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Interest in the observation and characterization of organic isomers in astronomical environments has grown rapidly with an increase in the sensitivity of detection techniques. Accurate modeling and interpretation of these environments require experimental isomer-specific reactivity and spectroscopic measurements. Given the abundance of formaldehyde (H 2 CO) in various astrophysical objects, the properties and reactivities of its cation isomers H 2 CO •+ and HCOH •+ are of significant interest. However, for the hydroxymethylene radical cation HCOH •+ (and its isotopologue DCOH •+ ), detailed reactivity studies have been limited by the lack of suitable experimental methods to generate this isomer with high purity. Here, potential approaches to the isomer-selective generation of HCOH •+ and DCOH •+ are characterized through differential reactivity measurements. While the dissociative photoionization of cyclopropanol (c-CH 2 CH 2 CHOH) is determined to be unsuitable, the dissociation of methanol- d 3 (CD 3 OH) allows for the formation of DCOH •+ with a fractional abundance of >99% at photon energies below 14.8 eV. These results will allow future spectroscopic and reactivity measurements of HCOH •+ /DCOH •+ to be conducted, laying the groundwork for future detection and incorporation into models of the interstellar medium.

The gas phase reactivity of the radical cation isomers H 2 CNH˙ + (methanimine) and HCNH 2 ˙ + (aminomethylene) with propene (CH 3 CHCH 2 ) has been investigated by measuring absolute reactive cross sections and product branching ratios, under single collision conditions, as a function of collision energy (in the range ∼0.07-11.80 eV) using guided ion beam mass spectrometry coupled with VUV photoionization for selective isomer generation. Experimental results have been merged with theoretical calculations to elucidate reaction pathways and structures of products. The H 2 CNH˙ + isomer is over a factor two more reactive than HCNH 2 ˙ + . A major channel from both isomers is production of protonated methanimine CH 2 NH 2 + via hydrogen-atom transfer reaction but, while H 2 CNH˙ + additionally gives charge and proton transfer products, the HCNH 2 ˙ + isomer leads instead to protonated vinylimine CH 2 CHCHNH 2 + , produced alongside CH 3 ˙ radicals. The reactions have astrochemical implications in the build up of chemical complexity in both the interstellar medium and the hydrocarbon-rich atmospheres of planets and satellites.

We report a study on the reactive collision of S+(4S) with H2, HD, and D2 combining guided ion beam experiments and quantum-mechanical calculations. It is found that the reactive cross sections reflect the existence of two different mechanisms, one being spin-forbidden. Using different models, we demonstrate that the spin-forbidden pathway follows a complex mechanism involving three electronic states instead of two as previously thought. The good agreement between theory and experiment validates the methodology employed and allows us to fully understand the reaction mechanism. This study also provides new fundamental insights into the intersystem crossing process., (© 2024 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution-NonCommercial 4.0 International (CC BY-NC) license (https://creativecommons.org/licenses/by-nc/4.0/).)

Structural change of ions induced by collision with a neutral has been studied in a guided ion beam tandem mass spectrometer, using Time-Of-Flight measurements and SIMION simulation. The exothermic catalytic isomerization of HOC + to HCO + is used to explore the new methodology. Isomerization is catalyzed via a proton transport mechanism through the interplay of a neutral molecule, the catalyst. Four different potential catalysts, Ne, D 2 , CH 4, and C 18 O, were studied at different collision energies. SIMION simulation of the ion path and collision in the instrument leads to the highlight of a specific signature related to the catalytic isomerization in the time-of-flight spectra. This signature is used to identify the experimental conditions where isomerization takes place. Only C 18 O, at low collision energies, gives a clear signature of catalytic isomerization, and a quantitative estimate of the catalyzed isomerization cross-section and rate constant is derived. This new methodology is sensitive to clear presence of catalyzed isomerization and can be used in instruments designed for cross-section measurements, provided low collision energy is used and ion bunching is available., (© 2024 The Author(s). Journal of Mass Spectrometry published by John Wiley & Sons Ltd.)

The gas-phase reaction between the ethyl cation (C 2 H 5 ) and ethyne (C + ) and ethyne (C 2 ) is re-investigated by measuring absolute reactive cross sections (CSs) and branching ratios (BRs) as a function of collision energy, in the thermal and hyperthermal energy range, via tandem-guided ion beam mass spectrometry under single collision conditions. Dissociative photoionization of C 2 ) is re-investigated by measuring absolute reactive cross sections (CSs) and branching ratios (BRs) as a function of collision energy, in the thermal and hyperthermal energy range, via tandem-guided ion beam mass spectrometry under single collision conditions. Dissociative photoionization of C 2 Br using tuneable VUV radiation in the range 10.5-14.0 eV is employed to generate C 5 Br using tuneable VUV radiation in the range 10.5-14.0 eV is employed to generate C 2 , which has also allowed us to explore the impact of increasing (vibrational) excitation on the reactivity. Reactivity experiments are complemented by theoretical calculations, at the G4 level of theory, of the relative energies and structures of the most relevant stationary points on the reactive potential energy hypersurface (PES) and by mass-analyzed ion kinetic energy (MIKE) spectrometry experiments to probe the metastable decomposition from the [C 5 + , which has also allowed us to explore the impact of increasing (vibrational) excitation on the reactivity. Reactivity experiments are complemented by theoretical calculations, at the G4 level of theory, of the relative energies and structures of the most relevant stationary points on the reactive potential energy hypersurface (PES) and by mass-analyzed ion kinetic energy (MIKE) spectrometry experiments to probe the metastable decomposition from the [C 4 H 7 C + PES and elucidate the underlying reaction mechanisms. Two main product channels have been identified at a centre-of-mass collision energy of ∼0.1 eV: (a) C 3 H 3 C + +CH 4 , with BR = 0.76±0.05 and (b) , with BR = 0.22±0.02. A third channel giving C 4 H 5 in association with C + +H 2 is shown to emerge at both high internal excitation of C 2 H 3 + in association with C 2 H 4 , from which barrierless and exothermic pathways exist for the production of both cyclic c-C 2 H 5 + and high collision energies. From CS measurements, energy-dependent total rate constants in the range 4.3×10-11-5.2×10-10 cm3·molecule-1·s-1 have been obtained. Theoretical calculations indicate that both channels stem from a common covalently bound intermediate, CH 3 isomers of the main product channel. For the minor C 2 CHCH + product, two isomers are energetically accessible: the three-member cyclic isomer c-C 3 H 3 + and linear H 2 CCCH + isomers of the main product channel. For the minor C 4 H 5 + product, two isomers are energetically accessible: the three-member cyclic isomer c-C 3 H 2 (CH 3 ) + and the higher energy linear structure CH 2 CHCCH 2 + , but their formation requires multiple isomerization steps and passages via transition states lying only 0.11 eV below the reagents' energy, thus explaining the smaller BR. Results have implications for the modeling of hydrocarbon chemistry in the interstellar medium and the atmospheres of planets and satellites as well as in laboratory plasmas (e.g., plasma-enhanced chemical vapor deposition of carbon nanotubes and diamond-like carbon films).

Most polycyclic aromatic hydrocarbons (PAHs) can isomerize with internal energies near to or below the dissociation threshold. The C 10 H + 8 group of ions, made up of the naphthalene (Naph + ) radical cations, is a prototypical example. C + ) radical cations, is a prototypical example. C 8 isomers are important species in the growth kinetics and formation of complex organic molecules, and more generally fragments from larger PAHs, yet information about C + 6 structures is scarce and contradictory. Here, ion-molecule reactions were carried out and the tunable photoionization chemical monitoring technique was used to probe the C 8 H + isomers formed upon C 6 structures is scarce and contradictory. Here, ion-molecule reactions were carried out and the tunable photoionization chemical monitoring technique was used to probe the C 8 H + 6 isomers formed upon C 2 and kinetics calculations. To facilitate the interpretation of these data, chemical reactivity starting from phenylacetylene (PA) was studied. It was found that most of the C 2 -loss from Naph + ions formed from C + . The experimental findings were interpreted with the support of ab initio and kinetics calculations. To facilitate the interpretation of these data, chemical reactivity starting from phenylacetylene (PA) was studied. It was found that most of the C 8 H + 6 ions formed from C 10 H 8 , in a timescale of 40 μs, are PA + in the vicinity of the dissociation threshold. No evidence of the pentalene radical cation (PE + ) was observed and explanations to reconcile previous results are presented., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Being a potential process that could explain gas phase abundances of so-called complex organic molecules (COMs) in the cold interstellar medium (ISM), the UV photon-induced desorption from organics-containing molecular ices has been experimentally studied. In this work, we focused on the observation of the photodesorbed products and the measurement of the associated photodesorption yields from pure and mixed molecular ices, each containing organic molecules whose detection has been achieved in the gas phase of the cold ISM, namely formic acid HCOOH and methyl formate HCOOCH 3 . Each molecule, in pure ice or in ice mixed with CO or water, was irradiated at 15 K with monochromatic vacuum UV photons in the 7-14 eV range using synchrotron radiation from the SOLEIL synchrotron facility, DESIRS beamline. Photodesorption yields of the intact molecules and of the photoproducts were derived as a function of the incident photon energy. Experiments have revealed that the desorbing species match the photodissociation pattern of each isolated molecule, with little influence of the kind of ice (pure or mixed in CO or H 2 O-rich environment). For both species, the photodesorption of the intact organics is found to be negligible in our experimental conditions, resulting in yields typically below 10 -5 ejected molecules per incident photon. The results obtained on HCOOH and HCOOCH 3 -containing ices are similar to what has already been found for methanol-containing ices, but contrast with the case of another complex molecule, CH 3 CN, photodesorption of which has been recently studied. Such experimental results may be linked to the observation of COMs in protoplanetary disks, in which CH 3 CN is commonly observed whereas HCOOH or methanol are detected only in some sources, HCOOCH 3 not being detected at all.

Radiotherapy is one of the most widespread and efficient strategies to fight malignant tumors. Despite its broad application, the mechanisms of radiation-DNA interaction are still under investigation. Theoretical models to predict the effects of a particular delivered dose are still in their infancy due to the difficulty of simulating a real cell environment, as well as the inclusion of a large variety of secondary processes. This work reports the first experimental study of the ion-molecule reactions of the H 2 O˙ + and OH + ions, produced by photoionization with synchrotron radiation, with a furan (c-C 4 H 4 O) molecule, a template for deoxyribose sugar in DNA. The present experiments, performed as a function of the collision energy of the ions and the tunable photoionization energy, provide key parameters for the theoretical modelling of the effect of radiation dose, like the absolute cross sections for producing protonated furan (furanH + ) and a radical cation (furan˙ + ), the most abundant products, which can amount up to 200 Å 2 at very low collision energies (<1.0 eV). The experimental results show that furanH + is more fragile , indicating how the protonation of the sugar component of the DNA may favor its dissociation with possible major radiosensitizing effects. Moreover, the ring opening of furanH + isomers and the potential energy surface of the most important fragmentation channels have been explored by molecular dynamics simulations and quantum chemistry calculations. The results show that, in the most stable isomer of furanH + , the ring opening occurs via a low energy pathway with carbon-oxygen bond cleavage, followed by the loss of neutral carbon monoxide and the formation of the allyl cation CH 2 CHCH 2 + , which instead is not observed in the fragmentation of furan˙ + . At higher energies the ring opening through the carbon-carbon bond is accompanied by the loss of formaldehyde, producing HCCCH 2 + , the most intense fragment ion detected in the experiments. This work highlights the importance of the secondary processes, like the ion-molecule reactions at low energies in the radiation damage due to their very large cross sections, and it aims to provide benchmark data for the development of suitable models to approach this low collision energy range.

Structural rearrangements in ions are essential for understanding the composition and evolution of energetic and chemically active environments. This study explores the interconversion routes for simple polycyclic aromatic hydrocarbons, namely naphthalene and azulene radical cations (C 10 H 8 ), by combining mass spectrometry and vacuum ultraviolet tunable synchrotron radiation through the chemical monitoring technique. Products of ion-molecule reactions are used to probe C + ), by combining mass spectrometry and vacuum ultraviolet tunable synchrotron radiation through the chemical monitoring technique. Products of ion-molecule reactions are used to probe C 10 structures that are formed as a function of their internal energies. Isomerisation from azulene radical cation towards naphthalene radical cation in a timescale faster than 80 μs was monitored, whereas no reverse isomerisation was observed in the same time window. When energising C 8 + with more than 6 eV, the reactivity of C 10 H 8 + with more than 6 eV, the reactivity of C 10 H 8 + unveils the formation of a new isomeric group with a contrasted reactivity compared with naphthalene and azulene cations. We tentatively assigned these structures to phenylvinylacetylene cations., (© 2022 The Authors. ChemPhysChem published by Wiley-VCH GmbH.)

Mass spectrometry is a powerful tool but when used on its own, without specific activation of ions, the ion mass is the single observable and the structural information is absent. One way of retrieving this information is by using ion-molecule reactions. We propose a general method to disentangle isomeric structures by combining mass spectrometry, tunable synchrotron light source, and quantum-chemistry calculations. We use reactive chemical monitoring technique, which consists in tracking reactivity changes as a function of photoionization energy i. e. the ionic structure. We illustrate the power of this technique with charge transfer reactions of C 6 H 4 +. isomers with allene and propyne and discuss its universal applicability. Furthermore, we emphasize the special reactivity characteristics of distonic ions, where strong charge transfer reactivity but very limited reactivity involving bond formation and following cleavages were observed and attributed to the unconventional ortho-benzyne distonic cation., (© 2021 Wiley-VCH GmbH.)

Acetamide, a small organic compound containing a peptide bond, was observed in the interstellar medium, but reaction pathways leading to the formation of this prebiotic molecule remain uncertain. We investigated the possible formation of a peptide-like bond from the reaction between acetic acid (CH 3 -COOH) and methylamine (CH 3 -NH 2 ) that were identified in the interstellar medium. From an experimental point of view, a quadrupole/octopole/quadrupole mass spectrometer was used in combination with synchrotron radiation as a tunable source of VUV photons for monitoring the reactivity of selected ions. Acetic acid was photoionized, and the reactivity of CH 3 COOH +• as well as COOH + (produced from either acetic acid or formic acid) ions with neutral CH 3 NH 2 was further studied. With no surprise, charge transfer, proton transfer, and concomitant dissociation processes were found to largely dominate the reactivity. However, a C(O)-N bond formation process between the two reactants was also evidenced, with a weak cross section reaction. From a theoretical point of view, results concerning reactivity and barrier heights were obtained using density functional theory, with the LC-ωPBE range-separated functional in combination with the 6-311++G(d,p) Pople basis set and are in perfect agreement with the experimental data.

The reactivity of CO 2 + with CD 4 has been experimentally investigated for its relevance in the chemistry of plasmas used for the conversion of CO 2 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 CO 2 + with methane by changing either the kinetic energy of CO 2 + 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 CO 2 + . Concerning the product branching ratios ( CD 4 + / CD 3 + /DOCO + ) there is substantial disagreement among the values reported in the literature. We find that the dominant channel is the production of CD 4 + , followed by DOCO + and CD 3 + , as a minor endothermic channel.

We report the vacuum-ultraviolet threshold-photoelectron spectrum of HC 5 N recorded over a wide spectral range, from 84 000 to 120 000 cm -1 , with a 120 cm -1 spectral resolution, better than what was achieved in previous photoelectron studies, and with mass selectivity. The adiabatic ionization potential of cyanobutadiyne is measured at 85 366 (±40) cm -1 . Assignment of the vibrational bands of the four lowest electronic states X +2 Π, A +2 Π, B +2 Σ + , and C +2 Π are performed, supported by high level ab initio calculations which are fully detailed in Paper II [B. Gans et al., J. Chem. Phys. 150, 244303 (2019)] and by Franck-Condon simulations. Only vibrational stretching modes are observed in the threshold-photoelectron spectra. The ground state of HC 5 N + exhibits a vibrational progression in the ν 2 stretching mode involving mainly the elongation of the C≡C triple bonds, whereas the A + and C + excited electronic states show a progression in the stretching mode mainly associated with the elongation of the C≡N bond, i.e., ν 4 and ν 3 , respectively. The B + state appears almost as a vibrationless structure in close vicinity to the A + state.

The methyl carbocation is ubiquitous in gaseous environments, such as planetary ionospheres, cometary comae, and the interstellar medium, as well as combustion systems and plasma setups for technological applications. Here we report on a joint experimental and theoretical study on the mechanism of the reaction CH 3 + + CH 3 (but-2-yne, also known as dimethylacetylene), by combining guided ion beam mass spectrometry experiments with ab initio calculations of the potential energy hypersurface. Such a reaction is relevant in understanding the chemical evolution of Saturn's largest satellite, Titan. Two complementary setups have been used: in one case, methyl cations are generated via electron ionization, while in the other case, direct vacuum ultraviolet photoionization with synchrotron radiation of methyl radicals is used to study internal energy effects on the reactivity. Absolute reactive cross sections have been measured as a function of collision energy, and product branching ratios have been derived. The two most abundant products result from electron and hydride transfer, occurring via direct and barrierless mechanisms, while other channels are initiated by the electrophilic addition of the methyl cation to the triple bond of but-2-yne. Among the minor channels, special relevance is placed on the formation of C 3 (but-2-yne, also known as dimethylacetylene), by combining guided ion beam mass spectrometry experiments with ab initio calculations of the potential energy hypersurface. Such a reaction is relevant in understanding the chemical evolution of Saturn's largest satellite, Titan. Two complementary setups have been used: in one case, methyl cations are generated via electron ionization, while in the other case, direct vacuum ultraviolet photoionization with synchrotron radiation of methyl radicals is used to study internal energy effects on the reactivity. Absolute reactive cross sections have been measured as a function of collision energy, and product branching ratios have been derived. The two most abundant products result from electron and hydride transfer, occurring via direct and barrierless mechanisms, while other channels are initiated by the electrophilic addition of the methyl cation to the triple bond of but-2-yne. Among the minor channels, special relevance is placed on the formation of C 5 H 7 loss from the addition complex. This is the only observed condensation product with the formation of new C-C bonds, and it might represent a viable pathway for the synthesis of complex organic species in astronomical environments and laboratory plasmas.+ , stemming from H 2 loss from the addition complex. This is the only observed condensation product with the formation of new C-C bonds, and it might represent a viable pathway for the synthesis of complex organic species in astronomical environments and laboratory plasmas.

Threshold-photoionization spectroscopy of cyanoacetylene (HC 3 N) and its 15 N isotopologue has been investigated in the vacuum-ultraviolet range with a synchrotron-based experiment allowing to record threshold-photoelectron spectrum and photoion yield over a large energy range (from 88 500 to 177 500 cm -1 , i.e., from 11 to 22 eV). Adiabatic ionization energies towards the three lowest electronic states X + 2 Π, A + Σ+2, and B + Π2 are derived from the threshold-photoelectron spectrum. A detailed description of the vibrational structure of these states is proposed leading to the determination of the vibrational frequencies for most modes. The vibrational assignments and the discussion about the electronic structure are supported by multireference ab initio calculations (CASPT2, MRCI). Unprecedented structures are resolved and tentatively assigned in the region of the B + ← X transition. Exploratory calculations highlight the complexity of the electronic landscape of the cation up to approximately 10 eV above its ground state.

The reaction of C3N(-) with acetylene was studied using three different experimental setups, a triple quadrupole mass spectrometer (Trento), a tandem quadrupole mass spectrometer (Prague), and the "CERISES" guided ion beam apparatus at Orsay. The process is of astrophysical interest because it can function as a chain elongation mechanism to produce larger anions that have been detected in Titan's ionosphere by the Cassini Plasma Spectrometer. Three major products of primary processes, C2H(-), CN(-), and C5N(-), have been identified, whereby the production of the cyanide anion is probably partly due to collisional induced dissociation. The formations of all these products show considerable reaction thresholds and also display comparatively small cross sections. Also, no strong signals of anionic products for collision energies lower than 1 eV have been observed. Ab initio calculations have been performed to identify possible pathways leading to the observed products of the title reaction and to elucidate the thermodynamics of these processes. Although the productions of CN(-) and C5N(-) are exoergic, all reaction pathways have considerable barriers. Overall, the results of these computations are in agreement with the observed reaction thresholds. Due to the existence of considerable reaction energy barriers and the small observed cross sections, the title reaction is not very likely to play a major role in the buildup of large anions in cold environments like the interstellar medium or planetary and satellite ionospheres.

An experimental study has been carried out on the reactions of state selected O(+)((4)S, (2)D, (2)P) ions with methane with the aims of characterizing the effects of both the parent ion internal energy and collision energy on the reaction dynamics and determining the fate of oxygen species in complex media, in particular the Titan ionosphere. Absolute cross sections and product velocity distributions have been determined for the reactions of (16)O(+) or (18)O(+) ions with CH4 or CD4 from thermal to 5 eV collision energies by using the guided ion beam (GIB) technique. Dissociative photoionization of O2 with vacuum ultraviolet (VUV) synchrotron radiation delivered by the DESIRS beamline at the SOLEIL storage ring and the threshold photoion photoelectron coincidence (TPEPICO) technique are used for the preparation of purely state-selected O(+)((4)S, (2)D, (2)P) ions. A complete inversion of the product branching ratio between CH4(+) and CH3(+) ions in favor of the latter is observed for excitation of O(+) ions from the (4)S ground state to either the (2)D or the (2)P metastable state. CH4(+) and CH3(+) ions, which are by far the major products for the reaction of ground state and excited states, are strongly backward scattered in the center of mass frame relative to O(+) parent ions. For the reaction of O(+)((4)S), CH3(+) production also rises with increasing collision energy but with much less efficiency than with O(+) excitation. We found that a mechanism of dissociative charge transfer, mediated by an initial charge transfer step, can account very well for all the observations, indicating that CH3(+) production is associated with the formation of H and O atoms (CH3(+) + H + O) rather than with OH formation by an hydride transfer process (CH3(+) + OH). Therefore, as the CH4(+) production by charge transfer is also associated with O atoms, the fate of oxygen species in these reactions is essentially the O production, except for the reaction of O(+)((4)S), which also produces appreciable amounts of H2O(+) ions but only at very low collision energy. The production of O atoms and the nature of the states in which they are formed are discussed for the reactions of O(+) ions with CH4 and N2.

The threshold photoelectron spectra (TPES) of two unstable nitrogen-containing species, HNCO and NCO, were recorded utilizing vacuum ultraviolet synchrotron radiation. Both are intermediates in combustion processes and play a role in the removal of nitrogen oxides from exhaust gases. The rovibronic structure of the first band in the TPES of HNCO(+) was analyzed within the framework of an orbital ionization model, and the resolved structure of the origin band was assigned to ΔK subbands. An ionization energy of 11.602 ± 0.005 eV was determined and the vibrational structure of the cationic ground state was analyzed by a Franck-Condon fit. Low lying electronically excited states of HNCO(+) were also observed. In a second series of experiments, the NCO radical was generated by flash pyrolysis from chlorine isocyanate. The ionization energy to the X(+) (3)Σ(-) ground state was determined to be 11.76 ± 0.02 eV, while for the a(+) (1)Δ state, a value of 12.93 ± 0.02 eV was obtained. Vibrational structure was observed for both states, and bands were assigned by Franck-Condon simulations.