Amirouche Oumaziz

Emmanuel Sarraute

Frédéric Richardeau

Abdelhakim Bourennane

Céline Combettes

Vincent Bley

Ayad Ghannam

Fast cut-off, low I 2 T and high temperature monolithic on-chip fuse on silicon substrate for new fail-safe embedded power switch

Keywords: On-chip fuse, Power Converters, fuse design, 3D finite elements method

In this paper, a first concept of monolithic semiconductor fuses on silicon substrate, is realized and experimentally characterized. These new compact devices are, able to perform fast and irreversible current cut-off at medium voltage (200V), relatively high current (10A), with very low pre-arcing time (~ 4 µs to 5 µs). The fuses are intended for fail-safe power converter capabilities applications. Design and 3D simulation by finite elements method, taking into account static and dynamic specifications have been carried out. Thermal management in steady state is improved by dielectric epoxy thermal insulation under each constriction of the fuse. Implementation and practical tests are reported.

I. INTRODUCTION

Power electronic devices are widely used in different applications requiring high reliability, like automotive. Then, it is very important to improve the availability in order to guarantee fault tolerance and continuity of service. Fuses are simple, passive and inexpensive components, capable if they are well designed to protect electrical devices in the event of short-circuits. In this paper, a new concept of fuses is presented, for power semiconductors devices protection. These fuses could be monolithically integrated within active power devices [START_REF] Berberich | Active fuse[END_REF] [START_REF] Vom Dorp | DCarc behavior of a novel active fuse[END_REF], in fail-safe power inverters capabilities [START_REF] Richardeau | Complete short-circuit failure mode properties and comparison based on IGBT standard packaging. Application to new fault-tolerant inverter and interleaved chopper with reduced parts count[END_REF] [4] [START_REF] Dou | PCB dual-switch fuse with energetic materials embedded: Application for new fail-safe and fault-tolerant converters[END_REF] using SMD commercial fuses or PCB fuses prototypes. An example of a fail-safe capability is presented in Figure 1. The configuration considers a back-up half-bridge, allowing inverter's post-fault reconfiguration at full power.

II. FUSES CHARATERISTICS & DESIGN

A. Charasteristic of the targeted monolithic fuses

Usually, to protect semiconductor components, fuses are made from thick or thin metal films (copper or aluminum) on commercial SMD fuses [START_REF] Labrousse | First Practical evaluation of a complete fail-safe and 100 % faulttolerant inverter for critical load in aerospace application[END_REF], Printed Circuit Board (PCB) substrates [START_REF] Dou | PCB dual-switch fuse with energetic materials embedded: Application for new fail-safe and fault-tolerant converters[END_REF] [7] [START_REF] Tsuchiya | Basic research on the fuse element pattern changing a current pathway in the process of current interruption[END_REF] or printed on PCB using conductive composites (silver) [START_REF] Yang | Low-Temperature Fusible Silver Micro/Nanodendrites-Based Electrically Conductive Composites for Next-Generation Printed Fuse-Links[END_REF] for low power applications. The proposed fuses (Figure 2) consist of a thin (18 µm) film of copper deposited on a massive 400 μm silicone substrate. When the energy let-through (I 2 t in [A 2 .s]) is exceeded, in case of a short-circuit occurrence, the metal constituting the constrictions evaporates. The distance across the pads must be sufficiently long (1mm here) to withstand the supply voltage (up to 400 VDC). The evaporation must be irreversible in order to minimize the presence of metallic particles, which could cause re-arcing phenomena. A major difficulty of fuse technology is to create an insulating coating to trap the vaporized metal particles and passivate them in order to maintain a recovery voltage at least equal to that of the power supply without the risk of current leakage and arc recovery. The choice of material and its deposition mode is essential. This aspect is the subject of this article. Another important point is the control of the cutoff speed to limit the overvoltage and to avoid breakdown other components in parallel. This aspect is even more complex and will not be dealt with in this article.

The fuses design simulations have been carried out using "Comsol Multiphysics TM (software version 5.3a)". The fuses have been designed for 10 A nominal current at 85 °C substrate's maximum temperature, allowing 115 °C maximum constriction's temperature, because electronic power devices could operate in extreme temperature conditions [START_REF] Smet | Ageing and failure modes of IGBT modules in high-temperature power cycling[END_REF]. Substrate's passivation is ensured by using thin film of Nitride below the copper layer in order to avoid leakage currents after the constriction's blows. Optimized epoxy (low thermal conductivity, 1.16 W.m -1 .K -1 ) thickness (25 µm), thermally insulates the constriction from the substrate (higher thermal conductivity), which allows to focus the energy on its center, optimizing constriction's evaporation.

(a) (b) (c) (d) (e) (f) (g) (h)

B. Design and electro-thermal simulations

Geometrical, electrical, physical, thermal and mechanical parameters, like constrictions length and thickness, materials thermal and electrical conductivity, rating current, temperature, are combined and built up in Comsol simulations to adjust the constrictions maximum temperature by processing their widths. Four fuses topologies were designed, depending on the number of serial (Ns) and parallel (Np) constrictions. Fuses topologies, constriction and longitudinal substrate temperatures distribution are shown in Figure 3.

A preliminary theoretical calculation from the Fourier's law, allowed us to set up the initial constriction's width value for the mono-constriction design, with respect to the maximum allowed temperature (115 °C).

Φ = -𝜆 ∆𝑇 ∆𝑥 (1)
Where Φ represents the heat flux density, 𝜆 the thermal conductivity and ∆𝑇 ∆𝑥 the thermal gradient.

The reference design (Ns=Np=1, Figure 3.a) required a constriction's width of 240 µm to fit the temperature parameter (115 °C). Furthermore, the optimized epoxy's thickness (25 µm) ensures thermal decoupling between the constriction (red line) and the silicon substrate (blue line) (Figure 3.e). Power losses through the constriction are estimated to 394 mW, which is comparable with the average losses in commercially available substrate fuses for the rated current. In order to minimize the fuse's chip area, we added a parallel constriction (Ns=1 Np=2, Figure 3.b) to the first model. As a consequence, power dissipation has been improved, due to an improved heat distribution. To fit the specification of 115 °C constriction's maximum temperature at the rated current, a 117 µm constriction's width was required. The spacing between the two parallel constrictions is chosen so the substrate's temperature distribution between them, is close enough to set-up temperature (85 °C), to avoid thermal coupling (Figure 3.f). This improved configuration afforded an area gain of 37 % as compared to the reference fuse design described above, with approximatively the same power losses (404 mW). A serial constriction is added to the reference design to build up a two serial constrictions fuse (Ns=2 Np=1, Figure 3.c). The idea is to split the initial constriction's length (1 mm) in two equal constrictions of 500 µm length, which will split the electrical and mechanical constraints on the silicone substrate during the constrictions break-up and improve the thermal dissipation through the substrate. A shorter track has a reduced thermal deflection and therefore allows an even smaller thickness at maximum iso-temperature. To separate the constrictions, a middle pad is added, which unfortunately increases the fuses area, but it is size optimized to insure thermal decoupling between them. Figure 3.g shows a dissociate constrictions temperatures profile thanks to the middle pad. The estimated area loss is about 15 % compared to the reference design. Only 138 µm constriction's width is required to reach the constriction's maximum temperature specification with power losses estimated to 684 mW. The last configuration combines the advantages of parallelization and serialization in a structure made of four constrictions (Ns=Np=2), as shown in Figure 3.d. Parallelizing and serializing give noticeable improvement on the fuses area. Indeed, the estimated area gain is about 73 % as compared to the reference design (Ns=Np=1). Only 67 µm constriction's width was required to reach the maximum allowed temperature of 115 °C. This configuration shows an estimated power loss about 705 mW, which is still in the average as compared to the commercially available substrate fuses.

In the case of fault occurrence, the current ( 𝐼 𝑐𝑐 (𝑡) ) increases through the stray inductance to reach the maximum value (𝐼 𝑐𝑐 𝑚𝑎𝑥 ) that results fuse melting. Given the expression of the 𝐼 2 𝑇 𝑝 quantity in relation ( 2) and the current curve expression (𝐼 𝑐𝑐 (𝑡) = ⌈ 𝑑𝑖 𝑑𝑡 ⌉ 𝑡), the expression (3) of the pre-arcing delay time Tp can be obtained:

𝐼 2 𝑇 𝑝 = ∫ 𝐼 𝑐𝑐 2 𝑇𝑝 0 (𝑡)𝑑𝑡 (2) 𝑇 𝑝 = ( 3[𝐼 2 𝑇 𝑝 ] [ 𝑑𝑖 𝑑𝑡 ] 2 ) 1/3 (3)
Where 𝐼 𝑐𝑐 represents the maximum default current through the fuse element defined by the relation ( 4):

𝐼 𝑐𝑐 𝑚𝑎𝑥 = (3 𝑑𝑖 𝑑𝑡 [𝐼 2 𝑇 𝑝 ]) 1 3 (4)
Designs improvements made very fast fuses, Figure 4 shows actual fuses areas and I 2 T value for each fuse configuration. The serial configuration presents more occupied area, because of the middle separation pad. The highest improvement is achieved when both parallelization and serialization are combined simultaneously. The last design shows improvements in both occupied area and energy breakup requirements I 2 Tp (0.48 A 2 .s) with mainly low pre-arcing time (1.9 µs at 770 A). These integrated fuses occupy at least four times less area, and are three times faster, than one of the compact [START_REF] Eaton | Eeaton Fast-Acting SMD BrickFuses CB61F Series[END_REF] and faster [START_REF] Mersen | Fusibles Protistor Standard 250 V-URE-URZ calbres 5 a 180 A[END_REF] commercially available fuses. Such a low value suggests the possibility of directly protecting a transistor device in short-circuit operation with a fuse, provided that the fuse can be replaced.

III. FUSES REALIZATION

The monolithic on-chip fuse devices are fabricated using conformal 3D-RDL technology [START_REF] Ghannam | 3-D multilayer copper interconnects for high-performance monolithic devices and passives[END_REF]. This technology consists on forming 3D conformal interconnects over ICs or packages using wafer-level processing. It enables forming 3D interconnects, both horizontal and vertical parts simultaneously, using a single electroplating step. Hence, no multiple planar RDL with vias are involved, reducing thus the number of processing steps. In general, the technology involves forming first a 3D re-passivation layer over 3D topography (i.e. a die or a 3D feature) to isolate the 3D interconnects from their surrounding environment while keeping openings in desired locations for electrical connection with underlying device [START_REF] Ghannam | Ultra-thin QFN-like 3D package with 3D integrated passive devices[END_REF]. Then, the 3D interconnects are formed using 3D copper electroplating inside a photoresist. The particularity of this technology is its capability of forming high aspect ratio 3D-RDL/3D Interconnects over a vertical sidewall [START_REF] Ghannam | Ultra-thin QFN-like 3D package with 3D integrated passive devices[END_REF].

In the current work, fabricating the devices involved the following steps: 1/ a nitride layer was deposited on the silicon substrate to electrically isolate it from the 3D interconnects; 2/ a 25 µm-thick epoxy layer was formed above the substrate. This layer serves as a core to form the constriction of the fuse using 3D-RDL technology. No repassivation layer was needed as the core was already insulative; 3/ a seed layer was sputtered above the substrate; 4/ a photoresist was deposited and 3D patterned; 5/ 20 µmthick 3D copper interconnects were electroplated inside the photoresist. For the presented device, the copper laid-out partly on the epoxy core and on the substrate, without electrical connection to the later as it was passive; 6/ etching and cleaning. These steps are shown on Figure 6. When integrating the monolithic fuses above an active chip, the same processing steps could be applied. The first step of depositing a nitride layer can be omitted if the chip has one above it that answers design requirements. On the other hand, an organic re-passivation layer can be applied for electrical shielding from unwanted areas and/or to increase the separation distance between the fuse and some sensitive parts of the chip, such as RF areas. Given the small number of processing steps, the proposed herby solution is very advantageous for fabricating at wafer-level high-Rel active devices with fuses on-chip.

A. Fuses charaterization

The characterization process and experimental tests were carried out at Laplace laboratory. A four wires method was used for fuses characterization. Table 1 summaries fuses theoretical and measured nominal resistances. The theoretical values comprise only the constriction part resistances, but we can see that the tendency is respected. Even though the measured values comprise wire bonds, fuses pads (oversized for the experiments) and external contacts pads resistances, they still very low or at least even, compared to commercially available fuses, like compared in [START_REF] Oumaziz | Fail-safe switching-cells architectures based on monolithic on-chip fuse Keywords Fail-safe topologies using single-fuse or dual-fuse function[END_REF]. The specific resistance of the fuses is calculated using the relation [START_REF] Dou | PCB dual-switch fuse with energetic materials embedded: Application for new fail-safe and fault-tolerant converters[END_REF], where 𝑅 𝑃𝑎𝑑𝑠 and 𝑅 𝐶𝑜𝑛𝑠𝑡𝑟𝑖𝑐𝑡𝑖𝑜𝑛 represent the fuse pads and constriction resistances, 𝑆 𝑓𝑢𝑠𝑒 the actual fuse's occupied surface.

𝑅 𝑓𝑢𝑠𝑒 × 𝑆 𝑓𝑢𝑠𝑒 = (𝑅 𝑃𝑎𝑑𝑠 + 𝑅 𝐶𝑜𝑛𝑠𝑡𝑟𝑖𝑐𝑡𝑖𝑜𝑛 ) × 𝑆 𝑓𝑢𝑠𝑒 (5)
𝑅 𝑓𝑢𝑠𝑒 × 𝑆 𝑓𝑢𝑠𝑒 = (2.28 𝑚𝛺 + 3.98 𝑚𝛺) × 2.52. 10 -2 𝑐𝑚 2

The fuse's specific resistance (0.157 mΩ.cm 2 ) is at least 60 times less important than specific resistance (10 mΩ.cm 2 ) of commercially available power switches. Meaning that the fuses will not induce any important power losses when integrated on power switch.

B. Preliminary experimental test without passivation

The experimental test circuit (Figure 7) is mostly composed of a current-limited voltage source, four parallel capacitors which makes a total capacitance of 1880 µF. A crowbar thyristor is used to trigger the short-circuit. It is used to take advantage of its very high overcurrent capacity without being destroyed unlike a transistor. In order to limit the energy and to limit the di/dt max value, a 700 nH snubber inductance was added. When triggered, the serial thyristor creates a current path through the fuse, like a short-circuit. A freewheeling diode is included in parallel with the inductance as in real switching cell. This diode is important, it prevents the fuse from having to dissipate all the energy stored in the snubber inductance. Only the energy dissipated in the stray inductance, around 50 nH, will be dissipated in the fuse as in a real switching cell.

When the thyristor is triggered, the capacitors stored energy is partially discharged through the fuse, which increases its voltage and current until the required constriction's melting point energy is reached, resulting in its interruption. Finally, the initiated arc persists until the current reaches zero. This current cancellation must remain indefinitely and must not restrike. The primary fuses interruption process studies were carried out on non-passivated monoconstriction fuses in order to study the electrical behavior.

The results of the interruption, obtained for 50 V, microphotograph of the interrupted constriction and the measured leakage current are presented in Figure 8.

Unfortunately, the results (Figure 8.a) showed current restrike during the arcing period. As a result, both pads adjacent to the constriction were damaged (Figure 8.b).

That could be attributed to an excess of energy and without any top fuse passivation, it spreads over the pads. In order to achieve a satisfactory and controlled interruption without any pads damages, fuses passivation is required. To accomplish the passivation process, silicone gel, like used in power inverters modules, to help reduce mechanical stress, electrical breakdown, moisture, etc. seems to be the appropriate passivation product. Methods and technological passivation process are described below.

C. Fuses passivation technological process

During the fuses cut-off sequence at voltages higher than 100 V, the fuses have to be soldered on the bottom face, otherwise, the mechanical stresses induced by the electrical energy cause silicone substrate breakdown. The fuses were soldered on a FR4 substrate (Figure 9.a). Then, the wire bondings, made of aluminum (254 µm diameter), were designed with an I 2 T value, 200 times higher than the devices I 2 T values. This is a safety parameter, to support the rated current (10 A), measure the voltage across the constrictions and avoid current variations sensitivity. A specific PTFE mold, comprising a middle cavity, was designed (Figure 9.b) using a 3D printer in order to cast the silicone gel on the chip-fuse.

As a passivating silicone gel, Wacker SilGel 612 A/B, dedicated for electronic components and power electronics industries, is used. Wacker SilGel 612 A/B has a flame retardancy classification of UL 94 HB [START_REF] Wacker | Creating tomorrow ' s solutions today[END_REF]. Air impurities introduced during the dispensing sequence, are eliminated using vacuum machine.

D. Results and discussion

Using the experimental test circuit presented above (Figure 7), fuses cut-off tests have been carried out on the different topologies (np=2 ns=1, np=1 ns=2, ns=np=2). Figure 10 presents the cut-off sequence results for a two parallel constrictions fuse, with associate leakage current measure at rated voltage (200 V).

(a) The last fuses configuration (ns=np=2) showed very interesting results compared to the others. A perfect cut-off is achieved after a pre-arcing time of only ~4 µs at 720 A, without any current restrike (Figure 12.a) and no pads damaging (Figure 12.d). Measured leakage current (Figure 12.c) at rated voltage is very low (25 µA, 5 mW), providing an insulation resistance of 8 MΩ. This technology and structure therefore seem promising for a voltage rise and to be compatible with 600 V or even 1200 V rated components used under 300 V and 600 V respectively.

E. I 2 T theoretical and experimental compararative

In order to assess the process tolerance, theoretical and experimental I 2 T values are compared (Table 2). As expected, the required breaking energies (I 2 T) are very low. Theoretical and measured I 2 T values are quit even, with a slight difference only. This could be attributed to the process tolerance and the pre-arcing time calculation. Depending on components placement on the wafer, a process tolerance between 2.7% to 7% has been reported. Figure 13.a shows a four serial/parallel constrictions fuse cut-off results, with color identification of the pre-arcing period (green) and the arcing period (blue). Although, there is time lap (Figure 13.b, orange area) during the pre-arcing period, where the fuse current didn't reach yet its maximum value, meanwhile, the voltage across the fuse suddenly increases in non-linear way. This behavior means that the fuse is no longer in its ohmic state, thus the pre-arcing time definition (until the fuse current reaches its maximum value) no longer applies. During this time lap, the constriction metal is in liquid then gas state, which means the arcing period (blue area) has started. The presented measured values are calculated respecting the pre-arcing period definition (excluding the orange area). In addition, these fuses I 2 T values can be at least five times smaller than commercially available fuses (same rated current). It means that the fuses, in case of fault occurrence, will provide a safe current path cut-off, while avoiding circuit's healthy parts damages. In case of short-circuit occurrence, a power switch can withstand a designed saturation current for only few tens of microseconds. During a short-circuit fault, the switch component will withstand a saturation current (50 A), for example, five times the rated current (10 A), during ~10 µs.

Even with very low fuses I 2 T values (Table 2), the I 2 T values of a power switch are even smaller (0.025 A 2 .s here), meaning that the proposed fuses (even market fuses) cannot be blown first instead of the power switch, but are intended to isolate the power switch fault from spreading and causing further damages (source, load…etc).

V. CONCLUSION

In this paper, a new concept of monolithic on-chip fuses, based on silicone substrate, is simulated and experimentally characterized for the first time. Several fuses topologies were studied under medium voltage (200 V) and showed very good results. The devices are able to perform, fast and safe cut-off, with very low I 2 T energy. Especially the four serial and parallel configuration (ns=np=2) version, with very compact occupied area (1.46 mm 2 ). The insulation resistances after opening are very high (between 230 kΩ and 8 MΩ, depending on the fuses topologies), which guarantee very low leakage current at rated voltage (200 V). Fuses passivation process, using silicone gel, improved the cut-off process by absorbing excess energy. In addition, since power inverters mass production already uses this product, the passivation process using silicone gel will not require more expenses. These good results suggest that design of more compact fault-tolerant inverters, combining active components (IGBTs, SJ-SiC-Mosfet) and on-chip fuses, monolithically, is possible for medium voltage and medium power applications. Future work will focus on the lateral integration of these fuses on top-side connection-diodes for back-up phase-leg link. A 3D structure extension with low size is also in progress.

REFERENCE

Figure 1 :

 1 Figure 1 :Fail-safe topology Monolithic association of a top fuse on RC-IGBT (Figure 1, blue dashed box) would be very interesting, more secure IGBT can be provided, increasing their global reliability and applications.

Figure 2 :

 2 Figure 2 :On-chip fuse design

Figure 3 :

 3 10A fuses designs with one constriction (a), two parallel constrictions (b), two serial constrictions (c), four serial/parallel constrictions (d) and their respective temperature profile (blue for the substrate and red for the constrictions) ((e)-(h))

Figure 4 :

 4 Figure 4 : 10 A fuses surfaces (a) and I 2 Tp (b) versus fuses topologies @ I_Nominal= 10 A, fuse's maximum temperature=115 °C, substrate's maximum temperature=85 °C, copper's thickness=18 µm and epoxy's thickness=25 µm Both parallelization and serialization made very fast fuses (2.8 and 3.1 µs at 1420 A and 630 A respectively) with lower I 2 Tp (1.45 and 1.01 A 2 .s respectively) melting energy requirement compared to the reference design (1.53 A 2 .s).The serial configuration presents more occupied area, because of the middle separation pad. The highest improvement is achieved when both parallelization and serialization are combined simultaneously. The last design shows improvements in both occupied area and energy breakup requirements I 2 Tp (0.48 A 2 .s) with mainly low pre-arcing time (1.9 µs at 770 A). These integrated fuses occupy at least four times less area, and are three times faster, than one of the compact[START_REF] Eaton | Eeaton Fast-Acting SMD BrickFuses CB61F Series[END_REF] and faster[START_REF] Mersen | Fusibles Protistor Standard 250 V-URE-URZ calbres 5 a 180 A[END_REF] commercially available fuses. Such a low value suggests the possibility of directly protecting a transistor device in short-circuit operation with a fuse, provided that the fuse can be replaced.

Figure 5 :Figure 6 :

 56 Figure 5 : Microscopic view of a parallel fuses constrictions (a) and microphotography of the realized 10 A mono-constriction fuse (b). The fuses have been realized in collaboration with 3DiS™ [15] using facilities of the micro and nanotechnology platform of LAAS-CNRS laboratory. The pads are oversized (5x5 mm 2 ) in order to ease the measurements and experiments. A microscopic view of a parallel fuse constrictions and a mono-constriction fuse picture are shown in Figure 5.c and Figure 5.d.

Figure 7 :Figure 8 :

 78 Figure 7 : electrical schematic of the fuse's test circuit (a) and a picture of the experimental bench (b)

Figure 9 :

 9 Figure 9 : microphotography of a bonded two parallel constrictions fuse (a), encapsulation process using PTFE mold (b).

Figure 10 :Figure 11 :

 1011 Figure 10 : Electrical characteristics of a two parallel constrictions fuse (ns=1 np=2) cut-off (a), photography before the cut-off test (b), leakage current at rated voltage (c) and observed constrictions zone after fuse cut-off (d).

Figure 12 :

 12 Figure 12 : Electrical characteristics of a four serial/parallel constrictions fuse (ns=2 np=2) cut-off (a), photography before the cut-off test (b), leakage current at rated voltage (c) and observed constrictions zone after fuse cut-off (d).

Figure 13 :

 13 Figure 13 : Electrical characteristics of a four serial/parallel constrictions fuse (ns=2 np=2) cut-off (a) and zoom on the prearcing and arcing periods (b)

Table 1 :

 1 Fuses measured nominal resistances

	Nom.	Design	ns=np=1	ns=1 np=2	ns=2 np=1	ns=np=2
	Resistance (mΩ)
	Theoretical	3.98	4.08	6.92	7.13
	Measured	6.23	6.74	12.87	12.83

Measured at 10% of rated current

Table 2 :

 2 Theoretical and experimental I 2 T comparative

	Design
		ns=np=1	ns=1 np=2	ns=2 np=1 ns=np=2
	I 2 T(A 2 .s)
	Theoritical	1.53	1.45	1.01	0.47
	Measured	1.93	1.71	0.7	0.55

mm

Oumaziz et al. -2021, October 11.


J Cernicharo
email: jose.cernicharo@csic.es

C Cabezas

S Bailleux

L Margulès

R Motiyenko

L Zou

Y Endo

C Bermúdez

M Agúndez

N Marcelino

B Lefloch

B Tercero

P De Vicente

Discovery of the acetyl cation, CH 3 CO + , in space and in the laboratory

Keywords: astrochemistry -ISM, molecules -ISM, individual objects, TMC-1 -line, identification -molecular data

Using the Yebes 40 m and IRAM 30 m radiotelescopes, we detected two series of harmonically related lines in space that can be fitted to a symmetric rotor. The lines have been seen towards the cold dense cores TMC-1, L483, L1527, and L1544. High level of theory ab initio calculations indicate that the best possible candidate is the acetyl cation, CH 3 CO + , which is the most stable product resulting from the protonation of ketene. We have produced this species in the laboratory and observed its rotational transitions J u = 10 up to J u = 27. Hence, we report the discovery of CH 3 CO + in space based on our observations, theoretical calculations, and laboratory experiments. The derived rotational and distortion constants allow us to predict the spectrum of CH 3 CO + with high accuracy up to 500 GHz. We derive an abundance ratio N(H 2 CCO)/N(CH 3 CO + ) ∼ 44. The high abundance of the protonated form of H 2 CCO is due to the high proton affinity of the neutral species. The other isomer, H 2 CCOH + , is found to be 178.9 kJ mol -1 above CH 3 CO + . The observed intensity ratio between the K = 0 and K = 1 lines, ∼2.2, strongly suggests that the A and E symmetry states have suffered interconversion processes due to collisions with H and/or H 2 , or during their formation through the reaction of H + 3 with H 2 CCO.

Introduction

The cold dark core TMC-1 presents an interesting chemistry. It produces a significant number of the molecules detected in space, in particular long neutral carbon-chain radicals and their anions (see e.g. Cernicharo et al. 2020a;Marcelino et al. 2020, and references therein) as well as cyanopolyynes (see Cernicharo et al. 2020b andXue et al. 2020, andreferences therein). The presence in this object of O-bearing carbon chains, such as C 2 O (Ohishi et al. 1991) The abundance of polyatomic cations in cold interstellar clouds is relatively low because they react fast with electrons.

Full Table E.2 is only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc. u-strasbg.fr/viz-bin/cat/J/A+A/646/L7

Based on observations carried out with the Yebes 40 m telescope (projects 19A003, 20A014, and 20D15) and the Institut de Radioastronomie Millimétrique (IRAM) 30 m telescope. The 40 m radiotelescope at Yebes Observatory is operated by the Spanish Geographic Institute (IGN, Ministerio de Transportes, Movilidad y Agenda Urbana). IRAM is supported by INSU/CNRS (France), MPG (Germany), and IGN (Spain).

Interestingly, all polyatomic cations detected in cold clouds are protonated forms of stable and abundant molecules. Chemical models and observations suggest a trend in which the protonated-to-neutral abundance ratio [MH + ]/[M] increases with the proton affinity of M (Agúndez et al. 2015;Cernicharo et al. 2020cCernicharo et al. , 2021;;Marcelino et al. 2020).

It has been suggested that some O-bearing cations are sufficiently long-lived to be abundant (Petrie et al. 1993). We have recently reported the discovery of the cation HC 3 O + in TMC-1 (Cernicharo et al. 2020c). In this Letter, we report the detection of two series of lines that are harmonically related towards the cold dark core TMC-1. These lines can be fitted as the K = 0 and K = 1 lines of a symmetric rotor. From the astronomical data and the derived rotational constants, together with high-level ab initio calculations, we suggest CH 3 CO + as the best possible carrier. We have performed microwave laboratory experiments that fully support this hypothesis: we detected 79 rotational transitions near the predicted frequencies from the astronomical constants. Hence, we report the discovery in space and in the laboratory of CH 3 CO + (acetyl cation), which is the most stable isomer resulting from the protonation of ketene (H 2 CCO). The presence of CH 3 CO + can be expected on the basis of the high abundance of H 2 CCO in TMC-1 and its large proton affinity (825.3 kJ mol -1 ; Traeger et al. 1982). An anomalous abundance ratio of 2.2 is found between the A and E symmetry species of

Observations

New receivers, built as part of the Nanocosmos project1 and installed at the Yebes 40 m radio telescope, were used for the observations of TMC-1. The Q-band receiver consists of two high electron mobility transistor (HEMT) cold amplifiers that cover the 31.0-50.3 GHz band with horizontal and vertical polarizations. Receiver temperatures vary from 22 K at 32 GHz to 42 K at 50 GHz. The spectrometers are 2 × 8 × 2.5 GHz fast Fourier transform spectrometers (FFTs) with a spectral resolution of 38.1 kHz, providing the whole coverage of the Q-band in both polarizations. The main beam efficiency varies from 0.6 at 32 GHz to 0.43 at 50 GHz (Tercero et al. 2020).

The observations that led to the line survey in the Q-band towards TMC-1 (α J2000 = 4 h 41 m 41 s .9 and δ J2000 = +25 • 41 27 . 0) were performed in several sessions, between November 2019 and February 2020. The observing procedure was frequency switching with a frequency throw of 10 MHz. The nominal spectral resolution of 38.1 kHz was used for the final spectra. In these runs, two different frequency coverages were observed, 31. GHz. This permits the user to check that no spurious ghosts are produced in the down-conversion chain, in which the signal coming from the receiver is down-converted to 1-19.5 GHz and then split into eight bands with a coverage of 2.5 GHz, each of which are analysed by the FFTs. Additional data were taken in October 2020 to improve the line survey at some frequencies and to further check the consistency of all observed spectral features. These observations were also performed in frequency switching but with a throw of 8 MHz. The sensitivity varies along the Q-band between 0.5 and 2.5 mK, which is a considerable improvement compared to previous line surveys in the 31-50 GHz frequency range (Kaifu et al. 2004).

The IRAM 30 m data come from a line survey performed towards TMC-1 and B1, and the observations have been described by Marcelino et al. (2007) andCernicharo et al. (2012). The observations of L1527 and L1544 were obtained as part of the IRAM 30 m Large Program ASAI and were described by Lefloch et al. (2018). The intensity scale and antenna temperature (T * A ) for the two telescopes used in this work were calibrated using two absorbers at different temperatures as well as the atmospheric transmission model ATM (Cernicharo 1985;Pardo et al. 2001). Calibration uncertainties were adopted to be 10%. All data were analysed using the GILDAS package2 .

Results and discussion

The assignment of the observed features in our line surveys was done using the CDMS and JPL catalogues (Müller et al. 2005;Pickett et al. 1998) and the MADEX code (Cernicharo 2012). Most of the weak lines found in our survey of TMC-1 can be assigned to known species and their isotopologues. Nevertheless, many features remain unidentified. Frequencies for the unknown lines were derived by assuming a local standard of rest velocity of 5.83 km s -1 , a value that was derived from the observed transitions of HC 5 N and its isotopologues in our line survey (Cernicharo et al. 2020a,b). Our new data towards TMC-1 allowed us to detect C 3 N -and C 5 N -(Cernicharo et al. 2020a), as well as new species such as the isocyano isomer of HC 5 N, HC 4 NC Fig. 1. Observed lines of CH 3 CO + towards TMC-1. The abscissa corresponds to rest frequencies (in MHz) assuming a local standard of rest velocity of 5.83 km s -1 (Cernicharo et al. 2020a,b). Frequencies and intensities for the observed lines are given in Table 1. The ordinate is the antenna temperature (in mK). Spectral resolution is 38.1 kHz below 50 GHz and 48.8 kHz above. The blue labels correspond to the series of lines we assign to the A species of CH 3 CO + , while the red ones correspond to those of the E species. Within the unidentified features in our surveys in the 3 mm band and the Q-band, we found two series of four lines with a harmonic relation of 2:4:5:6 (see Fig. 1). Taking into account the line density in TMC-1, the possibility that the observed pattern is fortuitous is very small. The observed lines are shown in Fig. 1, and the derived line parameters are given in Table 1. In fact, the J = 5 -4 line at 91 342 MHz has intrigued us since 2017 when we detected it in TMC-1, L483, L1527, and L1544. We interpreted the K = 0, 1 lines and the U line at ∼91 344 (see Fig. A.1) as the hyperfine structure of a J = 1 -0 or J = 2 -1 transition of a molecule containing a nucleus with a spin of 1. Using the old receivers of the Yebes 40 m telescope, and assuming that the three lines around 91 342 MHz could correspond to J = 2 -1, we searched for lines at 45 671 MHz without success. Only when the new receivers covering the whole Q-band were available at the telescope, and we detected the doublet at 36 537 MHz (see Fig. 1), did we realized that two of the lines around 91 342 MHz correspond to a J = 5-4 transition in harmonic relation 2:5 with Notes. (a) Observed frequencies (in MHz) adopting a v LSR of 5.83 km s -1 for TMC-1. The uncertainty is 10 kHz for all the lines. (b) Observed minus calculated frequencies (in kHz) resulting from a fit to the observed frequencies. The J = 7 -6 K = 0, 1 lines observed in L1527 have been included in the fit (see text). (c) Antenna temperature (in mK). (d) Linewidth at half intensity (in km s -1 ). (e) Integrated line intensity (in mK km s -1 ).

the 36 537 MHz doublet. Moreover, the U line at 91 344 MHz is produced by another carrier as it is detected in B1, while the other lines are not. Once we relaxed the initial idea that these features were the hyperfine structure of a low-J transition, other features were found in the 3 mm domain (J = 4-3 and J = 6-5, as well as J = 7 -6 in L1527).

The two series of lines can be fitted to two linear rotors with rotational constants B = 9134.4738 ± 0.0006 MHz and B = 9134.2860 ± 0.0020 MHz. The distortion constant is exactly the same for both series, D = 4.00 ± 0.02 kHz. The observed spectra is reminiscent of the K = 0 and K = 1 components of the rotational transitions of a symmetric rotor. In fact, the eight observed lines in TMC-1 can be fitted with a single rotational constant and two distortion constants if we assume that the carrier is the same for both series and that it has a C 3v symmetry (i.e. that it is a symmetric rotor). Using the standard Hamiltonian for this kind of molecular rotor (Gordy & Cook 1984), we derived the rotational and distortion constants provided in Table 2.

From the derived rotational constant, 9134 MHz, the molecule should contain at least three atoms between C, N, and O. We analysed the possible candidates that could have a rotational constant similar to the observed one. Detailed ab initio calculations for the possible linear and asymmetric carriers are given in Appendix B. Concerning symmetric rotors, it is amazing to realize that all CH 3 X, with X = CN, NC, and CCH, have rotational constants close to our rotational and distortion constants. B.1), are too heavy. Hence, the best symmetric rotor candidate seems to be a species similar to CH 3 CN. The acetyl radical, CH 3 CO, has been observed in the laboratory by Hirota et al. (2006), but it is asymmetric and its lines show a very complex hyperfine structure. However, CH 3 CO + is a symmetric rotor (Mosley et al. 2014) and the lowest energy isomer of H 3 C 2 O + . Its possible precursor, if formed through protonation, is ketene, which is one of the most abundant O-bearing species in TMC-1 (see Cernicharo et al. 2020c).

Quantum chemical calculations and assignment to CH 3 CO +

Precise geometries and spectroscopic molecular parameters for the species mentioned above were computed using high-level Notes. (a) Fit to the lines of CH 3 CO + observed in TMC-1. In this fit, the J = 7 -6 and K = 0, 1 lines observed in L1527 (with frequencies of 127877.133 ± 0.025 and 127874.494 ± 0.050 MHz, respectively) have been included (see Fig.

A.1 and Appendix A). (b) Fit to the lines of CH 3 CO + observed in the laboratory. (c) Fit to the lines of CH 3 CO + observed in space and in the laboratory. (d) The standard deviation of the fit (in kHz). In addition to the geometry optimizations, we calculated the energy associated with the plausible formation of CH 3 CO + , starting from ketene and three proton donors; H + 3 , H 3 O + , and HCO + . All these calculations were carried out at the CCSD/cc-pVTZ level of theory. We found a total energy change in the protonation of ketene to form CH 3 CO + of -421.8, -130.9, and -244.8 kJ mol -1 when ketene reacts with H + 3 , H 3 O + , and HCO + , respectively. More details can be found in Appendix C. Notes. (a) This work. (b) (B+C)/2.

Laboratory detection of CH 3 CO +

We conducted an experiment to detect the CH 3 CO + cation in the laboratory using rotational spectroscopy below 500 GHz. To optimize the experimental setup, we first observed the J = 2 ← 1 transition of HCO + at 178 375.056 MHz, which was produced in the same gas mixture. We then searched for the J = 10 ← 9, K = 0 -2 series of lines of CH 3 CO + between 182.658 and 182.675 GHz based on the rotational constants derived from the lines observed in TMC-1. Weak spectra were observed within 500 kHz. The best experimental conditions were found to be P(CH 4 ) = P(CO) = 1.5 mTorr, P(Ar)=5.5 mTorr (gas mixture cooled using liquid nitrogen but pressures measured at room temperature), an electric discharge of 3.5 kV/10 mA, and an axial magnetic field of 200 G. These lines disappeared when one of the precursors was suppressed, or when the axial magnetic field was cut off. The latter phenomenon confirmed almost unambiguously that they were due to a cation. Subsequent measurements of higher-J transitions fully support the astrophysical assignment of the observed lines to CH 3 CO + .

In total, 79 lines were observed in the laboratory with quantum numbers in the ranges J = 10-27 and K ≤ 6 (see Table E.1). Transitions occurring below 330 GHz were measured by standard frequency modulation absorption spectroscopy, resulting in second-derivative lineshapes. These lines (K ≤ 3) were found unshifted from the first prediction. Those from 400 to 500 GHz were measured by emission spectroscopy (Zou & Motiyenko 2020), giving Voigt-profile lineshapes. Compared to the prediction, some deviations were observed up to 1 MHz for K = 6; these measurements led us to determine the H JK and H K J centrifugal distortion terms. For maximum sensitivity, these lines were measured using the single frequency excitation method with 5-20 million acquisitions (which took 1 to 5 min.). Additionally, a 120 MHz wide chirped excitation spectrum, measured with 67 million acquisitions, is given in Fig. 2 for illustration and comparison purposes. The uncertainty of the laboratory frequency measurements are estimated to be 50 kHz. Given the mass of the cation, and that the negative glow is a nearly electric field-free region, the reported laboratory frequencies are expected to be unshifted by the Doppler effect. The separate and merged least-squares analysis of all (astronomical and laboratory) measured transitions are provided in

Chemistry of CH 3 CO +

From the observed line intensities of CH 3 CO + , we derived a rotational temperature of ∼5 K and a total column density of (3.2 ± 0.3) × 10 11 cm -2 (see Appendix D). The column densities for the A and E species are (2.2 ± 0.2) × 10 11 cm -2 and (9.7 ± 0.9) × 10 10 cm -2 , respectively. Adopting the column density for ketene derived by Cernicharo et al. (2020c), we obtained a H 2 CCO/CH 3 CO + ratio of 44. Assuming the H 2 column density derived by Cernicharo & Guélin (1987), the abundance of CH 3 CO + is 3.2 × 10 -11 . The chemistry of protonated molecules in cold dense clouds has been discussed by Agúndez et al. (2015). Chemical model calculations similar to those that they presented predict that the abundance of protonated ketene is controlled by the typical routes operating for protonated molecules. That is, CH 3 CO + is mostly formed by proton transfer to H 2 CCO from HCO + , H + 3 , and H 3 O + , while it is destroyed through dissociative recombination with electrons. The radiative association between CH + 3 and CO is also an important route to CH 3 CO + . The abundance ratio H 2 CCO/CH 3 CO + predicted by the model is in the range 250-450 and depends on whether the UMIST RATE12 (McElroy et al. 2013) or KIDA kida.uva.2014 (Wakelam et al. 2015) chemical networks are used. As occurs for most protonated molecules observed in cold dense clouds, the abundance of the protonated form with respect to the neutral is underestimated by the chemical model. In this case, there is a factor of 5-10 difference between the model and observations. Incorrect estimates for the rate constants of the dominant reactions of the formation and destruction of CH 3 CO + may be behind this disagreement. Alternatively, the chemical network may miss some important formation route to CH 3 CO + , although it is difficult to identify reactions producing this ion from abundant reagents. For example, plausible reactions of CH + n ions with CO, H 2 CO, or CH 3 OH tend to form products other than CH 3 CO + (Adams & Smith 1978). In this context, it is worth noting that not all species resulting from the protonation of abundant molecules in TMC-1 are detected. For example, CH 3 CNH + is not detected in TMC-1 despite the CH 3 CN proton affinity of 787.4 ± 5.9 kJ mol -1 (Williams et al. 2001). The 3σ upper limit to the column density of CH 3 CNH + is 2.5 × 10 11 cm -2 . The column density of CH 3 CN is (3.2 ± 0.2) × 10 12 cm -2 (see Appendix A); hence, the abundance ratio between the neutral and its protonated form is ≥13. The low dipole moment of CH 3 CNH + compared to that of CH 3 CN (1.01 D versus 3.93 D) limits the chances of detecting this species.

A-CH 3 CO + /E-CH 3 CO + abundance ratio

The column densities derived for the A and E species of CH 3 CO + are not identical, as would be expected for a symmetric top. The A/E abundance ratio for this molecule is 2.27. However, all symmetric molecules of CH 3 X detected in TMC-1 have an abundance ratio between their A and E species that is close to unity (see Appendix D and Fig. D.2). In a symmetric top, the two symmetry states A and E are not connected radiatively nor through inelastic collisions with H 2 . Unlike the rest of the CH 3 X molecules detected in TMC-1, CH 3 CO + is a cation, and its reactive collisions with H 2 or H could produce a proton interchange if there is no barrier to the reaction. The lowest energy level of the E symmetry state is the J = 1, K = 1, which is 7.8 K above the ground J = 0, K = 0 level of the A state. Hence, the reaction of interchange of a proton

E-CH 3 CO + + H 2 /H → A-CH 3 CO + + H 2 /H + 7.8 K
is exothermic, although it is unknown if there is a barrier; this is something that has to be established via detailed theoretical calculations. At thermal equilibrium, and for a kinetic temperature of 10 K, the A/E abundance ratio could be e 0.78 = 2.18, which is very close to the observed value of 2.27. For neutral molecules with two or more symmetric hydrogens, the proton interchange could be mainly produced through collisions with H + , H + 3 , HCO + , and H 3 O + , which are much less abundant than H 2 and H. In Appendix D, we discuss the A/E abundance ratio of all neutral symmetric rotors that have been detected so far in TMC-1, including CH 3 NC, which has previously only been observed in two cold dense clouds: L1544 (Jiménez-Serra et al. 2016) andL483 (Agúndez et al. 2019). For all these species, the A/E abundance ratio is close to unity.

Alternatively, we could also consider the possibility that the collisional rates of the acetyl cation with H 2 or He are higher for the A species than for the E species. As the acetyl cation is isoelectronic to CH 3 CN, we could use the collisional rates of the latter species (Khalifa et al. 2020) to estimate possible differences in the excitation temperature of the K = 0 and K = 1 lines. We explored a density range of (4-10) × 10 4 cm -3 and a kinetic temperature range of 5-10 K. No significant differences were found in the predicted brightness temperature between these lines. Of course, if the effect is due to inelastic collisions, then methyl cyanide (CH 3 CN) would also show a similar behaviour. Nevertheless, although both species are isoelectronic, the fact that CH 3 CO + is positively charged could result in very different collisional rates with H 2 compared to CH 3 CN.

We could also consider that the A/E abundance ratio is affected by the formation process of the molecule. As shown in Sect. 3.1, the reaction of ketene with H + 3 is the most favourable for protonation from the thermodynamical point of view. Both species, ketene and H + 3 , could also have their ortho/para ratio affected by the low temperature of dense dark clouds, which will introduce a non-trivial spin statistic into the formation process of CH 3 CO + . Additional calculations are needed to evaluate the role of collisional excitation and of spin interchange in order to understand the anomalous behaviour exhibited by the A and E symmetry species of CH 3 CO + . (a) Calculations at the CCSD/cc-pVTZ level of theory. (b) The energy taken as reference is the lowest energy species within the same isomer family.

L7, page 7 of 12 A&A 646, L7 (2021) CH 3 CN CH 3 CO + Parameter Calc. (a) Exp. (b) Calc. (a) Scaled (c) Exp. (d) (A-B)×10 Notes. (a) Rotational constants calculated at the CCSD(T)-F12b/aug-cc-pVQZ level of theory, and centrifugal distortion constants calculated at the MP2/aug-cc-pVQZ level of theory. (b) Müller et al. ( 2009). (c) Scaled by the ratio Exp/Calc. of the corresponding parameter for CH 3 CN species. (d) This work. (e) Rodler et al. (1984). ( f ) Scaled by the ratio Exp/Calc. of the corresponding parameter for CH 2 CNH species. (g) Plíva et al. (1995). (h) Scaled by the ratio Exp/Calc. of the corresponding parameter for CH 3 NC species.

Appendix C: Additional quantum chemical calculations for CH 3 CO +

The potential energy surface (PES) for the protonation of ketene has been explored at the CCSD/cc-pVTZ level of theory. In the calculations, we considered three possible proton donors: H + 3 , H 3 O + , and HCO + , as well as the formation of the two isomers of protonated ketene, CH 3 CO + and CH 2 COH + . Figure C.1 depicts the PES along the reaction coordinate for the protonation of ketene and the relative energies for all the stationary points when ketene reacts with H + 3 , H 3 O + , or HCO + . For each reaction, the two reactants, ketene and the proton donor, that separated from each other were assumed to be the energy zero. The protonation of ketene in the CH 2 in the three cases is exothermic, and it proceeds without any transition state (TS) to form CH 3 CO + . This formation is more favourable in the case of H + 3 . On the other hand, the formation of CH 2 COH + is exothermic in the cases of H + 3 and HCO + , but endothermic in the case of H 3 O + . The less stable isomer, CH 2 COH + , can interconvert through a hydrogen migration to CH 3 CO + , which has a TS barrier height of 210.1 kJ mol -1 . As shown in Fig. C.1, the TS for this interconversion lies over the energy of the reactants in the protonation of ketene with H 3 O + and HCO + . In contrast, this TS lies below the energy of the reactants when ketene reacts with H + 3 . Total energies relative to those of the separated reactants, ketene and the proton donor X, are given in the enclosed table in kJ mol -1 . C* is the TS energy for the interconversion between CH 2 COH + and CH 3 CO + isomers. Y** is the energy difference between the reactants and the interconversion TS; a negative value indicates that the TS is submerged below the reactant energy, and a positive value implies that the TS lies above the reactant energy. Notes. The entire table is available at the CDS. (a) Upper and lower J K quantum numbers. (b) Predicted frequency (in MHz). (c) Uncertainty in the predicted frequency (in MHz). (d) Energy (in K) of the upper energy level of the transition. (e) Einstein coefficient of the transition (in s -1 ). ( f ) Line strength. (g) Degeneracy of the upper level. It is 2J+1 for all levels except for those with K = 3 × n (n = 1,2,. . . ), for which g u is 2 × (2J+1).

  , C 3 O (Matthews et al. 1984), HC 5 O (McGuire et al. 2017), HC 7 O (Cordiner et al. 2017), HCCO, and HC 3 O + (Cernicharo et al. 2020c), is a surprising result that has not yet been fully accounted for by chemical models.

(

  Cernicharo et al. 2020b), the cation HC 3 O + (Cernicharo et al. 2020c), the cation HC 3 S + (Cernicharo et al. 2021), and the cation HC 5 NH + (Marcelino et al. 2020), in addition to several tens of already known molecules and their isotopologues.

  For example, CH 3 CN has a rotational constant of 9198.9 MHz (Müller et al. 2009), which is really very close to our result. The other possible candidates, CH 3 CNH + (B = 8590.5 MHz; Amano et al. 2006) and CH 3 NCH + (see, Table

Fig. 2 .

 2 Fig. 2. J = 22 → 21, K = 0 -3 transitions obtained by chirped-pulsed excitation. The record corresponds to the average of 67 million spectra acquired in ∼20 min.

Fig

  Fig. C.1.Energy diagram for the protonation of ketene. Total energies relative to those of the separated reactants, ketene and the proton donor X, are given in the enclosed table in kJ mol -1 . C* is the TS energy for the interconversion between CH 2 COH + and CH 3 CO + isomers. Y** is the energy difference between the reactants and the interconversion TS; a negative value indicates that the TS is submerged below the reactant energy, and a positive value implies that the TS lies above the reactant energy.

Table 1 .

 1 Observed line parameters for CH 3 CO + in TMC-1.

	J u	K	ν obs (MHz) (a)	∆ν oc (kHz) (b)	T * A (mK) (c)	∆v (d) (km s -1 )	T * A dv (e) (mK km s -1 )
	2	0	36537.765	-3.5	39.0 ± 0.6	0.63 ± 0.01	26.1 ± 0.6
	2	1	36537.014	-0.6	11.8 ± 0.6	0.60 ± 0.03	7.5 ± 0.6
	4	0	73074.769	2.7	71.0 ± 3.5	0.47 ± 0.03	35.7 ± 2.0
	4	1	73073.252	-6.6	25.0 ± 3.5	0.65 ± 0.11	17.2 ± 2.0
	5	0	91342.732	-3.5	37.5 ± 3.0	0.46 ± 0.04	18.2 ± 1.0
	5	1	91340.865	14.2	11.9 ± 3.0	0.49 ± 0.11	6.3 ± 1.0
	6	0	109610.225	2.1	12.8 ± 2.8	0.45 ± 0.09	6.1 ± 1.0
	6	1	109607.954	-7.3	6.6 ± 2.8	0.60 ± 0.10	4.2 ± 1.0

Table 2 .

 2 Derived spectroscopic parameters (in MHz) for CH 3 CO + .

	Constant	Space (a)	Laboratory (b)	Merged (c)
	B		9134.47424(82)	9134.47083(27)	9134.47211(20)
	D J		4.014(12) 10 -3	3.99198(25) 10 -3	3.99307(21) 10 -3
	D JK		1.8847(53)10 -1	1.87616(41) 10 -1	1.87736(46) 10 -1
	H JK			8.66(33) 10 -7	9.56(37) 10 -7
	H K J			6.58(59) 10 -6	7.19(74) 10 -6
	rms(kHz) (d)	6.9	34.3	33.2
	J min /J max	1/7	10/27	1/27
	K min /K max	0/1	0/6	0/6
	N lines	(e)	10	79	89
	ν max (GHz)	127.87	492.95	492.95

  In our present case, we used CH 3 CN, CH 2 CNH, and CH 3 NC, which are isoelectronic species of CH 3 CO + , CH 2 COH + , and CH 3 NCH + , respectively, for this purpose. TableB.2 shows the results of these calculations, which are summarized in Table3. As can be seen, the employed level of theory reproduces the rotational parameters for CH 3 CN, CH 2 CNH, and CH 3 NC very well, with relative discrepancies around 0.08% and 0.04% for B in the cases of CH 3 CN and CH 3 NC, respectively. After correcting the calculated parameters for CH 3 CO + , CH 2 COH + , and CH 3 NCH + using the derived scaling ratios for CH 3 CN, CH 2 CNH, and CH 3 NC, respectively, we obtained a B constant of 9129.62 MHz for CH 3 CO + , which shows the best agreement with that derived from the TMC-1 lines. The centrifugal distortion values, obtained in the same manner but at the MP2/aug-cc-pVQZ level of theory for CH 3 CO + and CH 3 NCH + , are both compatible with those obtained from the fit of the lines. The agreement between the experimental constants and those calculated for CH 2 COH + is substantially worse. The calculated dipole moments for CH 3 CO + and CH 3 NCH + are 3.5 D and 2.0 D, respectively, while the µ a and µ b values for CH 2 COH + are 0.8 and 1.7 D, respectively.

(e)

Number of lines included in the fit.

ab initio calculations. The first screening for all plausible candidates (see Appendix B) was done at the CCSD/cc-pVTZ level of theory

(Cížek et al. 1969; Dunning 1989)

. These results are shown in Table

B

.1. In a second stage, the most promising candidates, namely CH 3 CO + , CH 2 COH + , and CH 3 NCH + , were calculated at the CCSD(T)-F12b/aug-cc-pVQZ levels of theory

(Raghavachari et al. 1989; Adler et al. 2007; Knizia et al. 2009)

. To obtain more precise values for the rotational parameters of these three species, we calibrated our calculations using experimental to theoretical scaling ratios for analogue molecular species. This method has been proved to be suitable to accurately reproduce the molecular geometry of other identified molecules

(Cernicharo et al. 2019(Cernicharo et al. , 2020c;; Marcelino et al. 2020

).

Table 3 .

 3 Scaled theoretical values for the spectroscopic parameters of CH 3 CO + , CH 2 COH + , and CH 3 NCH + together with the experimental values obtained in this work (all in MHz).

	Parameter	Exp. (a)	CH 3 CO + CH 2 COH + CH 3 NCH +
	B	9134.4742(8)	9129.6	9309.5 (b)	9105.5
	D J × 10 -3	4.014(13)	3.9	2.8	4.0
	D JK × 10 -3	188.47(50)	184.6	378.7	171.7

  The experimental setup was similar to the one used to detect NS + (Cernicharo et al. 2018). The cation was produced in a liquidnitrogen-cooled Pyrex absorption cell by glow-discharging a mixture of CH 4 , CO (1:1), and Ar. A solenoid coil wound on the cell can generate an axial magnetic field (up to 300 G) to magnetically extend the negative glow, the region known to produce the highest concentrations of cations (compared to the positive column discharge; DeLucia et al. 1983). We also tried acetone and acetaldehyde as precursors(Mosley et al. 2014), but without success.

Table 2 .

 2 The measured frequencies and the observed minus calculated values are given inTable E.1. Frequency predictions are given in Table E.2.

Table B .

 B 1. Rotational constants and electric dipole moments of potential candidates. Species (a) ∆E/kJ/mol (b) A, B, C/MHz D J , D JK /kHz µ a , µ b , µ c /D

			154355.3	3.9	2.5
	H 3 CCO +	0.0	9107.2 9107.2	178.7	0.0 0.0
			201325.3	2.7	1.6
	H 2 COH +	178.0	9411.2 9222.3	341.6	0.1 0.0
			155571.2	3.8	1.9
	H 3 CNCH +	0.0	9090.7 9090.7	160.6	0.0 0.0
			285625.5	2.6	0.5
	H 2 CCCH +	0.0	9673.5 9356.6	457.3	0.0 0.0
			280905.8	3.5	5.0
	H 2 CCN +	0.0	10338.2 9971.2	492.0	0.0 0.0
			277331.4	4.00	3.7
	H 2 CNC +	5.9	11582.1 11117.8	685.4	0.0 0.0
			595150.6	2.7	1.5
	HNCCH +	102.5	10359.3 10182.1	1285.8	1.9 0.0
			328396.0	3.6	4.6
	H 2 NCC +	176.4	10180.7 9874.5	593.3	0.0 0.0
	HCNCH +	196.2	11120.9	2.9	0.0
			324210.9	2.9	3.8
	H 2 NCO +	0.0	10295.5 9978.6	380.4	0.0 0.0
			723374.4	2.8	1.3
	HNCOH +	70.4	10034.0 9896.7	1255.3	1.7 0.0
			625942.1	3.8	2.4
	HCNOH +	285.4	10400.0 10230.0	1859.1	1.6 0.0
			268002.3	3.6	2.9
	H 2 CNO +	333.8	11140.1 10695.5	461.1	0.0 0.0
			259994.7	4.6	2.3
	H 2 CON +	613.2	11321.2 10848.8	484.7	0.0 0.0

Notes.

  Table B.2. Scaled theoretical values for the spectroscopic parameters of CH 3 CO + , CH 2 COH + , and CH 3 NCH + (all in MHz).

  Table E.1. Observed and calculated frequencies (in MHz) for CH 3 CO + .

	J u K Freq. Obser. (Unc) Freq. Calc. (Unc) Obs-Calc
	2	0	36537.765	0.010 36537.761 0.001	0.004
	2	1	36537.014	0.010 36537.010 0.001	0.004
	4	0	73074.769	0.010 73074.755 0.002	0.014
	4	1	73073.252	0.010 73073.253 0.002	-0.001
	5	0	91342.732	0.010 91342.725 0.003	0.007
	5	1	91340.865	0.010 91340.848 0.003	0.017
	6	0 109610.225 0.010 109610.215 0.003	0.010
	6	1 109607.954 0.010 109607.963 0.003	-0.009
	7	0 127877.133 0.025 127877.131 0.004	0.002
	7	1 127874.494 0.050 127874.504 0.004	-0.010
	10 3 182639.714 0.050 182639.724 0.011	-0.010
	10 2 182658.451 0.050 182658.469 0.006	-0.018
	10 1 182669.687 0.050 182669.719 0.005	-0.032
	10 0 182673.442 0.050 182673.470 0.005	-0.028
	11 3 200900.043 0.050 200900.014 0.012	0.029
	11 2 200920.617 0.050 200920.630 0.007	-0.013
	11 1 200933.010 0.050 200933.002 0.006	0.008
	11 0 200937.148 0.050 200937.127 0.006	0.021
	12 3 219159.236 0.050 219159.253 0.013	-0.017
	12 2 219181.747 0.050 219181.737 0.008	0.010
	12 1 219195.209 0.050 219195.232 0.006	-0.023
	12 0 219199.712 0.050 219199.731 0.006	-0.019
	13 3 237417.332 0.050 237417.345 0.014	-0.013
	13 2 237441.706 0.050 237441.696 0.008	0.010
	13 1 237456.320 0.050 237456.311 0.007	0.009
	13 0 237461.178 0.050 237461.184 0.007	-0.006
	14 3 255674.171 0.050 255674.193 0.015	-0.022
	14 2 255700.388 0.050 255700.410 0.009	-0.022
	14 1 255716.107 0.050 255716.145 0.007	-0.038
	14 0 255721.370 0.050 255721.391 0.007	-0.021
	16 3 292183.778 0.050 292183.777 0.017	0.001
	16 2 292213.646 0.050 292213.722 0.010	-0.075
	16 1 292231.636 0.050 292231.694 0.008	-0.058
	16 0 292237.659 0.050 292237.685 0.008	-0.026
	17 3 310436.291 0.050 310436.322 0.019	-0.031
	17 2 310468.201 0.100 310468.127 0.011	0.074
	17 1 310487.210 0.050 310487.216 0.009	-0.006
	17 0 310493.523 0.050 310493.580 0.009	-0.057
	18 3 328687.226 0.050 328687.241 0.020	-0.015
	18 2 328720.866 0.050 328720.905 0.012	-0.039
	18 1 328741.056 0.050 328741.110 0.010	-0.054
	18 0 328747.827 0.050 328747.846 0.010	-0.019
	21 6 383217.758 0.050 383217.721 0.096	0.037

L7, page 10 of 12

Table E .

 E 1. continued. Table E.2. Frequency predictions for CH 3 CO + .

							A&A 646, L7 (2021)
	J u	(a)	K u	(a)	J l	(a)	J u K Freq. Obser. (Unc) Freq. Calc. (Unc) Obs-Calc K l (a) ν(MHz) (b) Unc(MHz) (c) E up (K) (d) A i j (s -1 ) (e)	S i j	( f )	g u	(g)
	1 2 2 3 3 3 4 4 4 4 5		0 1 0 2 1 0 3 2 1 0 4	0 1 1 2 2 2 3 3 3 3 4	21 5 383303.931 0.050 383303.862 0.064 21 4 383374.395 0.050 383374.396 0.041 0 18268.92826 0.00039 0.9 21 3 383429.306 0.050 383429.289 0.025 1 36537.00979 0.00072 9.5 21 2 383468.517 0.050 383468.517 0.015 0 36537.76068 0.00079 2.6 21 1 383492.044 0.050 383492.060 0.012 2 54801.89688 0.00115 32.8 21 0 383499.892 0.050 383499.910 0.012 1 54805.27517 0.00107 12.1 22 6 401451.192 0.050 401451.205 0.103 0 54806.40143 0.00117 5.3 22 5 401541.411 0.050 401541.408 0.069 3 73061.24457 0.00265 70.7 22 4 401615.264 0.050 401615.268 0.044 2 73068.74904 0.00151 36.3 22 3 401672.742 0.050 401672.750 0.027 1 73073.25310 0.00141 15.7 22 2 401713.849 0.050 401713.827 0.017 0 73074.75469 0.00154 8.8 22 1 401738.482 0.050 401738.481 0.014 4 91312.71293 0.00563 123.3	0.069 2.898 × 10 -07 1.0000 -0.001 2.086 × 10 -06 1.5000 0.017 2.782 × 10 -06 2.0000 0.000 5.587 × 10 -06 1.6667 -0.016 8.941 × 10 -06 2.6667 -0.018 1.006 × 10 -05 3.0000 -0.013 1.081 × 10 -05 3.5000 0.003 1.854 × 10 -05 3.0000 -0.004 2.318 × 10 -05 3.7500 -0.008 2.473 × 10 -05 4.0000 0.022 0.001 1.776 × 10 -05 1.8000	3 5 5 7 7 7 18 9 9 9 11
	5		3	4	22 0 401746.707 0.050 401746.700 0.014 3 91325.83851 0.00326 75.1	0.007 3.159 × 10 -05	6.4000	22
	5		2	4	23 6 419682.601 0.050 419682.598 0.111 2 91335.21824 0.00186 40.7	0.003 4.148 × 10 -05	4.2000	11
	5		1	4	23 5 419776.829 0.050 419776.858 0.074 1 91340.84780 0.00173 20.0	-0.029 4.741 × 10 -05	4.8000	11
	5 6 6 6 6 6 6 7 7 7 7 7 7		0 5 4 3 2 1 0 6 5 4 3 2 1	4 5 5 5 5 5 5 6 6 6 6 6 6	23 4 419854.065 0.050 419854.039 0.048 23 3 419914.100 0.050 419914.106 0.029 0 91342.72461 0.00189 13.2 23 2 419957.037 0.050 419957.031 0.018 5 109553.96919 0.01021 190.5 23 1 419982.794 0.050 419982.794 0.015 4 109574.20539 0.00662 128.6 23 0 419991.387 0.050 419991.383 0.015 3 109589.95432 0.00384 80.4 24 6 437911.779 0.050 437911.807 0.119 2 109601.20873 0.00219 46.0 24 5 438010.092 0.050 438010.117 0.080 1 109607.96344 0.00203 25.3 24 4 438090.613 0.050 438090.615 0.052 0 109610.21536 0.00223 18.4 24 3 438153.261 0.050 438153.263 0.032 6 127782.68995 0.01698 272.3 24 2 438198.039 0.050 438198.033 0.020 5 127811.51930 0.01162 196.6 24 1 438224.913 0.050 438224.903 0.016 4 127835.12506 0.00755 134.7 24 0 438233.877 0.050 438233.861 0.016 3 127853.49637 0.00439 86.5 25 6 456138.764 0.150 456138.735 0.129 2 127866.62477 0.00249 52.1 25 5 456240.963 0.150 456241.090 0.087 1 127874.50422 0.00232 31.4	0.026 4.939 × 10 -05 -0.006 2.644 × 10 -05 0.006 4.810 × 10 -05 0.000 6.496 × 10 -05 0.004 7.701 × 10 -05 -0.026 8.425 × 10 -05 -0.025 8.666 × 10 -05 -0.002 3.683 × 10 -05 -0.002 6.805 × 10 -05 0.006 9.361 × 10 -05 0.010 1.135 × 10 -04 11.4286 5.0000 1.8333 3.3333 9.0000 5.3333 5.8333 6.0000 3.7143 3.4286 4.7143 0.016 1.278 × 10 -04 6.4286 0.029 -0.127 1.363 × 10 -04 6.8571	11 13 13 26 13 13 13 30 15 15 30 15 15
	7		0	6	25 4 456324.889 0.050 456324.900 0.056 0 127877.13111 0.00254 24.5	-0.011 1.391 × 10 -04	7.0000	15
							25 3 456390.167 0.050 456390.126 0.034	0.041
							25 2 456436.758 0.050 456436.737 0.022	0.021
							25 1 456464.703 0.050 456464.712 0.018	-0.009
							25 0 456474.048 0.050 456474.039 0.018	0.009
							26 5 474469.660 0.050 474469.681 0.094	-0.021
							26 4 474556.835 0.050 474556.798 0.061	0.037
							26 3 474624.604 0.050 474624.597 0.037	0.007
							26 2 474673.034 0.050 474673.048 0.024	-0.014
							26 1 474702.158 0.050 474702.127 0.020	0.031
							26 0 474711.811 0.050 474711.821 0.020	-0.010
							27 5 492695.931 0.150 492695.796 0.102	0.135
							27 4 492786.129 0.150 492786.214 0.066	-0.085
							27 3 492856.584 0.050 492856.582 0.041	0.002
							27 2 492906.862 0.050 492906.869 0.026	-0.007
							27 1 492937.060 0.050 492937.050 0.022	0.010
							27 0 492947.117 0.050 492947.112 0.022	0.005
											L7, page 11 of 12

https://nanocosmos.iff.csic.es/

http://www.iram.fr/IRAMFR/GILDAS

Access to the entire PRIMOS data set, specifics on the observing strategy, and overall frequency coverage information is available at http://www.cv.nrao.edu/~aremijan/PRIMOS/.

Acknowledgements. The Spanish authors thank Ministerio de Ciencia e Innovación for funding support through project AYA2016-75066-C2-1-P, PID2019-106235GB-I00 and PID2019-107115GB-C21/AEI/10.13039/501100011033. We also thank ERC for funding through grant ERC-2013-Syg-610256-NANOCOSMOS. MA and CB thanks Ministerio de Ciencia e Innovación for grants RyC-2014-16277 and FJCI-2016-27983, respectively. Y. Endo thanks Ministry of Science and Technology of Taiwan through grant MOST108-2113-M-009-25. We would like to thank Evelyne Roueff and Octavio Roncero for useful comments and suggestions.

Appendix A: CH 3 CO + in other sources Fig. A.1. Observations of the J = 5 -4 transition of CH 3 CO + towards L483, L1544, L1527, and B1 (top panels); bottom panel: J = 7 -6 line towards L1527. The abscissa corresponds to the rest frequency (in MHz) and the ordinate is the antenna temperature (in mK). spectral resolution is 48.8 kHz for all sources except L1527, for which it is 198 kHz. The vertical dashed blue lines indicate the position of the K = 0 and K = 1 lines (detected in all sources except B1), and the cyan line corresponds to the U feature at 91 344 MHz (detected in all sources except L1527). The rest velocities of L1527 and L1544 were taken as v LSR = 5.9 km s -1 and 7.2 km s -1 , respectively, based on Sakai et al. (2009) andVastel et al. (2015), respectively.

The acetyl cation, CH 3 CO + , has also been detected towards L483, L1544, and L1527 (see Fig. A.1). However, it is not observed towards Sgr B2 (PRIMOS 3 line survey; Neill et al. 2012), towards Orion-KL (Tercero et al. 2010, 2011), or in our line survey of B1 (Marcelino et al. 2007(Marcelino et al. , 2009(Marcelino et al. , 2010;;Cernicharo 2012). In the PRIMOS data on SgrB2, a very tentative detection of the J = 1 -0 K = 0 line could be claimed at a velocity of 80 km s -1 . However, only an upper limit can be obtained for the J = 2 -1 transition as this line is heavily blended with a strong line of acetone. It seems, hence, that CH 3 CO + is typical of cold interstellar clouds.

Appendix B: Potential carriers of the series of lines

All known diatomic species in cold dark clouds have overly large rotational constants compared with those derived from the lines in TMC-1. For example, a molecule containing one S (CS, NS, SO) will have a rotational constant that is too high by more than 12 GHz. Adding one or two H atoms to these combinations of sulphur produces radicals (with overly high rotational constants, such as HCS) or asymmetric rotors such as H 2 CS, which is too heavy (Müller et al. 2019). A molecule with two S atoms is, of course, too heavy (for example, B(S 2 ) = 8831 MHz; Pickett & Boyd 1979), and we do not expect to have Si-or Pbearing polyatomic species in this cloud. The first step in finding candidates is to exclude the possibility of having a slightly asymmetric species. In that case, we could expect to have lines corresponding to K= ± 1 at roughly ±(B-C) from the K = 0 lines. We searched in the Q-band survey (J = 2 -1) for such a pattern. No lines up to one-fourth of the intensity of the K = 0 line are observed. Moreover, taking into account that there is no evidence for a radical as a possible carrier, the species resulting from the addition of one hydrogen to the closed-shell asymmetric species HNCO, HCOOH, and H 2 CCC have to be excluded. However, their protonated species are also, at least in principle, closed-shell species. Hence, possible candidates are H 2 CCCH + , CH 3 CCH + 2 , HCNOH + , HNCOH + , HCOOH + 2 , and CH 3 OO + , all of which, with the exception of the last one, are protonated forms of known neutral and abundant species in TMC-1. Nevertheless, the resulting molecular structures will be highly asymmetric for most of them, or they are too light or too heavy, as are the cases for H 2 CCCH + and CH 3 CCH + 2 , respectively. HNCOH + is a linear species characterized in the laboratory with a rotational constant of 9955 MHz (Latanzi et al. 2012) and is not detected in our data. Other exotic species, such as NH 2 CHOH + , CH 2 ONH + 2 , and CH 3 NOH + , which could result from the protonation of interesting molecules (NH 2 CHO for example), are discarded for their molecular asymmetry and because the neutral species have not been observed in TMC-1. Ab initio calculations have been performed for the most promising candidates (see Table B.1), and their isomers and the results are discussed in Sect. 3.1. As noted above, the intensity of the J = 2 -1 K = 1 line in TMC-1 is well below the expected value if the rotational temperature for the A and E species is the same. In order to check this point, we show in Fig. D.1 the rotational diagrams for the A and E species of CH 3 CO + using the observed line parameters given in Table 1. The observed intensities have been corrected for beam dilution and the beam efficiencies of the Yebes 40 m and IRAM 30 m telescopes. We assumed a uniform source of radius 40 (Fossé et al. 2001). The derived rotational temperatures, T rot (A) = 4.4 ± 0.4 K and T rot (E) = 5.0 ± 0.5 K, are consistent with a common excitation through collisions with H 2 . The derived column densities are N(A-CH 3 CO + ) = (2.2 ± 0.2) × 10 11 cm -2 and N(E-CH 3 CO + ) = (9.7 ± 0.9) × 10 10 cm -2 . Hence, as discussed in Sect. 3.4, the A/E abundance ratio for CH 3 CO + has been modified through collisions with H + , H + 3 , HCO + , and H 3 O + . This is a similar effect to that found in cold molecular clouds for molecules having ortho and para symmetry species.

Appendix D: CH 3 X species in TMC-1

In order to check this peculiar result, we analysed all symmetric rotors having transitions within our line survey: CH 3 CCH, CH 3 C 4 H, CH 3 CN, and CH 3 NC. The cation CH 3 CNH + has not been detected in TMC-1 (see Sect. 3.3). The symmetric top CH 3 C 3 N is discussed in Marcelino et al. (2021). Figure D.2 shows the J = 2 -1 transition for CH 3 CN, CH 3 CCH, CH 3 NC, and CH 3 CO + (for this species, see also Fig. 1). The K = 0 and K = 1 lines of CH 3 CN exhibit the typical hyperfine structure introduced by the quadruple moment of the N nucleus. For all these additional molecules, we assumed a rotational temperature of 10 K and a source radius of 40 (Fossé et al. 2001), and we produced a synthetic spectrum that is compared to the observations. We found that the A/E abundance ratio is 1 for all species but CH 3 CO + . Adopting a lower rotational temperature has little effect on the derived A/E abundance ratio for these symmetric rotors.

Appendix E: Observed and calculated frequencies of CH 3 CO +

The frequencies observed in space and in the laboratory were merged to obtain the recommended rotational and distortion constants. A total of 89 rotational transitions, ten in space (see Table 1) and 79 in the laboratory (see Table E.1), were fitted to the standard Hamiltonian of a symmetric rotor (Gordy & Cook 1984). For the lines observed in TMC-1 and other dark clouds, only B, D J , and D JK can be obtained as only rotational transitions with K = 0 and 1 have been observed. The results are given in Table 2. For the 79 lines observed in the laboratory, the constants H JK and H K J were included in the fit, and the results are given in Table 2. Finally, the merged fit to the astronomical and laboratory lines produces the recommended set of rotational constants given in the last column of Table 2. The observed and calculated frequencies, together with the observed minus calculated values for the merged fit, are given in Table E.1. We used the rotational and distortion constants that resulted from the merged fit to the astronomical and laboratory lines (see Table 2) to produce frequency predictions, frequency uncertainties, line strengths, upper energy levels, and Einstein coefficients for all transitions involving levels with energies below 2000 K. They are given in Table E.2. The whole table is electronically available at the CDS. It should be noted that this table contains the transitions for the A and E species, and that the E lowest energy level, J K = 1 1 , is 7.8 K above the 0 0 level of the A species.