State-Selected Reactivity of Carbon Dioxide Cations (CO2+) With Methane

The reactivity of CO2+ with CD4 has been experimentally investigated for its relevance in the chemistry of plasmas used for the conversion of CO2 in carbon-neutral fuels. Non-equilibrium plasmas are currently explored for their capability to activate very stable molecules (such as methane and carbon dioxide) and initiate a series of reactions involving highly reactive species (e.g., radicals and ions) eventually leading to the desired products. Energy, in the form of kinetic or internal excitation of reagents, influences chemical reactions. However, putting the same amount of energy in a different form may affect the reactivity differently. In this paper, we investigate the reaction of CO2+ with methane by changing either the kinetic energy of CO2+ or its vibrational excitation. The experiments were performed by a guided ion beam apparatus coupled to synchrotron radiation in the VUV energy range to produce vibrationally excited ions. We find that the reactivity depends on the reagent collision energy, but not so much on the vibrational excitation of CO2+. Concerning the product branching ratios (CD4+/CD3+/DOCO+) there is substantial disagreement among the values reported in the literature. We find that the dominant channel is the production of CD4+, followed by DOCO+ and CD3+, as a minor endothermic channel.


INTRODUCTION
The chemistry of the CO + 2 cation attracts much attention because of the presence of this ion in planetary atmospheres (with particular reference to the Earth and Mars Matta et al., 2013;Tenewitz et al., 2018 as well as in laboratory plasmas for energetic and environmental applications Snoeckx and Bogaerts, 2017). In the latter field, the efficient conversion of greenhouse gases into valueadded chemicals is a central topic in current research on renewable and sustainable energies (Wang et al., 2017). In particular, the hydrogenation of CO 2 by technologies based on green electricity allows both the storage of renewable energy in value-added compounds and recycling CO 2 , thus paving the way to decarbonise the energy system. Non-thermal plasmas have been explored for their capability to activate very stable molecules with the potential of achieving a higher energy efficiency compared to purely thermal processes (Scapinello et al., 2016;Martini et al., 2018).
To improve the performances and to control the outcome of plasma-based processes, insight into the physical and chemical mechanisms at play is desired. According to a chemical kinetic model of the plasma-based dry reforming (Snoeckx et al., 2013), a key role is played by the reaction of CO + 2 with CH 4 . However, as described below, there is considerable uncertainty on the branching ratio, so that a reinvestigation of the reaction is desirable. Also, because vibrationally excited levels of CO + 2 can be populated in plasmas, this study aims at investigating the effect of the vibrational excitation of the CO + 2 cation on the reaction with CH 4 .
Energy, in the form of kinetic or internal motion of the reagents, is the driving force of chemical reactions. However, putting the same amount of energy in a different form (i.e., translational, vibrational, rotational or electronic energy) may affect the reactivity differently. For ion-molecule reactions, some state-selected experiments have shown that for endothermic charge-transfer (CT) processes, vibrational excitation is more effective than translational energy in driving the reactions (Viggiano and Morris, 1996). However, in other cases, the effect of vibrational excitation is more varied (see for example Candori et al., 2003;Boyle et al., 2011;Chang et al., 2012;Bell and Anderson, 2013a,b;Bell et al., 2014 and reference therein).
The effect of the internal excitation of CO + 2 in reactions with small molecules has been addressed in previous studies. It was found that the vibrational excitation of CO + 2 increases the reactivity with O 2 and NO (Alge et al., 1981;Durup-Ferguson et al., 1983;Derai et al., 1985;Ferguson et al., 1992;Nicolas et al., 2002), while it decreases the rate coefficient for the reaction with H 2 (Albritton, 1979;Borodi et al., 2009). However, no previous studies exist in which the CO + 2 cation is generated with a precise amount of internal energy (i.e., state-selection of a specific vibrational state) and reacted with CH 4 .

PREVIOUS STUDIES OF THE CO + 2 + CH 4 REACTION
The reaction of CO + 2 with methane in the gas phase has been studied by several groups, with the earliest experimental results dating back to the late 60s (Anicich, 2003). Rate constant and product branching ratio measurements were made using drift techniques, either flow drift tubes (FDT) (Rakshit and Warneck, 1980;Durup-Ferguson et al., 1983) or selected ion flow tube (SIFT) (Smith et al., 1978;Copp et al., 1982), ion cyclotron resonance (ICR) techniques (Huntress et al., 1980) and ion beam methods (Tsuji et al., 1994). Earlier determinations where done using high-pressure mass spectrometry (HPMS) (Harrison and Myher, 1967;Chong and Franklin, 1971;Kasper and Franklin, 1972) and electron space charge traps (SCT) (Ryan and Harland, 1974). The most relevant results are summarized in Table 1.
It has been shown that the reaction proceeds at thermal energy with a rate constant close to the Langevin collision rate constant k L =1.1 × 10 −9 cm 3 ·molecule −1 ·s −1 (Durup- Ferguson et al., 1983). There is fair agreement (within the experimental errors) among the total rate constants measured at thermal energies (in the range 280-340 K for the data reported in Table 1), with the exception of the HPMS study by Kasper and Franklin (1972), that gives a rate constant value more than a factor two higher than the others. The values for the branching ratios are quite scattered, with HOCO + being dominant in all studies except (Durup-Ferguson et al., 1983), where the CT is the only observed channel.

MATERIALS AND METHODS
The experiments have been performed using the CERISES apparatus, an associated experiment to the SOLEIL synchrotron radiation facility. Since the set-up was described in details previously (see Alcaraz et al., 2004;Cunha de Miranda et al., 2015), only the most relevant details will be given here. CERISES is a guided ion beam tandem mass spectrometer composed of two octopoles located between two quadrupole mass spectrometers in a Q1-O1-O2-Q2 configuration that permits investigation of bi-molecular reactions of mass-selected ions. By measuring the yields of parent-and product-ions, absolute reaction cross sections, branching ratios and product velocity distributions as a function of the collision energy are derived.
Vibrational state selection of CO + 2 is performed via the Threshold Photoelectron Photoion Coincidence (TPEPICO) method (Baer and Guyon, 1986) using the ion source of CERISES and the DESIRS beamline. The undulator based DESIRS beamline (Nahon et al., 2012) provides tunable radiation in the vacuum ultraviolet (VUV) range from about 5 eV to 40 eV. Photons at the desired wavelength are selected and scanned simultaneously with the undulator peak energy by a normal incidence monochromator equipped with a low dispersion uncoated SiC grating (200 grooves/mm) optimized to provide photon flux in the 10 12 photon/s to 10 13 photon/s range with an energy resolution down to 1 meV in the 5 eV to 20 eV range. In the present experiments, the photon energies (E phot ) required to produce the parent ion by photoionisation are in the range 13.7-18.3 eV. Depending on the operation mode, the monochromator slits were set in the range 25 to 400 µm, corresponding, at these photon energies, to a resolution of 3 to 44 meV.
In the TPEPICO mode, the CO + 2 ions are extracted in coincidence with threshold photoelectrons. Threshold photoelectrons are filtered first through geometrical discrimination of energetic photoelectrons by using a small extraction field of ≈ 1 V/cm and an extraction hole of 2 mm in diameter. Further time discrimination of energetic photoelectrons is made possible by recording the photoelectron arrival time on the detector and setting a time gate of 10 ns corresponding to the arrival time of threshold photoelectrons. The overall resolution of threshold electrons is about 25 meV. The source pressure and VUV flux are set to limit the false coincidence (FC) rate in the order of 10%. The FC are measured for each TPEPICO point for parent and product ions by replacing the true photoelectron signal by an arbitrary trigger. The FC contribution is then subtracted from the ion count signal. Some measurements have been carried out in the DC mode, i.e., without state-selection but with parent ions in a distribution of excitation that can vary with the photon energy. Prior to the reactivity experiment, the threshold photoelectron spectrum of CO 2 has been measured using the CERISES set up in the 13.7-18.3 eV photon energy range corresponding to the X 2 g (3/2, 1/2) ground state (I.E.=13.778 eV), the first excited state A 2 u (3/2, 1/2) with I.E.= 17.313 eV, up to the beginning of the B 2 + u state of the ion (I.E.=18.076 eV). For high-resolution VUV TPES spectra of CO 2 with the complete assignment of the spectral features to specific internal modes of the CO + 2 ion, the reader is referred to the papers by Baer and Guyon (1986), Merkt et al. (1993), Liu et al. (2000a,b). In Figure 1 we report our measured TPES spectrum in the VUV region where reactivity experiments have been performed. The red dashed lines indicate the photon energies chosen for the production of state-selected CO + 2 in the TPEPICO mode, while the black arrows point to the photon energies at which reactivity studies have been performed in the DC mode (without state-selection).
A sufficient number of ion counts on the threshold electron signal have been observed to perform reactivity experiments with the state-selected cations at the three photon energies (red lines in Figure 1) corresponding to the vibronic bands giving CO + 2 with either no or low vibrational excitation. The intensity of other vibronic bands corresponding to higher internal excitation of CO + 2 was not sufficient to study reactivity in coincidence. For this reason, we decided to perform some measurements not in the coincidence mode, i.e., without pure state-selection but with parent ions in a distribution of excitation that can vary with the photon energy. Two photon-energies were chosen: at E phot = 13.48 eV, corresponding to the CO + 2 (0,0,0) X 2 g,3/2 ← CO 2 (0,0,0) X 1 + g transition, the CO + 2 cation will be produced with no vibrational excitation; at E phot = 16.48 eV, corresponding to a strong resonant autoionization transition via Rydberg states converging to theB state of CO + 2 , the latter will be produced in the X electronic state but with a broad distribution of internal energies, hence in a mixture of low and high vibrational excitation (see Baer and Guyon, 1986 for details).
Deuterated methane (CD 4 ) was used for the reactivity study, to avoid partial mass overlap between the strong parent ion peak at m/z 44 and the one due to the product of the H-atom transfer process at m/z 45. CD 4 pressure in the scattering cell was kept in the range 1-2 × 10 −4 mbar throughout the experiments. Some considerations on possible isotope effects arising when CD 4 is replaced by CH 4 are addressed in the Conclusions.

Results in the DC Mode
As already mentioned in the previous section, in the DC mode we have measured absolute values of the cross sections as a function of the collision energy E CM at two selected photon energies: E phot = 13.78 eV (no vibrational excitation of the CO + 2 cation) and E phot = 16.48 eV (some excitation to high vibrational levels). The main reaction products observed are due to the CT and deuterium-atom-transfer channels (1) and (2), respectively: Also, small amounts of CD + 3 , and minimal amounts of CD 3 CO + 2 are detected and attributed to the following channels (3), (4) and (5): Using literature values (taken from NIST Chemistry Webbook 1 , with the exception of some products as specified in the following) for the standard enthalpies of formations ( f H • ) of reagents and products we can assess that both channels (1) and (2) are exothermic, while channels (3) and (4) are endothermic. In particular, the CT channel (1) has a reaction enthalpy r H • = −1.17 eV, while r H • for (2) is equal to −1.26 eV, assuming that HOCO + has the structure of the hydroxyformyl cation (Holmes et al., 2006). The production of CD + 3 can derive either from D − transfer process (3) or from dissociative CT (4). In the former case, the DOCO radical might be formed in association with the methyl cation, and the overall process is calculated to be endothermic by about 0.56 eV, using the heat of formation of HOCO as reported in Francisco et al. (2010). In the latter case, the methyl cation derives from dissociation of the CT product and the process is endothermic by about 0.61 eV.
The CD 3 CO + 2 formation can be due to D loss from the ionmolecule adduct CO + 2 -CD 4 (reaction 5). We can attempt to estimate the reaction enthalpy of (5) assuming that CD 3 CO + 2 has the structure of the methoxycarbonyl cation [an average value for its heat of formation f H • (CH 3 OCO + ) is 5.57±0.19 eV, as reported in Holmes et al., 2006] to get r H • = −1.10 eV. We note in passing that the adduct CO + 2 -CD 4 was not observed, as expected, as the CD 4 pressure in the scattering cell used throughout the experiments (about 2 × 10 −4 mbar) was too low to allow for secondary collisions for its stabilization.
Cross sections for products CD + 4 , DOCO + and CD + 3 as well as CD + 5 (from secondary reactions of the primary CD + 4 and DOCO + products) measured as a function of the collision energy when the reagent CO + 2 ion is in its ground vibrational state (i.e., at E phot = 13.78 eV) are reported in Figure 2, while results for E phot = 16.48 eV are shown in Figure 3. The cross section for CD + 5 was measured to correct the absolute value of the cross section for reactions (1) and (2) due to product ion losses via the highly efficient secondary reactions operative at the deuterated methane pressures used: with k = 1.1×10 −9 cm 3 ·molecule −1 ·sec −1 for reaction (6) (Anicich, 2003) and k = 7.2 × 10 −10 cm 3 ·molecule −1 ·sec −1 for reaction (7) (Anicich, 2003). The measured CD + 5 yields have been redistributed among the CD + 4 and DOCO + products on the basis of the CD + 4 /DOCO + yield ratio (that changes with collision energy from ∼2 at low energies up to ∼7 at high energies) and of the different rate coefficients for reactions (6) and (7). In Figures 2, 3 the uncorrected and corrected cross sections for CD + 4 are labeled as "CD + 4 " and "CD + 4 corr" respectively, and the same notation is used for DOCO + .
The sudden increase in the cross section shown for CD + 4 and CD + 5 products at E CM ∼ 0.3 eV (clearly visible in the data of Figure 2, but also present in the data of Figure 3) is an instrumental effect due to a decrease in the collection efficiency for "slow" products. In particular, when product ions are produced at very low velocities in the lab frame, they have a chance to move backwards in the 1st octopole (O1) in the opposite direction from the parent ions, and they then face the last electrode before this octopole, L3, which is set to a potential of −0.4 V. As these ions are produced in O1, their fate depends on their initial kinetic energy and the relative values of the O1 and L3 potentials. At high collision energies, the mean potential of O1 is very low (negative values) and all product ions going back in O1 are reflected on L3 and later detected. At low collision energies, the O1 potential can be higher than that of L3, and backward product ions can be lost, accounting for the step in the product yield observed below collision energies of ∼0.3 eV. To correct for such effect, we have rescaled the data measured at E CM ≤ 0.3 eV by a fixed multiplication factor, chosen equal to 1.12 to match the data measured at E CM < 0.3 eV with those measured at higher collision energy. Implicit in this way of rescaling data is the assumption that the number of product ions lost at low collision energy is independent on the collision energy. The corrected data (reported only for CD + 4 cross sections corrected for the presence of secondary reactions leading to CD + 5 ) are shown as dashed lines in Figures 2, 3.
According to the above-mentioned thermochemistry, the formation of CD + 3 via either (3) or (4) is endothermic and the cross-section for its formation, when CO + 2 is generated with no vibrational excitation (see green diamonds in Figure 2), shows the expected threshold behavior, with an appearance energy compatible with the endothermicity. Above threshold, the cross section increases accordingly with collision energy. The small amount of signal observed below the threshold is an artifact due to the tail of the very intense mass peak at 20 m/z (CD + 4 ). Data measured at E phot = 16.48 eV show non-negligible cross sections even at low E CM , thus indicating that vibrational excitation of the CO + 2 cation can promote the endothermic channel. At E phot = 13.78 eV and at the CD 4 pressure used, the yield of CD 3 CO + 2 was below the detection limit, while at E phot = 16.48 eV it was possible to measure a cross section for this very minor channel (data for CD 3 CO + 2 in Figure 3 are multiplied by 300 to be able to show them in the same scale of the other products).
In Table 2 results at the two photon energies are summarized by reporting the branching ratios (BRs) for the observed product channels and the energy-dependent rate constants. BR for the ith channel have been calculated from the absolute cross sections according to the expression: The energy-dependent total rate constants k tot (E ave ) have been estimated using the expression k tot (E ave ) = V · σ tot , were σ tot is the total reaction cross section (i.e., σ i ), as measured in this work (see data in Figures 3, 4) and <v> is the average relative velocity that can be estimated from the collision energy E CM (see Ervin andArmentrout, 1985 andNicolas et al., 2002 for a more detailed treatment). While the total rate constants do not change (within the error bars) when increasing the amount of internal excitation of the CO + 2 cation (compare the values at different photon energies but same E ave ), a slight change in the BRs is observed when increasing the collision energy, which favors the production of CD + 4 (and CD + 3 ) over that of DOCO + . Interestingly, our results for the branching ratios (see Table 2) show reaction (1) to be the dominant channel at all the explored collision energies, in net disagreement with some of the existing values for the branching ratios (see Table 1). As already mentioned, in fact, in the literature there is a spread of the branching ratios for reaction (1) and (2) ranging from 1:0 (Durup- Ferguson et al., 1983) to 0.6:0.4 (Smith et al., 1978), to 0.5:0.5 (Copp et al., 1982), to 0.28:0.72 (Tsuji et al., 1994) and 0.25:0.75 (Huntress et al., 1980), and finally to 0:1 (Harrison and Myher, 1967;Rakshit and Warneck, 1980). We provide here some explanations for the differences observed between our experiment and previous ones: 1. In addition to working with CD 4 , in our experiment we perform a mass selection of the parent ion before reaction. In this way, we eliminate the contribution due to the 13 CO + 2 parent ion that appears at the same m/z as the HOCO + product (from residual not fully deuterated methane) and represents ∼1% of the parent ion intensity. Additionally, our experimental procedure consists in measuring both parent and product ion yields first with the target gas in the collision cell, secondly with the target gas in the chamber. In this way, we correct for any contribution of "impurities" coming from the source at the same mass as the product (namely 13 CO + 2 /HOCO + ). In one of the a Total (i.e., summed over all the product channels) rate constant (in cm 3 ·molecule -1 ·s -1 ) at the specified average collision energy Eave, estimated as detailed in the text. earlier papers (Tsuji et al., 1994), the experiment is performed without parent ion mass selection, and no correction for 13 CO + 2 is mentioned. Hence the claimed HOCO + branching ratio (0.72) is most likely overestimated. In Durup-Ferguson et al. (1983), the parent ion mass selection is performed, although no indication is given about the mass resolution. In Rakshit and Warneck (1980), a mixing of CO + 2 , CO 2 CO + 2 and H 2 O + parent ions are used. The values from Huntress et al. (1980) are given without any experimental details, for which a reference is given to an earlier paper (Huntress, 1977) where the ICR set-up is described. However, the earlier paper does not contain data for the title reaction, and it is impossible to infer whether and how the 13 CO + 2 contribution was taken into account.
2. In our experiment, we keep the target gas pressure as low as possible to limit the number of secondary reactions. Some of the earlier works (Harrison and Myher, 1967;Kasper and Franklin, 1972;Tsuji et al., 1994) have a pressure in the reaction cell higher than ours by a factor 30-50, with about the same cell length.
Such differences may lead to underestimating the BR for CH + 4 if secondary reactions are not adequately accounted for (as done in our study).
3. In our experiment, we use a pure target gas, while some of the earlier works perform mass spectrometry studies in mixtures of gases. If some CO 2 is present in the region where CH + 4 products are generated by the CO + 2 + CH 4 reaction, the CH + 4 will be easily consumed by the efficient reaction CH + 4 + CO 2 → HOCO + + CH 3 (k = 1.2 × 10 −9 cm 3 ·molecule −1 ·s −1 ) that will produce HOCO + , leading to a negative bias in the CH + 4 /HOCO + ratio. This is an issue in Rakshit and Warneck (1980) as highlighted by Copp et al. (1982), where CO 2 is present in the reaction cell, as well as in Ryan and Harland (1974), where mixing of CH4 and CO 2 occurs in the reaction region, in Tsuji et al. (1994), Harrison and Myher (1967) and Kasper and Franklin (1972).
4. In our experiments we mass select ionic products. Hence we can directly give BRs among different channels. In some of the flow/drift tube experiments, the reaction rate constants are measured by observing the decline of the primary ion signal upon addition of the neutral gas. For instance, in Durup- Ferguson et al. (1983) no mention is made about the mass detection of products, as if the authors have assumed the exclusive formation of CH + 4 via non-dissociative CT, not considering the possibility that HOCO + might be produced.

Results in the TPEPICO Mode
In the TPEPICO mode, we recorded cross sections for the reaction of CO + 2 ions in the (0,0,0) ground state and in two vibrationally excited states: (0,1,0) with one quantum of bending vibration and [(1,0,0) + (0,0,1)] corresponding to a combination of the symmetric and antisymmetric stretching vibration. Cross sections were measured for products CD + 4 , DOCO + and CD + 3 as well as CD + 5 (from secondary reactions of the primary CD + 4 products, see above). The ion yield for product CD 3 CO + 2 was too low to be detectable in coincidence. For most of the other products, reactive cross sections were measured at two different collision energies E CM = 0.17 ± 0.02 eV and 1.34 ± 0.01 eV and results are shown in Figures 4, 5. Despite the larger uncertainties (due to the low S/N ratio in the coincidence mode) cross section measurements are consistent with results obtained in the DC mode. In particular, when converting cross sections reported in Figure 4 for CO + 2 at low collision energy and in the (0,0,0) ground state we obtain the following BRs: CD + 4 (0.75 ± 0.25), DOCO + (0.25 ± 0.11) and CD + 3 (0.00 ± 0.03), entirely consistent, within the error bars, with the data obtained in the DC mode (see Table 2, second column from the left).
In addition, the TPEPICO results confirm that cross sections for the exothermic CT (1) and deuterium-atom-transfer (2) channels change very little with increasing vibrational excitation, and the increase is more evident in data taken at high collision energy (Figure 5). Cross sections for the endothermic channel leading to CD + 3 increase with collision energy but they do not seem to depend strongly on the internal excitation of the ionic parent, at least for the low internal excitations here explored (i.e., maximum one quantum of vibrational excitation).
The observed small dependence of reactions (1) and (2) from CO + 2 vibrational excitation requires some consideration. First of all, we note that our result is in agreement with a previous study (Durup-Ferguson et al., 1983) in which a drift tube technique is used, and the dependence of the reaction rate constant on the internal energy of the ions is examined by varying the mass of the buffer gas. Despite the limitations of the technique compared to our truly state-selection method, in Durup- Ferguson et al. (1983) the non-dissociative CT giving CH + 4 plus CO 2 is found to occur at near the collision rate and to have little energy dependence and no measurable vibrational dependence.
We also note that, differently from the CH 4 case, vibrational excitation of CO + 2 ions was found to enhance the CT reaction probability with O 2 (Durup-Ferguson et al., 1983;Ferguson et al., 1992;Viggiano and Morris, 1996;Nicolas et al., 2002). To rationalize such results, it should be considered that the two systems present several differences: • CT channels have different exothermicities (−1.17 eV for CH 4 and −1.71 for O 2 ).
The inefficiency of the CT with O 2 is attributed to the nonresonant character of the reaction, i.e., to the fact that Franck-Condon factors for O 2 ionization at the ionization potential of CO 2 (13.78 eV) are close to zero (see for instance Wacks, 1964). On the other hand, it is known that for CH 4 the Franck-Condon factors are low at the ionization threshold and increase reaching a maximum in the region 13.5-14.5 eV (see for instance Stockbauer and Inghram, 1971). Thus, while in the CH 4 case CT can occur efficiently at long range via the direct mechanism previously described, for O 2 it should involve the formation of a collision complex. As tentatively explained and demonstrated in Ferguson et al. (1992), in the O 2 case a small amount in the stretching excitation of CO + 2 can lead to an increase in the CT probability. Hence, CH 4 and O 2 are quite different reaction systems, and in the latter case, the effect of CO + 2 excitation could be more pronounced than for a reaction already at the Langevin limit (as in CH 4 ).
More generally, the dynamics of CT processes is regulated by crossings among entrance and exit potential energy surfaces. When such crossings are occurring at distances for which the probability of electron transfer from one adiabatic PES to the other is unfavorable, the CT cross section will be very small even for exothermic processes (according to the Landau-Zener model for CT probability). The fact that increasing the vibrational excitation of the cation does not increase the CT cross section can be related to the fact that the crossing probability does not change much when increasing the vibrational excitation of CO + 2 , even though the exothermicity increases. Unfortunately, modeling the dynamics occurring on a CO + 2 -CH 4 multidimensional potential energy surface which includes vibrational excitation of CH 4 is not an easy task, and it is beyond the scope of our paper. We hope that our results will stimulate theoreticians and experts in ab-initio calculations to use this system as a test bench for theory.

CONCLUSIONS
The reactivity of CO + 2 with deuterated methane has been investigated experimentally by guided ion beam mass spectrometric techniques by changing either the kinetic energy of CO + 2 or its vibrational excitation (using synchrotron radiation in the VUV energy range to produce vibrationally excited reagent ions). The main products are CD + 4 , DOCO + , and CD + 3 and reactivity is found to depend on the reagent collision energy, but not so much on the vibrational excitation of CO + 2 . An interesting issue is whether reaction rates and dynamics change or remain the same when CD 4 is replaced by CH 4 . We do not expect charge transfer cross sections to be affected by a strong kinetic isotope effect (KIE). On the other hand, one can foresee a kinetic isotope effect in the H/D atom transfer channel leading to HOCO + /DOCO + . In particular, according to the semi-classical theory of primary KIE a normal effect (i.e., k H /k D > 1) is expected in the transfer of an H/D atom due to the vibrational zero-point energy differences for each of the vibrational modes of the reactants and transition state. Despite the limited mass resolution in our experimental set-up, we have managed to perform some tests using CH 4 in the reaction cell and measuring BRs and cross sections at a fixed collision energy of 0.11 eV in the DC mode at a photon energy E phot = 13.78 eV (i.e., same conditions of Figure 2). Products are observed at m/z values corresponding to CH + 4 , CH + 5 , and HOCO + . By correcting for secondary reactions (as detailed in the text) we obtain a BR of 0.74:0.26 = CH4 + : HOCO + and a total cross section of 199 (±30%) Å 2 , corresponding to an energy-dependent rate constant of (2.68 ± 0.8) × 10 −9 cm 3 ·molecule −1 ·s −1 to be compared with the CD 4 value of (1.8 ± 0.5) × 10 −9 at similar E ave (see our results in Table 2) and the BR of 0.63:0.36 = CD + 4 : DOCO + . This means a positive KIE k H /k D = 1.5(±0.6). On the other hand, our results show that when CD 4 is replaced by the lighter isotopolog, the CT is more favored than the H atom transfer. This effect can be explained assuming that the different vibrational spacings in CD 4 /CH 4 might change the Franck-Condon factors and the efficiencies of non-adiabatic transition probability among the entrance and exit potential energy surfaces. We note that in a similar reacting system CN + plus CH 4 /CD 4 a KIE in the total rate coefficient similar to the one observed in our case has been reported, namely k H /k D = 1.55 (±0.66) (Raksit et al., 1984).
To put our results into the context of plasma chemistry used for the conversion of CO 2 in carbon-neutral fuels (Snoeckx et al., 2013;Snoeckx and Bogaerts, 2017), both the products (CH + 4 and HOCO + ) of the reaction of CO + 2 with CH 4 eventually lead to the production of CH + 5 and CH 3 , as shown in the following scheme: CO + 2 + CH 4 → CO 2 + CH +

DATA AVAILABILITY
The datasets generated for this study are available on request to the corresponding author.

AUTHOR CONTRIBUTIONS
DA, CA, and PT contributed conception and design of the study. CA, CR, and JŽ planned and developed the experimental set-up. DA, CR, CA, AL, JŽ, MP, and CS contributed to data acquisition, data analysis, and interpretation of results. DA wrote the first draft of the manuscript. DA, PT, and CA wrote sections of the manuscript. All authors contributed to manuscript revision, read and approved the submitted version.