transmission electron microscopy using the focused ion beam sampling. A thin layer of sodium chromite (Na x CrO 2 with x # 1) is detected at the interface validating wetting on an oxide mechanism for the enhanced wetting after pre-exposure. Fracture micromechanisms and the crack path of liquid sodium-embrittled austenitic steel 304L at 573 K have been investigated down to the nanoscale. High-resolution orientation mapping analyses immediately below the fracture surface show that abundant martensitic transformations (c ! a) and twinning occur during deformation of austenite. The preferential crack path is intergranular along the newly formed c/c interfaces. It is concluded that these transformations play a major role in the fracture process.

I. INTRODUCTION

Liquid Metal Embrittlement (LME) is a special type of environmental effect where an otherwise ductile material has a brittle fracture mode when mechanically tested while in contact with a liquid metal. LME of austenitic steels by liquid sodium (Na) is of relevance for sodiumcooled nuclear power plants or solar thermal power plants using intermediate heat transfer sodium loops. It is therefore a subject of importance related to the structural safety of such structures and it is highly desirable to assess the potential sensitivity to LME. The recent outcome of LME research shows that to obtain a correct sensitivity assessment, great care must be taken to define proper testing conditions that take into account the physico-chemistry of the solid-liquid interface [START_REF] Fazio | Handbook on Lead-Bismuth Eutectic Alloy and Lead Properties, Materials Compatibility, Thermal-Hydraulics and Technologies[END_REF] and ensure good surface's wettability. [START_REF] Hémery | Effect of oxygen on liquid sodium embrittlement of T91 martensitic steel[END_REF] By correct assessment, one implies reproducible results in mechanical properties as well as a quantification of the maximum possible mechanical degradation. Nonobservance of these conditions, particularly the requirement of prewetting, potentially leads to a spread in the degree of measured embrittlement and at the extreme wrong null-results. The requirement of good wetting is crucial in particular to enable the supply of liquid at the crack tip during crack propagation (provided that the systems have a brittle crack initiation).

The earlier analysis of the interaction of austenitic steels with sodium [START_REF] Bhat | Corrosion behavior of structural materials in sodium influenced by formation of ternary oxides[END_REF] indicated that a high content of dissolved oxygen favors the formation of a NaCrO 2 scale in replacement of the passive chromium oxide that natively forms in air. These exposure conditions were observed to trigger a transition from nonwetting of liquid sodium on the steel's surface to total wetting by liquid sodium of the corroded layer. [START_REF] Longson | Some experiments on the wetting of stainless steel, nickel and iron in liquid sodium[END_REF] Consequently, in a prior work of ours, [START_REF] Hémery | Liquid metal embrittlement of an austenitic stainless steel in liquid sodium[END_REF] pre-exposure to oxygenated sodium at the level of 200 lg/g was used to promote wetting of the 304L steel by sodium. Alkali liquid metals such as sodium are particularly demanding in terms of experimental facilities due to their strong reactivity with di-oxygen and water vapor. The corrosion behavior depends largely upon the content of dissolved impurities (oxygen, hydrogen, nitrogen, or carbon). Therefore,

Contributing Editor: Jürgen Eckert a)
Address all correspondence to this author. a facility with a controlled impurity level allowing handling of sodium in satisfactory conditions was used that simplifies all operations required for preparation of the testing and handling of the liquid metal. [START_REF] Courouau | Corrosion by liquid sodium of materials for sodium fast reactors: The CORRONa testing device[END_REF] In addition, specific operating procedure and purification step were developed to ensure efficient control of the dissolved oxygen in the liquid sodium to a definite level. The issue of contamination during handling is particularly acute with alkali metals and requires glove boxes with a content of water and oxygen controlled to remain continuously below a level of 1 wppm. A similar facility for mechanical testing has been developed in parallel with transfer under cover gas ability. [START_REF] Hémery | Liquid metal embrittlement of an austenitic stainless steel in liquid sodium[END_REF] While handling and manipulations require high purity, it can be beneficial to re-introduce oxygen at a controlled level in sodium processes. Indeed, by applying pre-exposure in oxygenated sodium at a controlled level, a drastic change in the wettability of the 304L steel by sodium was observed as expected by prior know-how. [START_REF] Longson | Some experiments on the wetting of stainless steel, nickel and iron in liquid sodium[END_REF] The formation of NaCrO 2 , supposedly responsible for the observed wettability change, was only inferred from the thermodynamical analysis of our pre-exposure conditions (X-ray diffraction analysis did not convincingly detect this oxide even with a small angle configuration). We report here a dedicated analysis of the interface evolution particularly by transmission electron microscopy (TEM) on the focused ion beam (FIB)-extracted lamella. One of the aims of this study is to provide firm ground for the pre-exposure wetting on an oxide scenario that was proposed earlier. [START_REF] Hémery | Liquid metal embrittlement of an austenitic stainless steel in liquid sodium[END_REF] The pre-exposure conditions for the wetting transition were also the conditions in which a reproducible onset of brittle cracking of 304L steel was observed when tested in contact with liquid sodium at a low strain rate in the temperature range 573-673 K. [START_REF] Hémery | Liquid metal embrittlement of an austenitic stainless steel in liquid sodium[END_REF] This case of liquid sodium embrittlement of austenitic stainless steels presents a challenge to fracture analysis due to the complexity of the observed fracture surfaces. Some cracks observed on the fracture surface are clearly intergranular, but the vast majority of the fracture surface does not seem to have a clear interpretation in terms of the crack path relative to the microstructure. The fracture mode is conventionally qualified as the standardized quasi-cleavage mode. The current level of understanding prevents correlating the topographies of the fracture surface as observed by the standard scanning electron microscopy (SEM) surface analysis with microstructural features. A better knowledge of the fracture path would allow a major step forward to pin point the right modeling scheme to tackle this type of environmental fracture. Therefore, to improve the knowledge of this LME fracture type, we characterized the potential relationship between the fracture surface as observed in SEM and the underlying microstructure. Therefore, in the second part of this work, we report a nanoscale TEM analysis on the FIB lamellae extracted from selected areas of the fracture surface that allows us to give a clear interpretation of this otherwise ambiguous LME fracture mode. The implications of this work will be discussed further.

II. MATERIALS AND EXPERIMENTAL PROCEDURE

A. Materials

The chemical composition of the 304L austenitic stainless steel used in this study is listed in Table I. The as-received material was in the annealed and quenched metallurgical state. The reported yield stress was 377 MPa, the ultimate tensile strength was 596 MPa, and the Vickers hardness was 170 HV. The steel is primarily in the austenitic state with 1.8% of d-ferrite (measurement carried out by ferritoscope). The initial microstructure was analyzed using optical microscopy by electrolytic etching (10% oxalic acid solution) and TEM on a Jeol 1200-EX operated at 120 kV. The mean grain size was estimated to be of the order of 30 lm. TEM analysis of the as-received material indicates a low density of initial dislocations with evidences of stacking faults as expected from a low stacking fault energy steel like the 304L austenitic steel. No initial subgrains structures were found. The sodium used throughout this study is high-purity sodium (.99.95%-ER grade) provided by the Métaux Spéciaux company (Plombière St Marcel, France). Before testing, the sodium was melted and the free level was skimmed off to remove sodium oxides present in excess. A holding step at a temperature of 378 K was then done for ;300 h, analog to a decantation step, followed by the skimming off any residual oxides present at the free level. Then, a last purification step was achieved at high temperature (;903 K during 75 h) with a large surface zirconium foil (150 Â 300 Â 0.125 mm-Neyco, Vanves, France) to gather any residual oxygen. This foil was removed after the purification step. Oxygen was then assessed to be lower than 1 wppm. This was indeed checked with 316L(N) specimens whose oxidation kinetics is known in our conditions. The perfect mirror appearance of the free level qualitatively confirmed this assessment. The resulting total metallic impurity level is less than 10 lg/g (the main impurities are Cl, Fe, and K). The 200 lg/g oxygen condition was obtained by adding, after the zirconium purification step, the required amount of sodium oxide to the liquid metal (Na 2 O-Alfa Aesar, Haverhill, Massachusetts). After the corrosion test, an additional The 304L steel cylindrical notched tensile samples were machined from the transverse-longitudinal direction of the as-received steel's plate. The uniform gauge length was 20 mm, the diameter was 4 mm, and a notch was machined at the center of the gauge length. The notch depth was set to 600 lm, the tip angle was 60°, and the tip radius was 50 lm. No further polishing was carried out on the machined samples asides being cleaned in acetone in an ultrasonic bath. The samples were brought into an argon (Ar) glove box equipped with an isolated molybdenum crucible located at the bottom but inside of the glove box. [START_REF] Courouau | Corrosion by liquid sodium of materials for sodium fast reactors: The CORRONa testing device[END_REF] This crucible was filled with the oxygenated sodium. The samples were then dug in liquid oxygenated sodium and remained at 823 K for 250 h for pre-exposure. During corrosion pre-exposure, the system was perfectly tight toward both the atmosphere and the argon of the glove box so that any residual traces of dioxygen and water equilibrate with the liquid sodium. Such contamination is however negligible because of the large volume of liquid sodium compared to the trapped Argon volume. A small plate (;1 cm 2 ) of mechanically polished 304L steel (SiC P#1200) was added in the bath with the samples as a corrosion coupon. After this preexposure step, the samples were taken out of the liquid sodium bath kept at 383 K. The samples were totally covered with a shiny and perfectly wetting film of sodium (see Fig. 1). Most importantly, the notch was fully filled with the sodium's coating. The solidified sodium stuck in the threading was mechanically removed, and all the samples were brought into the test apparatus under the protection of a double inert gas airtight container.

The mechanical test apparatus consisted of a highpurity Ar glove box connected to a tensile test machine at the crosshead level with a specially designed steel flexible bellow for air insulation and load transmission. The transfer was carried out without exposure of the sample to any contamination from dioxygen or water vapor. The sample to be tested was mounted on the grips of the tensile machine located inside the glove box. The sample heating was carried out by the Joule effect via an internal current loop. The temperature was regulated using a pyrometer. Additional monitoring by a type K thermocouple was used to have a temperature measurement at the notch location. The mechanical tests were performed at a temperature of 573 K and at a crosshead displacement rate of 0.025 mm/min. Because of the notch, the results are not presented as normalized stress-strain curves. Instead, the data analysis is carried out by integrating the mechanical force-displacement curve to obtain the energy to fracture of the test (in Joule). It is then compared with the reference mechanical test obtained at the same temperature and crosshead displacement rate. The reference measurement is carried out with a sample having also been pre-exposed to sodium but later completely cleaned by ethanol washing from any sodium remaining on the surface. This allows us to take into account the possible evolution of the interface, a parameter that might reduce by a small but noticeable amount the energy to fracture of the reference test. Doing so, one avoids overestimating the effect of sodium on mechanical properties, an effect induced by the surface effect at the notch that is particularly sensitive with small-size tensile specimens. In the conditions for this work (temperature of 573 K and a crosshead displacement rate of 0.025 mm/min), the reduction in energy to fracture reads from 20 to 25% depending on the details of brittle crack initiation (several tests were carried out with sodium in these conditions).

After mechanical testing, the test specimen was cleaned in absolute ethanol to dissolve the sodium covering the fracture surface. Within a few minutes exposure, all of the sodium is removed from the fracture surface allowing easy handling in air. One specimen was then cut into pieces to allow for interface analysis. The corroded surface of the corrosion coupon was analyzed on a Ultra 55 SEM (Zeiss, Oberkochen, Germany). A Dual Beam HELIOS Nanolab 650 FIB (FEI, Hillsboro, Oregon) was used to extract TEM lamella using the lift out technique. One TEM foil was extracted, perpendicular to the surface, at a location that was not affected by the plastic deformation near or at the notch (some place along the gauge length but away from the notch). The other ones were extracted from the fracture surface at places representative of the morphology of the fracture surface. First a protective platinum (Pt) layer (of typically 2 lm thickness) was deposited on the surface to protect it from damage during subsequent ion beam sectioning. Two trenches are then milled on both sides of the protected area using a focused Ga 1 ion source operated at 30 kV. A U-cut was then made, and the sample was lifted out and attached on a TEM grid by a Pt deposit. A molybdenum grid instead of a copper one was chosen for the lift out devoted to EDX analysis to allow an easier deconvolution of the Na K a X-ray line (copper has an Lb line at 949.8 eV while Ga has a La line at 1097.9 eV surrounding the Na K a line at 1041 eV). A low Gallium ion current of 80 pA was used for the final thinning of the lamella to reduce surface artifacts. The TEM lamella was analyzed at 200 kV in an Tecnai F20 TEM (FEI, Hillsboro, Oregon) equipped with an EDX detector. A chemical mapping was carried out on the TEM lamella and treated after background subtraction with a deconvolution algorithm for the sodium K a X-ray line (EDAX TEAM software, Draper, Utah). The composition profiles of the corroded layers have been assessed semi-quantitatively by Glow Discharge-Optical Electron Spectroscopy (GD-OES) using the Horiba Jobin-Yvon (Kyoto, Japan) GD-Profiler 2 in the radiofrequency mode (30 W & 800 Pa). The emission responses from the excited sputtered elements (O, Fe, Cr, Ni, Mo, C, Si, Mn, etc.) were detected using a polychromator with a focal length of 500 mm, while a monochromator was used to determine the sodium concentration. Reference alloys were used to calibrate the measurements.

To characterize the crack path, two TEM foils were sampled from the brittle fracture surface area of the 304L steel. One lift out was carried out at a location with step ridges on the fracture surface and the other one at a location where an arrested crack was likely to be captured in the foil (see Sec. III.B for further details on the experimental procedure). These areas represented typical morphologies of the quasi-cleavage fracture surface. The crystallographic orientations of the interesting zones in the FIB lamellas were then investigated by a TECNAI F20 (FEI, Hillsboro, Oregon) operating at 200 kV and equipped with the precession automated crystal orientation mapping (ACOM-TEM) using the Astar package. [START_REF] Rauch | Automatic crystal orientation and phase mapping in TEM by precession diffraction[END_REF] Two phases (c. c. and c.f.c.) were allowed in the automated indexing due to the possibility of martensitic transformation of the 304L austenitic steel. A phase map was built whenever necessary.

III. RESULTS

A. Pre-exposure

The initial surface of the mechanical test specimen was in the state left by machining i.e., a bright and shiny surface. After exposure to liquid sodium for 250 h at 550 °C with 200 lg/g dissolved oxygen, the surface had a dark aspect indicating that either roughness had developed on the surface or that a compound had formed (sodium was removed by a few minutes immersion in ethanol). The SEM analysis of the surface of the exposed sample is shown in Fig. 2. One can see a continuous coverage of the surface by small crystallites that formed during the immersion in liquid sodium. Therefore, a clear change in the surface phase composition induced by preexposure had occurred. Grazing angle X-ray analysis was carried out but did not allow identification of the phase forming this very thin layer.

The GD-OES profile carried out on the flat plate introduced with the samples is shown in Fig. 3. At the FIG. 2. SEM images of the 304L surface after pre-exposure. FIG. 3. GD-OES profile of sodium-exposed 304L's surface. surface (at the left side of the graph), one notices a drop in the steel's iron and nickel constituents correlated with a plateau in chromium, oxygen, and sodium. Other minor elements (carbon and molybdenum) have a peak concentration at the extreme surface potentially indicative of mass transfer between the liquid, the molybdenum crucible, and the steel during pre-exposure. The increase in manganese could be explained by the formation of NaMnO 2 since it is possible at a high temperature above 80 wppm oxygen. [START_REF] Bhat | Corrosion behavior of structural materials in sodium influenced by formation of ternary oxides[END_REF] One notices a secondary peak for sodium between 1 and 1.5 lm depth.

The nanoscale-resolved chemical mapping performed on the TEM lift out is shown in Fig. 4. It indicates that a layer composed mainly of sodium, chromium, and oxygen elements is clearly separated from the underlying matrix elements (iron, chromium, and nickel). The thickness of that layer is of the order of one hundred nm, and the steel/oxide interface is rough. One can note as well that chromium-rich particles formed below the outer NaCrO 2 . This constitutes an unambiguous evidence of the formation of a continuous layer of this phase over a short-term exposure time in oxygenated sodium. This is the key to explain the role of dissolved oxygen on the wettability of sodium over 304L's steel.

Immediately, underneath this oxide layer, a Crdepleted zone is found which is certainly caused by the oxidation reaction. The TEM bright field (BF) image of the interface is shown in Fig. 4(f). This corrosion zone presents intergranular pores that are clearly seen on the TEM foil. These porosities were probably filled with liquid sodium so that it explains the secondary sodium peak on the GD-OES profile. The preferential dissolution of iron along the grain boundaries could be the result of the formation of soluble Na 4 FeO 3 .

The interface is constituted of small grains of a few hundred nm with holes in between at over a depth of several lm. The microstructure at the surface therefore evolved remarkably from a large grain size to small grains in the machining-affected area. This finding bears similitude with the early finding of Moberly et al. of intergranular precipitates forming during the contact with liquid sodium in the 304L steel. [START_REF] Moberly | Planting: Electron microscope observations of liquid sodium interaction with 304 stainless steel[END_REF] The corroded area below the oxide layer is then porous and not protective. One can note that this constitutes a posteriori justification for taking the reference measurement from a batch of sodium-exposed samples since the corrosion-affected area develops some voids that weaken the surface layer.

B. Crack path

The SEM analysis of the fracture surface is shown in Fig. 5. A brittle ring of 200-300 lm starting at the notch is visible, while the final fracture formed a ductile central ring. Therefore, the fracture surface has a mixed brittle/ ductile character that can be explained by the strain rate acceleration at the center during final cracking. A higher magnification view of the fracture surface in the brittle area reveals that some clear intergranular cracks occurred, but the majority of the fracture surface is covered with a quasi-cleavage type fracture surface and numerous step ridges. This is the type of fracture surface that we aimed at investigating further.

The standard lift out procedure is applied here on carefully selected areas of the fracture surface. The sites were chosen to allow for a potential analysis of the microstructure lying underneath specific features of the fracture surface. The TEM foil 1 was extracted to look for an explanation of the numerous step ridges that are seen on the fracture surface. Arrested cracks were also looked for to analyze the crack path by preserving its crack tip. It was selected looking for cracks seemingly going in the material's bulk that might have a chance to be short cracks fully captured within the thinned area of the TEM lamella. Such a case is presented here in the TEM foil 2. Figure 6 shows the fracture surface from the FIB preparation of that TEM lamella.

The microstructures indexed by the ACOM-TEM tool of foil 1 and foil 2 are shown in Figs. 7 and8. The thin foil 1 analysis detected numerous areas with a9 (bcc) phase transition underneath the fracture surface [Fig. 7(c)]. At the same time, twinning and grain refinement was induced by plastic deformation prior to cracking creating a wealth of newly formed interfaces. One can note the clear correlation between the presence of step ridges at the fracture surface [top part of the orientation map in Fig. 7(b)] and large abrupt orientation changes just below the surface ridges. This indicates that the step ridges visible on the fracture surface are a legacy of the deformed microstructure lying below. Indeed, the ridges are correlated with newly formed large grains transverse misorientations or eventually mechanical twin boundaries.

An arrested crack was captured in the TEM foil 2 and subsequently indexed (see Fig. 8). The crack has an opening angle of roughly 20°. The crack is clearly intergranular in the c matrix and was stopped at an interface (a twin boundary) that was slanted toward the presumed vertical loading axis (therefore not oriented favorably for crack propagation). Several measurements of the misorientation across the cracked interface show that the interface belongs to the general type, the one with a misorientation higher than 15°(not corrected for the crack opening angle). The rapid variation of orientations above and below the cracked interface also indicates highly inhomogeneous plastic deformation. The crack stopped at a twin boundary oriented diagonally but did not go further by a cleavage process. The crack path in this case is fully c/c interfacial.

IV. DISCUSSION

Austenitic steels such as the 304L spontaneously develop a passive layer with a thickness of few nanometers. It is known that this passive oxide layer has a duplex structure composed of an inner chromium oxide layer (Cr 2 O 3 ) and an outer layer composed of a mix of iron oxides and hydroxides (Fe 3 O 4 , Fe 2 O 3 , Fe(OH) 2 , and FeOOH). [START_REF] Lorang | Chemical composition of passive films on AISI 304 stainless steel[END_REF] Liquid sodium does not spontaneously wet such a passive layer unless the temperature is raised above a threshold temperature (773 K for at a least a few hours in Ref. 10). The analysis of the interfacial layer built up during sodium pre-exposure gave the unambiguous outcome that being exposed to oxygenated sodium leads to the formation of a thin NaCrO 2 -type oxide fully covering the 304L stainless steel surface. Chromium is leached from the 304L steel to form this stable oxide as evidenced by the Cr depletion behind this layer seen in GD-OES and in the EDX mapping of the TEM foil. The threshold level of dissolved oxygen in sodium for the formation of binary oxides (Cr 2 O 3 or FeO) is very high and exceeds its solubility in sodium. [START_REF] Bhat | Corrosion behavior of structural materials in sodium influenced by formation of ternary oxides[END_REF] Therefore, the initial passive layer is thermodynamically unstable at a dissolved oxygen content of 200 lg/g and at a temperature of 823 K. On the other hand, the formation of sodium chromite ternary oxide (NaCrO 2 ) is known to be thermodynamically favored in such conditions. [START_REF] Bhat | Corrosion behavior of structural materials in sodium influenced by formation of ternary oxides[END_REF] Therefore, the layer found in this study is in agreement with the thermodynamics of the steel in sodium. The relatively small thickness of the layer explains the difficulty when trying to detect it by X-ray diffraction. One notes that the NaCrO 2 -type oxide has a layered rock-salt structure. Sodium is intercalated between planes formed of chains of CrO and it can be nonstoechiometric while keeping its structure (R-3m). [START_REF] Braconnier | Etude par désintercalation électrochimique des systèmes Na x CrO 2 et Na x NiO 2[END_REF] As a matter of fact, one should rather consider an oxide of type Na x CrO 2 with theoretically x # 1.

The kinetics of formation of this oxide has been studied at low oxygen dissolved content (1-24 ppm) on 316L steel. [START_REF] Cavell | Some observations concerned with the formation of sodium chromite on AISI 316 exposed to oxygenated sodium[END_REF] The kinetics is rather slow with the onset of oxidation starting above grain boundaries then fully covering the remainder of intragranular areas with an assumed diffusion limited parabolic time dependence. This slow growth (limited by the diffusion of chromium) and the fact that the grown layer has a thickness of the order of one hundred nanometers imply that our conditions (200 lg/g, 823 K during 250 h) are almost optimal in building up a compact and fully covering oxide as the inspection by SEM could confirm. Since the wettability has transitioned from initially non-wetting to total wetting, one can infer that sodium wets the Na x CrO 2 oxide layer. The very reproducible wetting condition is due to the fact that the Na x CrO 2 oxide layer is continuous without interruption. This validates, a posteriori, the preexposure conditions that were used in Ref. 5 to enhance the wettability of the steel in sodium. These conditions are able to promote the build up of an interfacial ternary oxide whose wettability warrants good and reproducible LME sensitivity studies. One can elaborate on why sodium totally wets Na x CrO 2 while the initial 304L passive oxide is not wetted. Because there is an interfacial phase formation, this wetting transition should belong to the reactive wetting category (liquid metal/oxide reactive wetting). However, the observed transition to a nearly perfect wetting indicates larger improvement than usually reported in reactive wetting. This possibly could imply that the sodium bonding of this new phase is bonded metallically rather than iono-covalently as categorized by Eustathopoulos. [START_REF] Eustathopoulos | Interfacial bonding, wettability and reactivity in metal/oxide systems[END_REF] It remains to be clarified at this point such that wettability studies of different oxides by liquid sodium would be necessary.

Early on, it was shown that the 304L steel interacted with sodium by the formation of an intergranular phase. [START_REF] Moberly | Planting: Electron microscope observations of liquid sodium interaction with 304 stainless steel[END_REF] Here, it is found that the first few microns of the corroded layer develop some porosity and Cr-rich precipitates (see BF TEM image in Fig. 4). This might also be an ingredient favoring early crack initiation. Within a wetted crack, the capillary flow of sodium induced by good wetting is then the key to supply liquid sodium at the crack tip at the earliest stage of LME in a self-maintained process. Once started, a crack is filled and will remain filled as it propagates.

The pre-exposure conditions of this study imply that the sodium that fills the notch of the test specimen is oxygenated at some level (but not necessarily as much as the initial oxygen content if some oxygen was consumed by corrosion in the course of the crack propagation). The effect of dissolved oxygen on the fatigue crack growth rate has been studied between 1 ppm up to 40 ppm with the 304L austenitic steel. [START_REF] James | Fatigue-crack propagation behavior of type 304 stainless steel in a liquid sodium environment[END_REF] In the conditions of that study (high test frequency and the lowest temperature starting at 700 K), it was concluded that the dissolved oxygen level did not affect the crack growth rate. In another study, the oxygen influence on crack growth under constant load was studied with the 304L austenitic steel. It was also shown that while varying the oxygen content of sodium from 0.7 to 14 ppm, the intergranular crack growth had only a slight increase and so was deemed largely independent upon the dissolved oxygen concentration. [START_REF] Abe | Time dependent crack growth of type 304 stainless steel in elevated temperature sodium[END_REF] It is therefore anticipated that the oxygenated condition is no more detrimental than ultrapure sodium.

The brittle fracture mode evidenced by 304L in contact with oxygenated sodium above 573 K, albeit with limited mechanical degradation (20-25% reduction of the energy to fracture), has possibly important implications. It therefore deserves some close attention. The fracture surface has the typical appearance of the quasi-cleavage brittle fracture with a few intergranular cracking spots. The knowledge of the crack path relative to the microstructure, or at least its categorization in terms of intergranular or intragranular, is decisive for the subsequent understanding and modeling of LME. The crack path was analyzed in typical areas of the fracture surface where the fracture mode is not seemingly intergranular. The fracture surface of this LME case has numerous areas with regular step ridges and buried cracks. Two TEM foils were extracted to analyze the microstructure below regular step ridges and at a location with a microcrack going inside the material. Two main conclusions can be drawn from the analysis that was carried out via the ACOM-TEM technique.

Firstly, the 304L steel microstructure is heavily deformed with a large amount of a9 phase transformed material. Twinning at the microscale is also observed right below the fracture surface. These processes created a microstructural refinement with a large increase in the density of sub-grain boundaries. The step ridges structure is correlated with a large underlying orientation change (subgrains or twin boundaries) and is the trace of the interplay of twinning or phase transform and fracture.

Secondly, the arrested crack of foil 2 shows a typical case of c/c interfacial fracture. The underlying microstructure is extremely complex with numerous twins and subgrain boundaries. The fracture surface in the analyzed area was typical of the quasi-cleavage-like fracture area. We can infer that a large part of the crack propagation proceeds as an interfacial fracture.

There are several possibilities to analyze the interplay between fracture and twinning or a9 (bcc) phase transform. Under stress, interface formation via twinning or phase transformation may simply be an independent response over brittle cracking. Another possibility is that the crack may nucleate twins or trigger phase change due to the high stress triaxiality near the crack tip. The third possibility is that these newly formed interfaces may provide a preferred path for the growth of cracks.

In studies of the ductile fracture of the 304LN stainless steel, a clear correlation between deformation induced martensite and the nucleation of microvoids has been found. [START_REF] Das | Experimental investigation on martensitic transformation and fracture morphologies of austenitic stainless steel[END_REF] In a detailed study of the deformation at room temperature, examples of shear-band intersection, shearband grain boundary intersection or grain boundary triple point have been found as nucleation sites. [START_REF] Das | Experimental investigation on martensitic transformation and fracture morphologies of austenitic stainless steel[END_REF] Therefore, the location of a9 (bcc) phase transform or twinning can be found inhomogeneously but everywhere in the microstructure beyond some threshold strain level so that the interface nucleation process takes place at relatively low deformation levels. LME occurs clearly at a later stage of plastic deformation in our case such that a large quantity of interfaces must already be present so that one may think of twinning or phase change as an independent process relative to brittle cracking. It means that it is likely not while twinning that cracking takes place.

We infer that the crack propagation was likely to proceed in a discontinuous manner associated with crack propagation along twinned and c (fcc) to a9 (bcc) boundaries induced by phase transform. The step ridges of the fracture surface in particular are leftovers of this process. This is similar to the association of twinning and fracture in bcc metals where twin-matrix interfaces are found to be the preferred propagation path. [START_REF] Reid | The association of twinning and fracture in bcc metals[END_REF] The specificity of LME is that here sodium induces crack propagation along newly formed interfaces.

These characterizations document further the LME case of 304L in contact with sodium as being due to the embrittlement of plastically generated interfaces. We highlight the similarity of these findings with the demonstration of the importance of subgrain defect structures in understanding hydrogen embrittlement of steels. [START_REF] Martin | On the formation and nature of quasi-cleavage fracture surfaces in hydrogen embrittled steels[END_REF] Additional insight calls for in situ investigation of the propagation of a crack induced by liquid sodium. This would provide further indications into the details of the intertwining of phase transform and cracking in austenitic steel's LME.

V. CONCLUSION

The LME case of the 304L austenitic steel has been analyzed down to the nanoscale in two crucial points. The first one is the wettability and the role of pre-exposure in oxygenated sodium. The second one is related to the correlation of the crack path with the deformed microstructure. The main findings are the followings:

(1) The wettability is enhanced due to the formation of a Na x CrO 2 (x # 1) layer in direct contact with liquid sodium. The oxygenated sodium pre-exposure conditions that were used allow for the in situ buildup of this oxide and validate the wetting of an oxide scenario that was inferred from the thermodynamical analysis in Ref. 5.

(2) A quasi-cleavage-like fracture surface with step ridges is to be understood as a product of microstructural changes (phase transform and grain refinement) induced by prior plastic deformation. The step ridges are the markers of discontinuous crack propagation intertwined with phase transform. This strongly hints at the quasicleavage being essentially an interfacial cracking in the case of LME of austenitic steels.

(3) The crack path was characterized on arrested cracks. It is found fully interfacial. The crack path is found to follow the newly formed interfaces created by plastic deformation prior to cracking that are embrittled by sodium.

We highlight the need to characterize fracture surfaces with the underlying material's microstructure when assessing the fracture mode of unstable steels like the 304L austenitic steel.

FIG. 1 .

 1 FIG. 1. Image of sodium wetted specimens extracted from the sodium bath after 250 h pre-exposure at 550 °C (sodium wetted notches indicated by arrows).

FIG. 4 .

 4 FIG. 4. (a-e) EDX mapping of main elements (f) TEM BF micrograph of the area (the rectangle indicates the area of EDX mapping).

FIG. 5 .

 5 FIG.5. Fracture surface analysis after tensile test in sodium at 573 K for 304L.

FIG. 7 .

 7 FIG. 7. Microstructure beneath the fracture surface in thin foil 1: (a) TEM BF micrograph (b, c) corresponding orientation map superposed with image quality (IQ) with respect to the normal direction and phase mapping.

TABLE I .

 I Composition of the as-received 304L austenitic steel. Zr foil purification step is repeated to measure the oxygen content at the end of the test by measuring the weight gain. It was found close to the initial one (200 lg/g).

	high-temperature
	Element	C	Cr	Co Mn	P	Ni	Si	S	Fe
	wt%	0.015 18.6 0.10 1.48 0.023 9.0 0.42 0.001 Balance

B. Experimental procedure

ACKNOWLEDGMENTS

Financial support for this work by the CEA-Generation 4/Structural material project is acknowledged. Exposure of the specimen to liquid sodium was carried out in the Corrona-1 test bench with the help of V. Lorentz (CEA Saclay). The FIB work was carried out using the equipment of the MATMECA consortium partially funded by the ANR under contract number ANR-10-EQUIPEX-37. M. Tabarant (CEA Saclay) is thanked for performing the GD-OES analysis.


Joanna Zapała
email: jzapala@ichf.edu.pl

Thomas Custer

Jean-Claude Guillemin

Marcin Gronowski

Photochemistry of XCH 2 CN (X=-Cl,-SH) in Argon Matrices

We report infrared spectra and photochemical behavior of the potentially astrochemically significant species, mercaptoacetonitrile (HS-CH 2 C≡N) and, for comparison purposes, chloroacetonitrile (Cl-CH 2 C≡N), both suspended in an argon matrix at 6K. Photolytic formation of the isocyano products HS-CH 2 -NC and Cl-CH 2 -NC were observed as well as CH 3 NSC and CH 3 SCN (in HS-CH 2 CN photolysis). While no dissociation products were observed for Cl-CH 2 -CN, photolysis of HS-CH 2 -CN produced compounds necessitating the loss of the CN group to form CH 2 =S, the SH group to form H 2 C-CN and HC-CN, or both CN and SH to form CH 3 and CH 4 . Observation of emission spectra upon annealing indicates the presence of free sulfur atom in matrices of photolyzed HS-CH 2 -CN.

Introduction

Theoretical studies 1 show that mercaptoacetonitrile (HS-CH 2 -CN), methyl isothiocyanate (CH 3 NCS), and methyl thiocyanate (CH 3 SCN) are the three most stable molecules of the C 2 H 3 NS family having energies of 0, 7.8, and 12.4 kJ/mol respectively.

Although not yet detected in the interstellar medium (ISM), each of these can be

considered as a potential candidate. The structurally similar species CH 3 SH 2 , HSCN [START_REF] Frerking | Interstellar isothiocyanic acid[END_REF][START_REF] Halfen | Detection of a New Interstellar Molecule: Thiocyanic Acid HSCN[END_REF] , HNCS [START_REF] Frerking | Interstellar isothiocyanic acid[END_REF][START_REF] Halfen | Detection of a New Interstellar Molecule: Thiocyanic Acid HSCN[END_REF] , and CH 3 CN 5,[START_REF] Thiel | Complex organic molecules in diffuse clouds along the line of sight to Sagittarius B2[END_REF] have all been detected in the ISM. Hydroxyacetonitrile (HO-CH 2 -CN), the closest oxygen-bearing analogue of mercaptoacetonitrile, has been observed in laboratory generated interstellar and cometary ice analogs [START_REF] Danger | Formation of hydroxyacetonitrile (HOCH2CN) and polyoxymethylene (POM)-derivatives in comets from formaldehyde (CH2O) and hydrogen cyanide (HCN) activated by water[END_REF] and recently detected in solar-type protostar IRAS16293-2422 B [START_REF] Zeng | First detection of the pre-biotic molecule glycolonitrile (HOCH2CN) in the interstellar medium[END_REF] . It is not difficult to imagine replacement of an H atom with a CN, NC, or SH group, or addition of a CH 2 or S to one of these known astrochemical species to reach the lowest energy members of the C 2 H 3 NS chemical family.

While structural analogies are interesting, explicit pathways for formation or decomposition of S containing species in space, whether they have already been detected or are promising candidates, should be elucidated in the laboratory and through calculations. Existing models and the ratio of abundances of different sulfur species have already been used to evaluate the age of interstellar clouds [START_REF] Wakelam | Sulfur Chemistry: 1d Modeling in Massive Dense Cores[END_REF][START_REF] Liu | Fragmentation and Ob Star Formation in High-Mass Molecular Hub-Filament Systems[END_REF][START_REF] Li | Sulfur-Bearing Molecules in Massive Star-Forming Regions: Observations of Ocs, Cs, H2s, and So[END_REF] and to estimate the mass of an emerging star [START_REF] Esplugues | Modelling the Sulphur Chemistry Evolution in Orion Kl[END_REF] . However, problems encountered in determination of sulfur abundance in the interstellar medium [START_REF] Woods | A New Study of an Old Sink of Sulphur in Hot Molecular Cores: The Sulphur Residue[END_REF] , describing the interstellar synthesis of methanethiol (detected in Sagittarius B2 core [START_REF] Linke | Interstellar Methyl Mercaptan[END_REF] and hot core of G327.3-0.6 [START_REF] Gibb | Chemistry of the Organic-Rich Hot Core G327.3-0.6[END_REF] but absent in TMC-1), and prediction of abundances of nearly all sulfur-containing molecules detected in TMC-1 [START_REF] Wakelam | The 2014 Kida Network for Interstellar Chemistry[END_REF][START_REF] Goicoechea | Low Sulfur Depletion in the Horsehead Pdr[END_REF][START_REF] Wakelam | Sulfur Chemistry: 1d Modeling in Massive Dense Cores[END_REF] suggest that our current understanding of sulfur chemistry remains incomplete.

Here we explore one aspect of sulfur chemistry by describing the computational and experimental results of the UV photolysis of mercaptoacetonitrile (HS-CH 2 -CN, MAN) and its closely related synthetic precursor chloroacetonitrile (Cl-CH 2 -CN, CAN) in noble gas matrices. Although CAN is not of obvious astrochemical significance (Cl abundance is generally not high in the universe and only a handful of species including AlCl, KCl, NaCl, HCl, HCl + , H 2 Cl + , and CH 3 Cl [START_REF] Fayolle | Protostellar and cometary detections of organohalogens[END_REF] have so far been detected), no noble gas matrix photolysis experiments have been performed on this species. As detailed spectroscopic information for this molecule is available, photolysis of this chemical provides a convenient comparison with mercaptoacetonitrile. While chemically inert noble gas matrices are not considered good models for authentic interstellar ices [START_REF] Öberg | Photochemistry and Astrochemistry: Photochemical Pathways to Interstellar Complex Organic Molecules[END_REF] , photochemistry measurements in this environment can be treated as a first, necessary step toward understanding the transformations that might result in a more representative but chemically complex ice environment. They also provide a means to produce more exotic isomers for spectroscopic characterization. Photochemistry in pertinent interstellar ice analogues (e.g., containing CO, H 2 O, or other species or mixtures [START_REF] Hendecourt | Interstellar matrices: the chemical composition and evolution of interstellar ices as observed by ISO[END_REF] ) is the next step along the path towards understanding photochemical processing in space and will be the subject of a future publication.

Of the three lowest energy C 2 H 3 SN family members, mercaptoacetonitrile (HS-CH 2 -CN), which has a weak chemical stability at room temperature, is the only one not commercially available and is therefore less well studied. Nevertheless, this species has been explicitly discussed as a likely astrochemical species whose potential routes of formation in the interstellar medium were explored and microwave spectrum experimentally measured at both -30 °C and room temperature [START_REF] Møllendal | Rotational Spectrum, Conformational Composition, Intramolecular Hydrogen Bonding, and Quantum Chemical Calculations of Mercaptoacetonitrile (HSCH2C≡N), a Compound of Potential Astrochemical Interest[END_REF] . Synclinal and antiperiplanar conformers of mercaptoacetonitrile were identified and their energetic separation measured to be 3.8±0.3 kJ/mol. A theoretical study of these two conformers has also recently been made in an effort to understand their potential behavior in harsh astrophysical environments [START_REF] Bacchus-Montabonel | Proton-induced collision dynamics on potential prebiotic sulfur species[END_REF] . The charge transfer dynamics resulting from collisions between energetic protons and these isomers was explored and compared to the two lowest-energy HCN dimers (Z and E isomers of cyanomethanimine). Charge transfer of the synclinal conformer turned out to be more efficient than for the corresponding cyanomethanimine.

Outside of these recent works, only a handful of studies on mercaptoacetonitrile are available in the literature. Several synthetic routes to its formation have been explored and routine [START_REF] Gronowski | A Theoretical Study on the Spectroscopy, Structure, and Stability of C 2 H 3 NS Molecules[END_REF] H and [START_REF] Woods | A New Study of an Old Sink of Sulphur in Hot Molecular Cores: The Sulphur Residue[END_REF] C NMR as well as mass spectrometry and IR spectroscopy were used to characterize the products [START_REF] Mathias | Synthesis of mercaptoacetonitrile under mild conditions[END_REF][START_REF] Wepplo | Synthesis of Mercaptoacetonitrile and Cyanomethyl Thioesters[END_REF][START_REF] Gaumont | Thiocyanohydrins, a New Class of Compounds, Precursors of Unstabilized Thiocarbonyl Derivatives[END_REF][START_REF] Alexander | Direct aqueous synthesis of cyanomethyl thioglycosides from reducing sugars; ready access to reagents for protein glycosylation[END_REF] . Most pertinent to the work at hand are spectroscopic measurements in the infrared and all reported spectra to date are for the pure liquid. Early work of Mathais et al. [START_REF] Mathias | Synthesis of mercaptoacetonitrile under mild conditions[END_REF] reported six vibrational bands: the SH group at 2560 and 920 cm -1 ; the CN group at 2240 cm -1 ; and the CH 2 group at 2980, 2940 and 1400 cm -1 . Wepplo [START_REF] Wepplo | Synthesis of Mercaptoacetonitrile and Cyanomethyl Thioesters[END_REF] gave five unassigned vibrations: 2680, 2260, 990, 925 and 710 cm - 1 .Gaumont et al. [START_REF] Gaumont | Thiocyanohydrins, a New Class of Compounds, Precursors of Unstabilized Thiocarbonyl Derivatives[END_REF] reported the SH vibration at 2560 cm -1 and CN vibration at 2220 cm -1 .

Finally, Alexander et al. [START_REF] Alexander | Direct aqueous synthesis of cyanomethyl thioglycosides from reducing sugars; ready access to reagents for protein glycosylation[END_REF] reported a CN vibration at 2247cm -1 . Aside from the vibration at 2680 cm -124 , possibly meant to be 2980 cm -1 , these comprise a consistent summary of vibrations for this molecule in the liquid phase. They are collected and presented with our results in Table1 Supporting Information. No more detailed IR spectral data of mercaptoacetonitrille is available.

As for the commercially available chloroacetonitrile (Cl-CH 2 -CN), numerous publications are available and include measurements using a variety of spectroscopic methods. Infrared data was first available in the 1950's, when Zeil et al. [START_REF] Zeil | Die Schwingungsspektren der chlorierten Acetonitrile[END_REF] performed measurements of pure Cl-CH 2 -CN using IR and Raman spectroscopy and gave a comprehensive list of fundamental vibrations as well as some of their combinations. Since 50's, various other groups have performed measurements in the gas-phase 28, 29 or in the liquid-phase [START_REF] Van Der Kelen | Studies on Halogenated Aliphatic Compounds VII. Dipole Moments and Infrared Spectra of Halogenated Acetonitriles[END_REF][START_REF] Jones | Spectroscopic studies. Part V. The spectra and structure of monohalogenoacetonitriles[END_REF] . Researchers have described the effects of solvation 31 , 28, 32 , complexation [START_REF] Augdahl | Spectrocopic studies of charge transfer complexes-VI: Nitriles and iodine monochloride[END_REF] , or change of chlorine substituent [START_REF] Butt | The transmission of resonance effects through a methylene group-spectroscopic studies on some α-substituted-acetonitriles and ptoluonitriles[END_REF] on the position of various bands. IR band positions [START_REF] Crowder | Force constants and normal vibrations of the haloacetonitriles[END_REF] and intensities [START_REF] Wladkowski | The SN2 identity exchange reaction ClCH2CN + Cl-.fwdarw. Cl-+ ClCH2CN: experiment and theory[END_REF] have also been calculated. Nemes et al. [START_REF] Nemes | Infra-red dispersion studies. Part 3.-Band intensities of acetonitrile and its chlorinated derivatives[END_REF] estimated absolute intensities for eight vibrations found in the liquid phase and, later the same year [START_REF] Nemes | Infra-red dispersion studies. Part 5.-Variation of intensity with change of phase for acetonitrile and its chlorinated derivatives[END_REF] , refined the position of the vibrations and compared their intensities with what was measured in the gas phase as well as in carbon disulfide (CS 2 ). Thomas et al. [START_REF] Thomas | Infrared intensities of V(CN) bands[END_REF] investigated the CN stretching band in more detail and estimated its absolute intensity, focusing on the effect of environment and phase change on its intensity. Finally, George et al. [START_REF] George | Aggregation and association of polar molecules in low temperature matrices and in the gaseous phase[END_REF] reported hydrogen chloride mode of Cl-CH 2 -CN:HCl complex at 2709 cm -1 in an Ar matrix at 7K although no band positions for the matrix isolated Cl-CH 2 -CN species alone were given. The positions and intensities of Cl-CH 2 -CN vibrations published by abovementioned authors are collected in Table2 Supporting Information.

While the photochemistry of both HS-CH 2 -CN and Cl-CH 2 -CN species has not been described, a rich literature concerning transformations of small molecules is available to suggest potential processes that might occur upon UV irradiation. Due to the matrix cage effect [START_REF] Apkarian | Molecular Photodynamics in Rare Gas Solids[END_REF][START_REF] Bondybey | Chapter 10. Rare-gas matrices, their photochemistry and dynamics: recent advances in selected areas[END_REF] loss of larger moities (e.g., fragments such as CN or CH 3 ) following bond breakage is generally not observed and isomerization is the most probable process.

Nevertheless some fraction of these fragments may escape. Depending on the energy available and rare gas used, H, Cl or S atoms can end up being separated to leave radicals, ions, or other isomeric combinations.

Concerning isomerization of cyano species, UV irradiation of cyanoacetylene (HC 3 N) trapped in noble gas matrix leads to the formation of several isomers of the parent

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The Journal of Physical Chemistry molecule, including isocyanoacetylene (HCCNC) and an imine (HNC 3 ) [START_REF] Kołos | The Infrared Spectroscopy of Hnccc: Matrix Isolation and Density Functional Theory Study[END_REF] . Both of these isomers have been already detected in the interstellar medium accompanying the more abundant cyanoacetylene. Kawaguchi et al. identified HCCNC [START_REF] Kawaguchi | Detection of Isocyanoacetylene HCCNC in TMC-1[END_REF] and later the same year HNCCC [START_REF] Kawaguchi | Detection of HNCCC in TMC-1[END_REF] in Taurus Molecular Cloud 1 (TMC-1). Gensheimer et al. [START_REF] Gensheimer | Observations of HCCNC and HNCCC in IRC+10216[END_REF] identified both isomers in IRC +10216 and Vastel et al. in the L1544 pre-stellar core [START_REF] Vastel | High spectral resolution observations of HNC3 and HCCNC in the L1544 pre-stellar core[END_REF] . The formation of isocyanides from cyano precursors in a matrix environment has been observed for a variety of unsaturated cyanoacetylenes including HC 3 N, HC 5 N [START_REF] Coupeaud | Spectroscopy of Cyanodiacetylene in Solid Argon and the Photochemical Generation of Isocyanodiacetylene[END_REF] , NC 4 N 49 , NC 6 N 50 , and C 2 H 3 CN 51 among others. Nevertheless, it is not a foregone conclusion that such cyano/isocyano rearrangement must occur for any arbitrary cyanide containing species.

Photolysis of thiol species of the form R-SH results mainly in scission of C-S and S-H bonds [START_REF] Callear | Transient spectra and primary processes in the flash photolysis of CH3SSCH3, CH3SCH3, CH3SH and C2H5SH[END_REF][START_REF] Bridges | Photochemistry of methanethiol at 254 and 214 nm[END_REF][START_REF] Jensen | Dissociation dynamics of CH3SH at 222, 248, and 193 nm: An analog for probing nonadiabaticity in the transition state region of bimolecular reactions[END_REF] with the main products being radicals and, depending on photolysis conditions, various radical recombination products. In 195 nm flash photolysis experiments on gaseous CH 3 SH Callear et al. [START_REF] Callear | Transient spectra and primary processes in the flash photolysis of CH3SSCH3, CH3SCH3, CH3SH and C2H5SH[END_REF] Hydrogen atom production and escape of H 2 from the matrix cage has been reported as well, producing even more complex, dehydrogenated species, where photolysis of methanethiol at 121nm in a nitrogen matrix at 14 K leads to formation of CH 4 , CS, CS 2 , CH 3 and H 2 CS as the main products [START_REF] Jacox | Matrix isolation study of the infrared spectrum of thioformaldehyde[END_REF] .

Lastly, studies of photolysis of alkyl halides in matrixes [START_REF] Jacox | Matrix-Isolation Study of the Vacuum-Ultraviolet Photolysis of Methyl Fluoride. The Infrared Spectra of the Free Radicals CF, HCF, and H2CF[END_REF][START_REF] Jacox | Matrix-Isolation Study of the Vacuum-Ultraviolet Photolysis of Methyl Chloride and Methylene Chloride. Infrared and Ultraviolet Spectra of the Free Radicals CCl, H2CCl, and CCl2[END_REF] show that the cage effect can inhibit halogen detachment processes. Photolysis of methyl chloride CH 3 Cl in Ar and N 2 matrixes 60 was found to yield CCl, HCCl and H 2 CCl as major products in addition to smaller amounts of CH 3 , HCl and CH 4 .

Based on these works, the most likely result of mercaptoacetonitrile or chloroacetonitrile photolysis should involve CN/NC isomerization or be products of -H or -SH elimination.

Theoretical methods

A combination of two methods of solving the electronic Schrödinger equation was used for these studies to arrive at vibrational frequencies. Density functional theory with the 3term correlation functional of Becke [START_REF] Becke | Density-Functional Thermochemistry. III. The Role of Exact Exchange[END_REF] and the exchange functional of Lee, Yang, and Parr 62 (B3LYP) were used with vibrational second-order perturbation theory [START_REF] Cossi | Energies, structures, and electronic properties of molecules in solution with the C-PCM solvation model[END_REF][START_REF] Barone | Vibrational zero-point energies and thermodynamic functions beyond the harmonic approximation[END_REF][START_REF] Barone | Anharmonic vibrational properties by a fully automated second-order perturbative approach[END_REF][START_REF] Bloino | A second-order perturbation theory route to vibrational averages and transition properties of molecules: General formulation and application to infrared and vibrational circular dichroism spectroscopies[END_REF] to calculate anharmonic frequencies and intensities. The ab initio coupled cluster method, truncated to iterative treatment of single and double excitations and the perturbative treatment of triple excitations (CCSD(T)) [START_REF] Bloino | A second-order perturbation theory route to vibrational averages and transition properties of molecules: General formulation and application to infrared and vibrational circular dichroism spectroscopies[END_REF][START_REF] Bartlett | Many-body perturbation theory, coupled-pair many-electron theory, and the importance of quadruple excitations for the correlation problem[END_REF][START_REF] Pople | Electron correlation theories and their application to the study of simple reaction potential surfaces[END_REF][START_REF] Purvis | A full coupled-cluster singles and doubles model: The inclusion of disconnected triples[END_REF][START_REF] Scuseria | An efficient reformulation of the closed-shell coupled cluster single and double excitation (CCSD) equations[END_REF][START_REF] Pople | Quadratic configuration interaction. A general technique for determining electron correlation energies[END_REF] were used during computation of harmonic frequencies. The triple-ζ Dunning-type basis set (cc-pVTZ) [START_REF] Dunning | Gaussian Basis Sets for Use in Correlated Molecular Calculations. I. The Atoms Boron through Neon and Hydrogen[END_REF] was chosen for the CCSD(T) calculations while its augmented version (aug-cc-pVTZ) [START_REF] Kendall | Electron Affinities of the First-Row Atoms Revisited. Systematic Basis Sets and Wave Functions[END_REF] was used for B3LYP computations. All vibrational calculations were preceded by geometry optimization at the same level of electronic structure theory and the Gaussian 09 74 program was used for all computations.Vibrational frequencies ultimately reported were calculated using the following equation:

;

(1) 𝜈 𝑟𝑒𝑐 = 𝜔 𝐶𝐶𝑆𝐷(𝑇) +(𝜈 𝐵3𝐿𝑌𝑃 -𝜔 𝐵3𝐿𝑌𝑃 ) where and are harmonic frequencies computed either using CCSD(T) or 𝜔 𝐶𝐶𝑆𝐷(𝑇) 𝜔 𝐵3𝐿𝑌𝑃

B3LYP methods and is the anharmonic frequency computed using B3LYP as 𝜈 𝐵3𝐿𝑌𝑃 described previously. This methodology has been successfully applied in earlier studies [START_REF] Puzzarini | Accurate Harmonic/Anharmonic Vibrational Frequencies for Open-Shell Systems: Performances of the B3LYP/N07D Model for Semirigid Free Radicals Benchmarked by CCSD(T) Computations[END_REF][START_REF] Coupeaud | C5N-anion and new carbenic isomers of cyanodiacetylene: A matrix isolation IR study[END_REF][START_REF] Kołos | Prospects for the Detection of Interstellar Cyanovinylidene[END_REF] and in other systems where the resulting vibrational frequencies generally had an accuracy better than 10 cm -1 78 .

Experimental methods

The HS-CH 2 -CN used in these experiments was synthesized according to Gaumont et al. [START_REF] Gaumont | Thiocyanohydrins, a New Class of Compounds, Precursors of Unstabilized Thiocarbonyl Derivatives[END_REF] , purified as reported by Møllendal et al. [START_REF] Møllendal | Rotational Spectrum, Conformational Composition, Intramolecular Hydrogen Bonding, and Quantum Chemical Calculations of Mercaptoacetonitrile (HSCH2C≡N), a Compound of Potential Astrochemical Interest[END_REF] and stored either with or without Amberlyst at -80°C. This malodorous compound decomposes vigorously at room temperature or under basic conditions. The kinetic stability of this compound is strongly dependent on the temperature and the presence of acidic compounds. Although Amberlyst may stabilize the chemical for long-term storage, it also proved to be a strong source of methanol which was trapped in its pore spaces during previous synthetic steps.

The chemical proved to be sufficiently stable without Amberlyst for use in these photolysis experiments.

Chloroacetonitrile, Cl-CH 2 -CN was purchased (98%, abcr GmBH). Spectroscopic measurements were performed on a Bruker Vertex 70 instrument equipped with a KBr beam splitter and liquid-nitrogen-cooled MCT (HgCdTe) detector in the 400-6000 cm -1 spectral range at the maximum available resolution of 0.16 cm -1 .

Degassed HS-CH 2 -CN or Cl-CH 2 -CN was combined with Ar (5.0 Multax s.c.) at a ratio of 1:1000 in a stainless steel vacuum manifold with partial pressures measured using The Journal of Physical Chemistry capacitance manometers (10 torr and 1000 torr, MKS instruments). Following two hours to allow for mixing of HS-CH 2 -CN or Cl-CH 2 -CN with Ar in the manifold, samples were deposited onto a CsI window pre-cooled to 5-6 K inside a vacuum chamber (~10 -7 Torr).

Cooling was accomplished using a closed-cycle helium refrigerator (DE-202SE Advanced Research Systems) whose temperature was controlled using a LakeShore 325 temperature controller. The cold window was positioned at 45° with respect to the path of IR radiation of the spectrometer and deposition was carried out at a flow rate of ca. 0.1 mmol/minute through a capillary nozzle of 1 mm diameter terminating ~2cm from the cold window. The total amount of the mixture deposited never exceeded 8 mmol (~300 Torr).

Lower limits of absolute intensities were calculated using the formula from [START_REF] Szczepaniak | Measurement of the absolute infrared intensity of the fundamental vibration of HCl in low temperature matrices[END_REF] , by Szczepaniak et al.: . In this expression, is the molar

𝐴 [ 𝑘𝑚 𝑚𝑜𝑙𝑒 ] = 2.3031 100 * 𝑐 * 𝐷 ∫𝑙𝑜𝑔 𝐼 0 𝐼 𝑑𝑣 𝑐 concentration of HS-CH 2 -CN or Cl-CH 2 -CN in , is the film thickness , is [ 𝑚𝑚𝑜𝑙𝑒 𝑐𝑚 3 ] 𝐷 [𝑐𝑚] 𝑙𝑜𝑔 𝐼 0

𝐼

the measured absorbance of the band, and is the wavenumber [cm -1 ]. The molar 𝜈 concentration was calculated using = where ρ is the density of solid 𝑐 𝑝(HSCH2CN or ClCH2CN)

𝑝(argon)

• ρ argon with a value of 44 [START_REF] Szczepaniak | Measurement of the absolute infrared intensity of the fundamental vibration of HCl in low temperature matrices[END_REF] and is the measured partial pressure of the pertinent

𝑚𝑚𝑜𝑙𝑒 𝑐𝑚 3
𝑝 species. It is assumed that both the chemical and the rare gas are sufficiently pure that the partial pressures measured are not affected by any other species and that this ratio is conserved through deposition and in the ice itself. Observed bands were integrated using the OPUS program (Bruker Optik GmbH 2014). The thicknesses of the deposited matrices were determined by monitoring reflected He-Ne laser light 80 as well as using interference fringes observable in IR spectra. For the laser thickness measurements, a fixed-frequency He-Ne laser (ThorLabs HNL050R model, λ = 632.5 nm) was directed towards a growing matrix. Interference was produced in the laser light reflected by the vacuum-ice and ice-substrate interfaces which have different and changing optical paths as film growth proceeds. Reflected light intensity as a function of time was recorded using a portable CCD spectrometer equipped with a fiber optic that could be adjusted outside of the vacuum chamber to capture the reflected laser light (Mightex CCD, HRS series).

The intensity of the reflected laser light oscillates between periods of constructive and destructive interference as a function of time. While the time of one oscillatory period is a function of deposition rate, the change in thickness produced over the course of one period is constant. Because of this, the total thickness of a matrix can be estimated by counting the number of fringes starting from time zero until deposition is stopped (or an arbitrary time of interest) and using the following relationship presented by Urso et al. [START_REF] Urso | Combined infrared and Raman study of solid CO[END_REF] :

. In this equation, is the film thickness (total thickness of the

𝐷 = 𝑚∆𝑑 = 𝑚𝜆 0 2𝑛 𝑓 1 -𝑠𝑖𝑛 2 𝜃 𝑖 /𝑛 2 𝑓 𝐷 matrix) [cm]
, is the number of fringes observed, is the thickness increase between 𝑚 ∆𝑑 two fringes [cm], is incidence angle of the laser with respect to the cold target, is the 𝜃 𝑖 𝑛 𝑓 refractive index of the film, and is the wavelength of laser light: 6.325x10 -5 [cm]. We 𝜆 0 assumed to be similar to pure argon for our experimental conditions, a reasonable 𝑛 𝑓 assumption given the 1:1000 ratio of chemical to Ar. A refractive index value of 1.52 was used for solid Ar at 6 K 81 . Thickness information can also be retrieved from IR spectra directly using interference fringes which, in a well ordered ice, manifest as an undulating baseline in our spectra. In this method, a modified equation was used [START_REF] Urso | Combined infrared and Raman study of solid CO[END_REF]

: 𝐷 = 1 𝑙 * 2𝑛 𝑓 1 - 𝑠𝑖𝑛 2 𝜃 𝑖 𝑛 2 𝑓
, where is the film thickness [cm], is the distance between the fringes [cm -1 ], is 𝐷 𝑙 𝜃 𝑖 incidence angle of the IR light with respect to the cold target, and is the refractive index 𝑛 𝑓 of the film. The equivalence of these two methods of thickness determination was verified for pure Ar matrices by depositing a measured amount of noble gas while monitoring using laser interference fringes. At specified points, deposition was interrupted (0.625 mmol, 1.25 mmol, 2.5 mmol, 3.75 mmol, and 5 mmol) so IR spectral fringes could be measured. Good agreement between both methods was obtained with a difference of approximately 10%. In another series of experiments, thickness estimated from the pattern in our IR spectra was measured as a function of incident angle and gave an 𝜃 𝑖 error typically less than 2%.

Both HS-CH 2 -CN and Cl-CH 2 -CN were exposed to UV irradiation generated by an excimer laser. HS-CH 2 -CN samples were irradiated either using at 248 nm (KrF excimer) for 2.5 hours or 193 nm (ArF excimer) for 1.5-2.5 hours with an average laser pulse frequency of 10 Hz and power of 1 mJ per pulse (approx. 0.2 mJ/cm 3 ) measured just in front of the quartz window of the vacuum chamber using a pyroelectric sensor (Ophir Optronics PE50B). Cl-CH 2 -CN was irradiated at 193 nm (ArF excimer) for 2 hours with an average laser pulse frequency of 10 Hz and power of 0.5 mJ per pulse (approx. 0.1 mJ/cm 3 ). Annealing was performed as the last step of sample processing and consisted of raising the temperature of the sample to ~35 K for a period of 20 min and then recooling to the original value. Emission observed during annealing was recorded using a Mightex CCD spectrometer (HRS series, 300-1070 nm spectral range, 1.7 nm resolution).

IR spectra taken following annealing were used to confirm that various groups of product signals belonged to the same species (either increasing or decreasing together). in space, the microwave spectrum of CAN has already been reported and the rotational constants and dipole moments for the three lowest energy isomers of CAN given in Table 3 may prove useful for an eventual radioastronomical search or microwave measurements. In this table, ground state rotational constants are calculated at the CCSD(T)/cc-pVTZ level of theory and differences between ground state and equilibrium rotational constants at B3LYP/aug-cc-pVTZ. Without including core electron correlation and approaching the complete basis set limit, the precision of these rotational constants is likely a little more than 1%, a limitation that will be addressed in a future publication.

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The predicted dipole moments for these molecules are high, which suggests that their detection during any eventual experimental rotational spectroscopy studies should be facile. Freq.

[cm -1 ] Inten. [km mol -1 a) combined CCSD(T)/anharmonic B3LYP calculation, see equation (1) b) anharmonic B3LYP c) in parentheses, results of anharmonic CCSD(T) computations 1 d) two sites were observed with the most intense reported in the table (Fig. 1) e) out of detection range f) Relative intensities given with respect to ν 5 located at 1419 cm -1 (experimental) rather than the most intense mode which is outside of our measurement range.

g None of the computed overtones has an intensity higher than 1 km mol -1 and are not reported h) lower limit of absolute intensity

Spectroscopy

Prior to photolysis, the spectrum of pure HS-CH 2 -CN in a noble gas matrix was recorded (Fig. 2) and analyzed. The measured IR band positions along with lower limits for absolute and relative intensities are presented in Table 1 along with theoretically predicted values for synclinal and anti-periplanar forms (for structures of synclinal and anti-periplanar forms, see Figure 1 from 83 ).

Fig2 Experimental IR spectra of HS-CH 2 -CN for pertinent regions of Table 1. Y scale is the intensity in arbitrary units.

* all regions share a common y-scale except for the feature at 1419 cm -1 which has been reduced by a factor of 2.

Since HS-CH 2 -CN is not a particularly rigid molecule, containing only a single, nonconjugated multiple bond, it might be expected to exhibit interesting site structures upon trapping in solid Ar or broadening due to allowed molecular motions within a matrix cage.

However, IR absorption bands were narrow and only the peaks located near 1250 cm -1 and 1419 cm -1 had easily discernable structure. For the doublet near 1250 cm -1 , a typical ratio between components of 3:2 was observed with the higher intensity signal at 1249.6 cm -1 .

The absolute value of the difference between measured and predicted frequencies of the synclinal conformer ranges between 0 and 26 cm -1 . The sign of the difference depends on the band being considered. Overall, these deviations are generally 1% or less of the experimentally measured frequency. In light of considerable differences between HS-CH 2 -CN as a pure liquid and in an Ar matrix environment, our matrix measurements are also remarkably consistent with reports for the pure liquid in the literature and reveal additional vibrations at 786, 1193, 1249. Microwave studies indicate that HS-CH 2 -CN can exist in either anti-periplanar or synclinal configurations in the gas phase at room temperature [START_REF] Mollendal | Rotational Spectrum, Conformational Composition, Intramolecular Hydrogen Bonding, and Quantum Chemical Calculations of Mercaptoacetonitrile (HSCH2CN), a Compound of Potential Astrochemical Interest[END_REF] and that these forms have an energy difference of 3.8±0.3 kJ/mol. Studies of L-isoserine [START_REF] Dobrowolski | IR low-temperature matrix, X-ray and ab initio study on l-isoserine conformations[END_REF] and β-alanine [START_REF] Dobrowolski | IR Low-Temperature Matrix and ab Initio Study on β-Alanine Conformers[END_REF] indicate that it is sometimes possible to conserve ratios between conformers during matrix deposition. Based on the computed differences in vibrational frequencies of antiperiplanar and synclinal forms of HS-CH 2 -CN (Table 1), the resolution of our spectrometer and observed peak widths, these conformers should be easily distinguishable in these measurements should they both become frozen in the Ar ice. The computed (CCSD(T)/cc-pVTZ) energetic difference between conformers is 4. at 745 and 755 cm -1 are assigned to the two matrix sites of [START_REF] Crowder | Force constants and normal vibrations of the haloacetonitriles[END_REF] Cl-CH 2 -CN and the less intense peaks at 741 and 751 cm -1 were attributed to matrix sites of the [START_REF] Nemes | Infra-red dispersion studies. Part 3.-Band intensities of acetonitrile and its chlorinated derivatives[END_REF] Cl-CH 2 -CN isomer. For v 7 , more intense features at 484 and 483 cm -1 were again assigned to [START_REF] Crowder | Force constants and normal vibrations of the haloacetonitriles[END_REF] Cl-CH 2 -CN and the smaller 482 and 481 cm -1 bands to [START_REF] Nemes | Infra-red dispersion studies. Part 3.-Band intensities of acetonitrile and its chlorinated derivatives[END_REF] Cl-CH 2 -CN.

Fig3 Experimental IR spectra of Cl-CH 2 -CN (see Table 2A). Y scale is the intensity in arbitrary units.

Matrix sites assigned to [START_REF] Crowder | Force constants and normal vibrations of the haloacetonitriles[END_REF] Cl-CH 2 -CN (black and red) and matrix sites for [START_REF] Nemes | Infra-red dispersion studies. Part 3.-Band intensities of acetonitrile and its chlorinated derivatives[END_REF] Cl-CH 2 -CN (green and yellow). * all regions share a common y-scale except for the region between 740 cm -1 and 760 cm -1 which was reduced by a factor of 2. **all regions share the same x scale except for the region between 3030 cm -1 and 3010 cm -1 which was reduced by a factor of 2

Comparing theoretically predicted values of IR absorption frequencies to experimentally observed values, differences range between 0.4 and 18 cm -1 with the sign of the deviation varying depending on the mode being considered. This amounts to less than 1% of the measured value for all modes except for that for at 476 cm -1 which 𝑣 7 differs by ~1.7%. The position of vibrations was also very similar to previously reported in liquid and in gas phase and ranges between 0 and 14 cm -1 (Table 2 Supporting Information). We were not able to detect any of the overtones or combinations reported in the work of Zeil et al. [START_REF] Zeil | Die Schwingungsspektren der chlorierten Acetonitrile[END_REF] . Our inability to observe such features is consistent with our calculations which indicate they should have very low intensities. While no new features of this molecule were detected in our study, no author has reported vibrations from [START_REF] Nemes | Infra-red dispersion studies. Part 3.-Band intensities of acetonitrile and its chlorinated derivatives[END_REF] Cl-CH 2 -CN isotopomer or peak locations in a noble gas matrix.

Measured absolute intensities are again smaller than theoretical predictions, although they are much closer than those measured for HS-CH 2 -CN. Measured values are only smaller than theoretical values by up to a factor of around 2.4 depending on the band being considered. Better agreement likely has to do with the higher purity and stability of the commercially available chemical.

Absolute intensities have been measured previously in the liquid phase by Nemes et al. [START_REF] Nemes | Infra-red dispersion studies. Part 5.-Variation of intensity with change of phase for acetonitrile and its chlorinated derivatives[END_REF] and by Thomas et al. [START_REF] Thomas | Infrared intensities of V(CN) bands[END_REF] for the CN vibration. They are several times larger than both our experimental and calculated values. Absolute intensities were also calculated by Wladkowski et al. [START_REF] Wladkowski | The SN2 identity exchange reaction ClCH2CN + Cl-.fwdarw. Cl-+ ClCH2CN: experiment and theory[END_REF] . Despite their moderate level of theory, their values are quite similar to our own. A variety of photoproducts were at least tentatively identified, some of which were common across photolysis wavelength and others of which were not. Formation of mercaptoisocyanomethane (HS-CH 2 -NC) was observed at both wavelengths with reasonable certainty and features associated with this species are shown in Fig 4 with positions listed in Table 5. Three peaks were associated with this species whose relative ratio is conserved across multiple experiments, suggesting that they belong to a single photoproduct. While these features are not strong, the agreement of observed line positions and intensities with ab initio theoretical results 1 for HS-CH 2 -NC (Table 5) form a good basis for attribution of these features. The prevalence of CN/NC conversion observed for other cyanide containing carbon chain molecules in a matrix environment also lead us to expect that this transformation will occur.

Fig4

Experimental IR spectrum of HS-CH 2 -NC formed as a photoproduct in an Ar matrix. Y scale is the intensity in arbitrary units. The most stable C 2 H 3 SN isomer, CH 3 NCS, has its most intense vibrations at 2142 cm -1 and 1419 cm -1 with a ratio of 100 to 6 [START_REF] Turowski | Isomerization of methyl isothiocyanate and methyl thiocyanate in argon matrices induced by UV irradiation[END_REF] . While we were able to distinguish a 2142 cm -1 vibration, this was assigned to HS-CH 2 -NC. A band of the parent molecule falls in the exact location of the 1419 cm -1 feature. While we cannot rule out the formation of this species, it is impossible to confirm its production here.

Positive identification of the second isomer, CH 3 SCN is also problematic. Peaks located at 2168 cm -1 , 1442 cm -1 , and 2951 cm -1 (with a ratio of intensities of 3:1:1) should be distinguishable [START_REF] Turowski | Isomerization of methyl isothiocyanate and methyl thiocyanate in argon matrices induced by UV irradiation[END_REF] . While there is photolytic formation of a species with a vibration at 2168 cm -1 which might be linked to CH 3 SCN, this peak had a very small intensity in all five photolysis experiments performed. The other two vibrations were not observable and may simply have been below the signal-to-noise limit. Stronger evidence is needed to confirm production of CH 3 SCN.

Some information exists concerning the potential production of the higher energy isomers CH 3 CNS and CH 3 SNC which were not available commercially or through synthesis. CH 3 CNS is likely not formed as the most intense peak, located at 2230 cm -1 86 , could not be detected.

CH 3 SNC has an experimentally assigned vibration at 2050 cm -1 accompanied by a smaller feature at 1442 cm -1 (intensity ratio of 100:9) [START_REF] Turowski | Isomerization of methyl isothiocyanate and methyl thiocyanate in argon matrices induced by UV irradiation[END_REF] . Although a vibration at 2050 cm -1 is often produced following 193 nm photolysis the second vibration at 1442 cm -1 is very close to our limit of detection. Without stronger evidence, CH 3 SNC detection cannot be confirmed.

The remainder of products observed for this system requires cleavage of at least one bond and separation of the resulting fragments into their own respective matrix cages The Journal of Physical Chemistry to be produced. For instance, evidence for formation of H 2 C=S following 248 nm irradiation suggests that at least some C-C bond cleavage occurs. This photolytic product was identified on the basis of its three most intense bands which were measured previously in an Ar matrix [START_REF] Jacox | Matrix isolation study of the infrared spectrum of thioformaldehyde[END_REF] . No H 2 C=S was observed following 193 nm irradiation. It may be that the higher photon energy promoted swift loss of the H-atoms from H2C=S to form CS (followed by CS conversion to CS 2 might also be detectable. However, based on IR observations of HS and H 2 S in an Ar matrix environment [START_REF] Isoniemi | Infrared Spectroscopy of H2S and SH in Rare-Gas Matrixes[END_REF] , there is no evidence of the formation of either of these in any of the photolyses performed here.. The photochemistry of Ar matrix isolated H 2 S and HS have been studied [START_REF] Koda | Photodissociation of H 2S doped in low temperature rare gas solids under UV irradiation[END_REF][START_REF] Continetti | Photodissociation of H2S and the HS radical at 193[END_REF][START_REF] Khriachtchev | 193 nm photolysis of H2S in rare-gas matrices: Luminescence spectroscopy of the products[END_REF] and H 2 S photolysis leads to production of HS followed eventually by atomic sulfur formation. While we have evidence of production of atomic sulfur, from our experiments, described later in discussion of CS 2 formation and annealing, we cannot say whether its production proceeds through H 2 S and/or HS, some other intermediate, or with deposition window (CsI and sapphire deposition windows also showed the same vibrations). As this peak is not mentioned in the literature either as a band of HCl isolated in a rare gas matrix or as a product of HCl complexation with water or some other species, we take the opportunity to report its presence here.

Fig7 Selected regions depicting bands of HCl in an Ar matrix (ratio 1:1000). Red labels are matrix sites. Y scale is the intensity in arbitrary units.

Annealing

The last step of sample processing was annealing. During annealing, softening of the noble gas matrix can allow some isolated, reactive species to become mobile and recombine, possibly emitting light in the process, and allow changes in specific matrix sites that might be visible in subsequent IR spectra. Aside from confirming photoproduct assignment using IR spectra, we also monitored for light emission during annealing using a low resolution CCD spectrometer. Following 193 nm irradiation of HS-CH 2 -CN, three different emission signals were observed, one starting at around 8 K, a second between 8 and 12 K, and a third above 12 K. Just above 8K, we were able to see a greenish glow (a broad weak line at 521 nm) (Fig8). Ar matrix which allows it to become mobile at a lower temperature.

Conclusions

We have investigated the photochemistry of mercaptoacetonitrile (HS-CH 2 -CN) and chloroacetonitrile (Cl-CH 2 -CN) in noble gas (Ar) matrices at 6K. We determined position and intensities of peaks and supported our conclusions with computational analysis. For

Page 8 of 44 ACS

 44 Paragon Plus Environment

Fig 1

 1 Fig 1 Structures of isomers of C 2 H 2 ClN stoichiometry. The B3LYP relative energies are given for all isomers, while CCSD(T) (bold in parenthesis) relative energies are given only for the three lowest energy isomers.

4. 3

 3 photoproduct. Values of intensities are based on their integrals.

Page 28 of 44 ACS

 44 Paragon Plus Environment

Fig 5 Fig 6

 56 Fig 5 Experimental IR spectrum of Cl-CH 2 -NC. Matrix sites assigned to[START_REF] Crowder | Force constants and normal vibrations of the haloacetonitriles[END_REF] Cl-CH 2 -CN (black and red) and[START_REF] Nemes | Infra-red dispersion studies. Part 3.-Band intensities of acetonitrile and its chlorinated derivatives[END_REF] Cl-CH 2 -CN (green) . Y scale is the intensity in arbitrary units.

Fig8Fig9

  Fig8 Glow after photolysis of HS-CH 2 -CN

  photolysis of HS-CH 2 -CN. Observation of CH 2 -CN and CH-CN show loss of HS. The

Table 1

 1 Comparison of predicted frequencies and intensities for two conformers of HS-CH 2 -CN (MAN) along with experimental values measured in an Ar matrix.

	Mode g)

Table 2A

 2A Comparison of predicted frequencies and intensities of Cl-CH 2 -CN (CAN) along with experimental values measured in an Ar matrix.

	Page 15 of 44		The Journal of Physical Chemistry
	Mode	Symmetry	Freq.	Freq. shift	Int. [km mol -1 ]
	f)		[cm -1 ]	35 Cl→ 37 Cl	(Rel. Int. [unitless])
					[cm -1 ]
			Calc. a)	Exp.	Calc. b) Exp.	Calc. c)	Exp. g)
	 1	A'	2990	2972.1	0		4 (12)	2.3 (8)
	 2	A'	2259	2263.4	0		0 (0)	0.6 (2)
	 3	A'	1437	1431.1	0		5 (15)	5.1 (18)
	 4	A'	1268	1271.1	0		18 (53)	9.3 (33)
	 5	A'	929	930.8	0		15 (44)	24.9 (89)
	 6	A'	750	745.4	-4	-4	34	28.0 (100)
							(100)
	 7	A'	476	484.2	-3	-3.5	5 (15)	2.1 (7)
	 8	A'	181	d)	-1		6 (17)	-
	 9	A''	3020	3014.3	0		0 (0)	3.1 (11)
	 10	A''	1183	1183.4	0		0 (0)	0.4 (1)
	 11	A''	908	906.7	0		1 (0)	0.9 (3)
	 12	A''	336	d)	0		1 (0)	-
	a) combined CCSD(T)/anharmonic B3LYP calculation, see equation (1)
	b) harmonic CCSD(T)
	c) anharmonic B3LYP
	d) out of detection range
	e) relative intensities with respect to ν 6 at 745 cm -1
	f) None of the computed overtones has an intensity higher than 1 km mol -1 and are not
	reported.
	g)lower limit of the absolute intensities
			ACS Paragon Plus Environment

Table 2B

 2B Comparison of predicted infrared frequencies and intensities of Cl-CH 2 -NC, CAN-1 along with experimental values measured in an Ar matrix.

				The Journal of Physical Chemistry	Page 16 of 44
	Mode		Freq.	Freq. shift	Rel. Int.	Rel. Int.
			[cm -1 ]	35 Cl→ 37 Cl	[unitless]	[unitless]
					[cm -1 ]
			Calc. a)	Exp.	Calc. b) Exp.	Calc. c)	Exp e)
	 1	A'	3002	3035.9	0		4	19
	 2	A'	2137	2143.8	0		100	100
	 3	A'	1457	1453.0	0		2	1
	 4	A'	1311	1306.2	0		24	12
	 5	A'	962	962.3	0		30	19
	 6	A'	746	777.4	-4	-1.2	43	87
	 7	A'	418	477.7	-3	-1.4	1	23
	 8	A'	160	d)	-1		2	-
	 9	A''	3032	3036.0	0		0	19
	 10	A''	1229	1228.2	0		1	1
	 11	A''	951	-	0		0	-
	 12	A''	242	d)	0		0	-
	Combination modes and overtones with intensities higher than 1 km × mol -1
	2 5		1908	-	0		1	-
	 5 + 4		2271	-	0		1	-
	 6 + 3		2202	2208.0	-5		6	1
	 8 + 2		2293	-	-1		1	-
	 8 + 4		1114	-	-1		1	-
	 9 + 3		4467	-	0		1	-
	 11 + 10		2180	2179.0	0		30	1
	 12 + 2		2370	-	0		1	-
	a) combined CCSD(T)/anharmonic B3LYP calculation, see equation (1)
	b) harmonic CCSD(T)
	c) anharmonic B3LYP, relative intensities with respect to ν 2 (178 km/mol)
	d) out of detection range
	e) relative intensities given with respect to ν 2
				ACS Paragon Plus Environment

Table 2C

 2C Predicted infrared frequencies and intensities of HNCCHCl, CAN-2

	Page 17 of 44		The Journal of Physical Chemistry
	Mode	Freq.	Freq. shift	Inten.
		[cm -1 ] a)	35 Cl→ 37 Cl	[km mol -1 ]
			[cm -1 ] b)
	 1	3281	0	15
	 2	3114	0	18
	 3	2047	0	227
	 4	1290	0	40
	 5	1125	0	9
	 6	1019	0	186
	 7	837	0	67
	 8	781	-4	34
	 9	536	-1	39
	 10	532	-3	2
	 11	409	0	26
	 12	182	-1	0
	Combination modes and overtones with intensities higher than 1 km × mol -1
	 1	6389	0	13
	 2	6112	0	1
	 3	4067	0	3
	 6	2005	0	35
	 8	1559	-7	1
	 9	1078	-1	1
	 10	1059	-6	5
	 4 + 3	3321	0	3
	 6 + 3	3071	0	2
	 6 + 4	2311	0	2
	 6 + 5	2145	0	8
	 9 + 7	1371	-1	7
	 10 + 9	1067	-4	1
	 11 + 6	1436	0	1
	 11 + 7	1249	0	1
	 11 + 9	943	-1	14
	 11 + 10	941	-3	2
	a) combined CCSD(T)/anharmonic B3LYP calculation, see equation (1)
	b) harmonic CCSD(T)
	c) anharmonic B3LYP results
			ACS Paragon Plus Environment

Table 3

 3 Rotational constants and dipole moment for selected isomers of CAN

		A 0 /MHz	B 0 /MHz	C 0 /MHz	μ e /D
	CAN	25045	3109	2811	3.28
		25271.3583(45) a)	3150.74897(102) a)	2789.4927(22) a)
	CAN-1	25758	3327	2999	3.04
	CAN-2	34114	2919	2705	1.89
	a) ground state experimental values 82

  The Journal of Physical Chemistry of their coexistence could be found in the measured spectra. Measured IR frequencies are in better agreement with what is predicted for the synclinal form, the more stable of the two conformations. As the absolute intensities are low for both compounds and IR signals from the most stable isomer were already small, it may be that we were simply unable to detect bands of the less abundant anti-periplanar conformer. The Cl-CH 2 -CN purchased for these experiments had excellent purity and identification of peaks was unambiguous based on previous reports. Comparison between calculated and experimentally determined values of intensities and position of peaks of Cl-CH 2 -CN are presented in Table 2A. The experimental IR spectrum of Cl-CH 2 -

	5 kJ/mol,
	close to the experimental value, and indicate a 4.1 kJ/mol difference between conformer
	free enthalpies at room temperature. Assuming thermal equilibrium between isomers, a

ratio of ~5:1 (synclinal to anti-periplanar) should be observed. However, no obvious sign ACS Paragon Plus Environment CN is presented in Fig 3

.

Most peaks appear as overlapping doublets suggesting existence of at least two different matrix sites. In Table

2A

, the more intense member of the doublet is reported. For the ν 6 and ν 7 vibrations a quartet is observable. For , peaks 𝑣 6

Table 4

 4 Peaks observed after HS-CH 2 -CN laser photolysis.

	Page 25 of 44			The Journal of Physical Chemistry
	Freq.	anneal Rel. Int a) [unitless]	Rel. Int. a) [unitless]	Assignment
	[cm -1 ]		photolysis 248 nm	photolysis 193 nm
	3611	↓	32	9
	3233	↓	47	-	HC-CN
	3219	↓	29	69
	3162	↓	-	100
	2168	↓	3	2	CH 3 SCN
	2142	↓	88	17	CH 3 NSC
	2091	↓	9	34
	2050	↓	-	8	CH 3 SNC
	2024	↓	18	6
	1930	↑	9	2
	1897	↓	-	3
	1888	↓	-	6
	1735	↓	12	-	HC-CN
	1528	↑	29	2	CS 2
	1328	↓	-	34
	1309	↓	12	22	CH 4
	1289	↓	6	59	HS-CH 2 -NC
	1148	↓	-	2
	1123	↓	-	2
	1063	↓	6	-	H 2 C-CN/CH 2 =S
	996	↓	100	-	CH 2 =S
	993	↓	76	-	CH 2 =S
	936	↓	11	2	HS-CH 2 -NC
	802	↓	-	13
	777	↑	4	6
	723	↓	12	50
	687	↓	29	-	H 2 C-CN
	617	↓	-	3	CH 3 * tentative detection
	483	↓	3	-	HC-CN
				ACS Paragon Plus Environment

Table 5

 5 Comparison of predicted infrared frequencies and intensities of HS-CH 2 -NC with experimental valuesPhotoisomerization of HS-CH 2 -CN into the more stable methyl thiocyanate (CH 3 SCN) and methyl isothiocyanate (CH 3 NCS) are also likely processes in a matrix environment. Although not presented here, we compared our HS-CH 2 -CN photoproduct spectra with those of pure CH 3 SCN and CH 3 NCS isolated in Ar (work in preparation[START_REF] Turowski | Isomerization of methyl isothiocyanate and methyl thiocyanate in argon matrices induced by UV irradiation[END_REF] ).The calculated and measured absolute intensities of IR bands of those two molecules are far larger than for HS-CH 2 -CN or HS-CH 2 -NC and, as a consequence, even a small amount of CH 3 NCS or CH 3 SCN formed via photolysis should be easily detectable.

	Page 27 of 44
	Mode	Frequency	Rel. Int.
		[cm -1 ]	[unitless]
		Calc a) .	Exp.	Calc. b) Exp.
	1	2998	-	0	-
	2	2993	-	2	-
	3	2670	-	0	-
	4	2135	2142	100	100
	5	1458	-	4	-
	6	1294	1288.9	15	7
	7	1247	-	4	-
	8	1030	-	1	-
	9	939	935.9	19	12
	10	804	-	1	-
	11	702	-	9	-
	12	416	-	1	-
	13	297	-	3	-
	14	278	-	6	-
	15	171	-	2	-
	a) combined CCSD(T)/anharmonic B3LYP calculation, see equation (1) and Ref. 1
	b)relative intensities with respect to v4 at 2142 cm -1

  ). H 2 C-CN and HC-CN, were likely formed following 248 nm irradiation but not following 193 nm exposure. Both were previously measured in Ar matrix environment, HC-CN by Dendramis et al. and by Maier et al. 87, 88 and H 2 C-CN by Cho et al.[START_REF] Cho | Matrix Infrared Spectra and Density Functional Calculations of the H2CCN and H2CNC Radicals Produced from CH3CN[END_REF] . HC-CN was identified in this work as a product on the basis of three unambiguous peaks. For H 2 C-CN, one peak was unambiguous while a second was very near the limit of detection (not included in table 5). A third peak which should also be visible shares the same location as a band of H 2 C=S. Overall, detection of H 2 C-CN is less certain than HC-CN.

	Formation of both HC-CN and H 2 C-CN requires cleavage of the S-C bond of HS-
	CH 2 -CN. Given that both HC-CN and H2C-CN were observed, It is reasonable to assume
	that some HS or H 2 S (possibly formed by an escaping HS taking with it another H-atom)

Table 5 .

 5 through sequential scission of H atom and then S atom from the H-S-CH 2 CN parent.A number of other products were also observed at both photolysis wavelengths suggesting that cleavage of single bonds is quite facile and that escape of the fragments thus produced from the matrix cage is also probable. Methane formation, identified here based on comparison with prior measurements in an Ar matrix[START_REF] Jacox | Matrix isolation study of the infrared spectrum of thioformaldehyde[END_REF] , requires cleavage of both S-C and C-C bonds as well as at least one free H atom coming from another matrix site or the possible presence of complexes in a single site. While H atom migration is not a surprising process in a noble gas matrix environment, the formation of CS 2 (identified as compared to measurements in Ar matrix by Szczepanski et al.[START_REF] Szczepanski | Infrared Absorption Spectroscopy of Small Carbon-Sulfur Clusters Isolated in Solid Ar[END_REF] ), possibly from CS and S, requires sulfur atoms coming from two different matrix cages or two parent molecules. This suggests either that free S atoms are somewhat mobile in an Ar matrix, that adjacent matrix sites were near enough to allow minimally mobile S atoms to find one another following photolysis, or a reasonable abundance of HS-CH 2 -CN dimers or larger complexes exist in the matrix. Finally, CH 3 identified based on IR measurements in Ar matrix by Pacansky et al. and by Snelson et al.[START_REF] Pacansky | Low temperature photochemical studies on acetyl benzoyl peroxide. Observation of methyl and phenyl radicals by matrix isolation infrared spectroscopy[END_REF][START_REF] Snelson | Infrared matrix isolation spectrum of the methyl radical produced by pyrolysis of methyl iodide and dimethyl mercury[END_REF] , was observed upon irradiation at 193 nm but not following exposure to 248 nm. Given the presence of CH 4 , production of CH 3 is logical upon exposure to higher energy photon radiation. Some peaks remain unassigned. Intense peaks, for which we did not find any convincing attribution are marked in the These lines do not fit with other potential products whose IR signatures in an Ar matrix environment have already been reported: S=CHCN[START_REF] Kappe | Matrix isolation and infrared spectrum of thioformyl cyanide[END_REF] ; CH 3 CN[START_REF] Kameneva | Radiation-induced transformations of isolated CH3CN molecules in noble gas matrices[END_REF] or further photolysis products (CH 3 NC, CH 2 NCH, CH 2 CNH, CH 2 NC)[START_REF] Kameneva | Radiation-induced transformations of isolated CH3CN molecules in noble gas matrices[END_REF] ; CH 3 SH 58 ; HS[START_REF] Isoniemi | Infrared Spectroscopy of H2S and SH in Rare-Gas Matrixes[END_REF] ; H 2 S 90 ; CH 2 (NIST gas); HS-CN[START_REF] Wierzejewska | Photolysis of isothiocyanic acid HNCS in low-temperature matrices. Infrared detection of HSCN and HSNC isomers[END_REF] ; or HS-NC[START_REF] Wierzejewska | Photolysis of isothiocyanic acid HNCS in low-temperature matrices. Infrared detection of HSCN and HSNC isomers[END_REF] .VIS spectrum of Cl-CH 2 -CN is available in the literature, we measured it and found no clear absorption bands between 200 nm and 900 nm, suggesting that two-photon processes were involved in photolysis.The main photolysis product of Cl-CH 2 -CN photolysis was Cl-CH 2 -NC.The position and intensities of peaks of FTIR measurements were compared with theoretical calculations and are presented in Table2band in Fig5. The yield of this product was high enough that[START_REF] Nemes | Infra-red dispersion studies. Part 3.-Band intensities of acetonitrile and its chlorinated derivatives[END_REF] Cl isotopomers for and vibrations could be discerned 𝑣 6 𝑣 7at 775.5 cm -1 and 476.3 cm -1 , with intensities proportional to the ratio of natural abundances of[START_REF] Crowder | Force constants and normal vibrations of the haloacetonitriles[END_REF] Cl and[START_REF] Nemes | Infra-red dispersion studies. Part 3.-Band intensities of acetonitrile and its chlorinated derivatives[END_REF] Cl. Additionally, a few small overtone vibrations could be distinguished in the spectra (ν 6 +ν 3 at 2208.0 cm -1 and ν 11 +ν 10 at 2179.0 cm -1 ). The anharmonic computations predicted that both these modes are in resonance with very intense . It seems that, the intensities of those combinational modes are overestimated, 𝑣 2 but the precise prediction for resonance are far beyond the applied theoretical methods.

	Page 29 of 44

We also performed photolysis of Cl-CH 2 -CN using 193 nm laser light. As no UV-

Table 2C ).

 2C

	Page 31 of 44	The Journal of Physical Chemistry
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ACS Paragon Plus Environment The Journal of Physical Chemistry ACS Paragon Plus Environment The Journal of Physical Chemistry Acknowledgments This work was supported by NCN grant 2015/17/B/ST4/03875. J.-C. G. thanks the Centre National d'Etudes Spatiales (CNES) and the French Programme National "Physique et Chimie du Milieu Interstellaire" for financial support. ACS Paragon Plus Environment The Journal of Physical Chemistry

Supporting information

Tables with comparison of results of this work with previous reports for HSCH 2