization channel to 2 + ) is open and thus the reaction cross section rises as the population of 2 + increases. This second case offers the advantage to be background-free, whereas the first one can have competing channels (long-lived excited state for example) also resulting in a decrease of the reaction cross section.

In actual experiments, ions of interest often exhibit a rich chemistry, generating diverse reaction productsespecially in their high-energy region. The CT reaction generally leads to the formation of one main reaction product so that the signal-to-noise ratio is maximized. Furthermore, one can genuinely consider that its reaction cross section is inversely proportional to the ∆IE of the two reactants so that even molecular ions exhibiting poor chemical reactivity can have strong suitable CT signals. Therefore, the CT reaction is the most attractive reaction mechanism for TPI-CM because of its inherent universal applicability.

Results

Two examples making use of the TPI-CM technique will be presented. The first one to monitor the structure of ionic photofragments formed upon dissociative ionization reactions and the second one on investigating different isomerization networks. In both experiments, the ions were generated using the tunable VUV radiation from the DESIRS beamline at the SOLEIL synchrotron. [14] with naphthalene (IE: 8.14 eV) [19] but not with azulene (IE: 7.41 eV). [20] The photoion yields of C 10 H + 8 from naphthalene and azulene are illustrated in panels a and c of Figure 4 The dissociative CT channel (purple squares) can serve as thermometer for the naphthalene ions. Indeed, the dissociative CT channel opens up only at 11.8 eV when naphthalene is used as precursor, whereas it appears right at the isomerization threshold from azulene (11.6 eV). The evolution of the ratio of the products resulting from CT and dissociative CT indicates that naphthalene radical cations formed via isomerization from azulene radical cations are produced with more internal energy than those produced from direct pho-toionization or alternatively that the temperature distribution of naphthalene radical cations formed from azulene radical cations is concentrated around hot temperatures.

In this study, we were not able to monitor the isomerization from naphthalene radical cation to azulene radical cation that would correspond to the first case of Figure 2. An explanation can be rooted in its slow isomerization time comparatively to the transit time in the CERISES apparatus and will be further discussed in the next section.

Conclusions and perspectives

This work details the principles of the tunable photoionization chemical monitoring technique and some of its abilities are illustrated. One with the structural elucidation of C 6 H + 4 ions and the other one studying the interconversion of the azulene/naphthalene radical cations. These two examples highlight how the charge transfer reaction is a very powerful tool in virtue of its universal nature and its ease of finding reactions with large cross sections.

The non-observation of the isomerization from the naphthalene cation to the azulene cation can be rationalized from the low isomerization rate predicted for a micro canonical rate of 10 4 s -1 [21] -corresponding to ≈ 80 µs. The travel time of the ions from the source to the reaction cell gives the upper limit for the timescale of unimolecular processes that can be monitored. This appears as a strong limitation as it renders the use of the TPI-CM technique with the CERISES apparatus blind to processes slower than 80µs. Extending the ion travel time would provide a larger observable time window. In addition, the implementation of a means to significantly vary the dynamic range of this travel time would be a major innovation as it will inform on the molecular dynamics involved.

Another limitation that could be risen is that the ions are not generated with a well defined internal energy at each photon energy but rather with a distributed internal energy, preventing direct comparison with RRKM modeling. This can be addressed using ion production in coincidence with threshold electrons.

At the moment, this way of generating ions cannot be used on exploratory studies of large molecular ions but could be used for one carefully chosen system at a time.

The ortho-benzyne isomer was successfully disentangled from C 6 H + 4 ions based on the difference of ionization energies for the different isomers. However, the spread in kinetics energy with which the ions are extracted from the source gives the minimal energy difference required between two isomers to be distinguished. Recently, the spread of kinetic energy of ions generated using the CERISES mass spectrometer

Figure 1 :

 1 Figure 1: Schematic representation of the CERISES apparatus that enables to record absolute reaction cross sections of ion-molecule reactions as a function of i) the internal energy of the ionic reactant and ii) collision energy.

Figure 2 :

 2 Figure 2: Schematic representation of the TPI-CM method.

  , respectively. The main reactivity results, i.e. the CT (m/z 134) and the dissociative CT (m/z 119) between naphthalene radical cation and TMB, are shown in panels b and d of Figure 4 starting from naphthalene and azulene neutral precursors, respectively. Panel b of Figure 4 displays a strong CT reactivity as soon as the photon energy corresponds to the IE of naphthalene, as expected. In panel d, no CT is observed once the ionization threshold of azulene is reached, as anticipated. However, at 11.6 eV (i.e, 4.2 eV of internal energy) the CT reaction kicks in. This is an unambiguous marker of the isomerization from azulene to naphthalene cations. 4.2 eV of internal energy renders accessible the six possible interconversion pathwaysbetween azulene and naphthalene radical cations calculated by Mebel and coworkers.[21]

Figure 4 :

 4 Figure 4: Panels a and c show the ion yield obtained for C 10 H + 8 using naphthalene and azulene, respectively, as a precursor. Panels b and d display the reaction cross sections of the corresponding C 10 H + 8 ions with TMB as a function of the photon energy. The collision energy in the centre-of-mass reference frame is indicated as ECM. Figure adapted from Ref. 13.

  1 * 1 Université Paris-Saclay, CNRS, Institut des Sciences Moléculaires d'Orsay, 91405 Orsay, France 2 Université Paris-Saclay, CNRS, Institut de Chimie Physique, UMR8000, 91405 Orsay, France

3

Synchrotron SOLEIL, L'Orme des Merisiers, 91192 Saint Aubin, Gif-sur-Yvette, France Abstract:

Acknowledgements C.A. acknowledges the synchrotron SOLEIL for the support to the associated CERISES setup of ICP since 2008 and subsistence expenses during beamtime periods. The ICP group acknowledges support from the LabEx PALM (ANR-10-LABX-0039-PALM, Project ERACOP).

Structural elucidation

The TPI-CM technique has been used successfully to elucidate the structure of C , resulting from a CT reaction, with a high cross section (about 200 Å2 ) while the high-energy region, from 14.5 to 20 eV, unveils a richer chemistry. We assumed that a change in the ion population is responsible for the transition between the two contrasting regions. To elucidate the C 6 H + 4 isomer responsible for the CT reaction, we computed the IEs of the 14 lowest energy isomers of C 6 H + 4 using the CBS-QB3 composite method [15] implemented in Gaussian 16 package [START_REF] Frisch | Gaussian 16 Revision B.01[END_REF] (see right part of Figure 3).

The IE of allene is 9.69 eV, [START_REF] Schulenburg | [END_REF] the necessary thermodynamical condition for the CT reaction to occur is that the IE of the C 6 H + 4 structure plus its kinetic energy (0.4 eV, pink shaded area in top panel of Figure 3) is greater than the one of allene. The six-membered ring ortho-benzyne structure is the only one fulfilling this condition.

Similar reasoning has been followed for C 6 H + 4 generated from dissociative ionization of naphthalene and azulene (loss of 2 C 2 H 2 units). We evidenced that ortho-benzyne was not or very inefficiently formed. The CT reaction with allene was thus used as proxy of the formation of the ortho-benzyne distonic ion.

Monitoring light-induced isomerization

The TPI-CM technique has proven to be an excellent tool to monitor the azulene to naphthalene isomerization. The reaction chosen to do so was the CT reaction with tetramethylbenzene (TMB) as neutral partner. TMB was chosen because of its IE of 8.07 eV, [18] which thermodynamically allows the CT only

Conflicts of interest

There are no conflicts to declare.