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1.
J Chem Phys ; 152(4): 041105, 2020 Jan 31.
Artigo em Inglês | MEDLINE | ID: mdl-32007030

RESUMO

The C2 carbon cluster is found in a large variety of environments including flames, electric discharges, and astrophysical media. Due to spin-selection rules, assessing a complete overview of the dense vibronic landscape of the C2 + cation starting from the ground electronic state X Σg+1 of the neutral is not possible, especially since the C2 + ground state is of X+ Σg-4 symmetry. In this work, a flow-tube reactor source is employed to generate the neutral C2 in a mixture of both the lowest singlet X Σg+1 and triplet a 3Πu electronic states. We have investigated the vibronic transitions in the vicinity of the first adiabatic ionization potential via one-photon ionization with vacuum ultraviolet synchrotron radiation coupled with electron/ion double imaging techniques. Using ab initio calculations and Franck-Condon simulations, three electronic transitions are identified and their adiabatic ionization energy is determined Ei(a+ 2Πu←X 1Σg +)=12.440(10) eV, Ei(X+ 4Σg -←a 3Πu)=11.795(10) eV, and Ei(a+2Πu ← a3Πu) = 12.361(10) eV. From the three origin bands, the following energy differences are extracted: ΔE(a - X) = 0.079(10) eV and ΔE(a+ - X+) = 0.567(10) eV. The adiabatic ionization potential corresponding to the forbidden one-photon transition X+ ← X is derived and amounts to 11.873(10) eV, in very good agreement with the most recent measurement by Krechkivska et al. [J. Chem. Phys. 144, 144305 (2016)]. The enthalpy of formation of the doublet ground state C2 + cation in the gas phase is determined at 0 K, ΔfH0(0K)(C2 +(Πu2))=2019.9(10) kJ mol-1. In addition, we report the first experimental ion yield of C2 for which only a simple estimate was used up to now in the photochemistry models of astrophysical media due to the lack of experimental data.

2.
Phys Chem Chem Phys ; 21(46): 25907-25915, 2019 Nov 27.
Artigo em Inglês | MEDLINE | ID: mdl-31742264

RESUMO

We present the absolute photoionization cross-section of the mercapto radical, SH, recorded from its first ionization energy at 10.4 eV up to a photon energy of 15 eV. The absolute scale was calibrated at the fixed photon energy of 11.2 eV using the known values of H2S and S as references. SH and S were produced in a microwave discharge flow-tube reactor by hydrogen abstraction of the H2S precursor. The measured photoionization cross-section of SH dramatically differs from the one currently employed to model the presence of this species in a number of astronomical environments, where SH along with its ionic counterpart SH+ have been detected. The cation spectroscopy and fragmentation of H2S, SH and S in the 9.2-15.0 eV energy range obtained using threshold photoelectron techniques is also presented and discussed in the context of existing literature.

3.
Phys Chem Chem Phys ; 21(40): 22230-22237, 2019 Oct 28.
Artigo em Inglês | MEDLINE | ID: mdl-31577290

RESUMO

The reactions of metastable atomic nitrogen N(2D) are important processes in the gas-phase chemistry of several planetary atmospheres. Here we present a combined experimental and theoretical investigation of the N(2D) + acetylene reaction due to its potential significance for the photochemistry of Titan's atmosphere. Experimentally, a continuous supersonic flow reactor was used to study this reaction over the 50-296 K temperature range employing pulsed laser photolysis and vacuum ultraviolet laser induced fluorescence to produce and detect N(2D) atoms, respectively. The measured rate constants are substantially larger than those obtained in earlier work and remain constant as a function of temperature. Moreover, these results are supported by new electronic structure calculations, which indicate that this process is likely to be barrierless. The impact of the new rate constants on Titan's atmospheric chemistry is tested through simulations using a 1D coupled ion-neutral photochemical model.

4.
J Phys Chem A ; 123(42): 9193-9198, 2019 Oct 24.
Artigo em Inglês | MEDLINE | ID: mdl-31556614

RESUMO

We report on the photoionization of the resonance-stabilized anilino radical (C6H5NH) formed by H atom abstraction from aniline by F atoms in a flow tube. The spectra were recorded from 7.8 to 9.7 eV by using a double-imaging photoelectron/photoion coincidence spectrometer with VUV radiation provided by the DESIRS beamline at the SOLEIL synchrotron. The vibrationally resolved recorded threshold photoelectron spectrum of the anilino radical showed transitions to the ground X+1A' ← X2A″ and first excited states a+3A″ ← X2A″ of the cation, which were assigned through comparison with theoretically simulated spectra, yielding an adiabatic ionization energy of 8.02 ± 0.02 eV. These results are discussed in light of existing data on the picolyl structural isomers and are of interest for the analytical applications of coincidence techniques in real-time combustion analysis where these intermediates are found.

5.
Mon Not R Astron Soc ; 485(4): 5777-5789, 2019 Jun.
Artigo em Inglês | MEDLINE | ID: mdl-31427830

RESUMO

We have developed the first gas-grain chemical model for oxygen fractionation (also including sulphur fractionation) in dense molecular clouds, demonstrating that gas-phase chemistry generates variable oxygen fractionation levels, with a particularly strong effect for NO, SO, O2, and SO2. This large effect is due to the efficiency of the neutral 18O + NO, 18O + SO, and 18O + O2 exchange reactions. The modeling results were compared to new and existing observed isotopic ratios in a selection of cold cores. The good agreement between model and observations requires that the gas-phase abundance of neutral oxygen atoms is large in the observed regions. The S16O/S18O ratio is predicted to vary substantially over time showing that it can be used as a sensitive chemical proxy for matter evolution in dense molecular clouds.

6.
Phys Chem Chem Phys ; 21(24): 12763-12766, 2019 Jun 28.
Artigo em Inglês | MEDLINE | ID: mdl-31179449

RESUMO

We present the photoelectron spectroscopy of the simplest Criegee intermediate, CH2OO, close to the first ionization energy. Comparison with existing theoretical data yields the experimental adiabatic ionization energy and provides a benchmark for theoretical studies on larger Criegee intermediates, which play an important role in the ozonolysis of alkenes.

7.
Phys Chem Chem Phys ; 21(12): 6574-6581, 2019 Mar 28.
Artigo em Inglês | MEDLINE | ID: mdl-30849164

RESUMO

The gas-phase reactions between metastable nitrogen atoms, N(2D) and saturated hydrocarbons CH4, C2H6 and C3H8 have been investigated using a supersonic flow reactor over the 296-75 K temperature range. N(2D) was generated as a product of the C(3P) + NO → N(2D) + CO reaction, with C(3P) atoms created in situ by pulsed laser photolysis of CBr4. The kinetics of N(2D) loss were followed by vacuum ultraviolet laser induced fluorescence. The measured rate constants for the N(2D) + CH4 reaction are in good agreement with earlier work and extend the available kinetic data for this process down to 127 K. The measured rate constants for the N(2D) + C2H6 and N(2D) + C3H6 reactions are in reasonable agreement with previous work at room temperature and extend the available kinetic data for these processes down to 75 K. The rate constants for all three reactions decrease as the temperature falls, indicating the presence of activation barriers for these processes. While the recommended values for the low temperature rate constants of the N(2D) + CH4 reaction are close to those reported here, the previous recommendations for the other saturated hydrocarbon reactions significantly overestimate the rate constants for these processes. The effects of the new rate constants on a coupled ion-neutral photochemical model of Titan's atmosphere are discussed.

8.
Phys Chem Chem Phys ; 21(5): 2337-2344, 2019 Jan 30.
Artigo em Inglês | MEDLINE | ID: mdl-30656348

RESUMO

The photoelectron spectrum of the X1Σ+ → X+2Σ+ ionizing transition of hydrogen isocyanide (HNC) is measured for the first time at a fixed photon energy (13 eV). The assignment of the spectrum is supported by wave-packet calculations simulating the photoionization transition spectrum and using ab initio calculations of the potential energy surfaces for the three lowest electronic states of the cation. The photoelectron spectrum allows the retrieval of the fundamental of the CN stretching mode of the cationic ground state ([small nu, Greek, tilde]3 = 2260 ± 80 cm-1) and the adiabatic ionization energy of hydrogen isocyanide: IE(HNC) = 12.011 ± 0.010 eV, which is far below that of HCN (IE(HCN) = 13.607 eV). In light of this latter result, the thermodynamics of the HCN+/HNC+ isomers is discussed and a short summary of the values available in the literature is given.

9.
J Phys Chem Lett ; 9(20): 5921-5925, 2018 Oct 18.
Artigo em Inglês | MEDLINE | ID: mdl-30234995

RESUMO

Diborenes, R-BB-R', are of current interest in inorganic chemistry because they offer the opportunity to tune the properties of a biradical by modifying the substituents of the diborene parent, HBBH. Here we synthesize the elusive diborene by H atom abstraction from diborane, B2H6, using fluorine atoms and report a vibrationally resolved photoelectron spectrum of the HBBH biradical. The spectrum is interpreted by comparison with high-level ab initio computations, taking into account the Renner-Teller splitting in the X+ 2Π ionic ground state, which show an excellent agreement with the experimental spectrum. An adiabatic ionization energy of 9.080 ± 0.015 eV was determined, and a vibrational progression in the boron-boron stretching vibration of 0.14 eV is visible. This is due to the reduction of bond order upon ionization, accompanied by an increase of the computed boron-boron bond length, RBB, from 1.514 to 1.606 Å.

10.
Phys Chem Chem Phys ; 20(13): 8707-8718, 2018 Mar 28.
Artigo em Inglês | MEDLINE | ID: mdl-29537039

RESUMO

We present the photoelectron spectra of C3Hx (x = 0-3) formed in a microwave discharge flow-tube reactor by consecutive H abstractions from C3H4 (C3Hx + F → C3Hx-1 + HF (x = 1-4)), but also from F + CH4 schemes by secondary reactions. The spectra were obtained combining tunable VUV synchrotron radiation with double imaging electron/ion coincidence techniques, yielding mass-selected threshold photoelectron spectra. The obtained results complement not only existing ones, but for the first time the photoelectron spectra of C3, cyclic and linear C3H (c,l-C3H) as well as of the excited states of C3H3 are reported. In the case of c-C3H, l,t-C3H2 and C3H3, Franck-Condon simulations have been performed in order to assign the vibrational structure. The adiabatic ionization energies of these radicals are reported and compared to ab initio calculated values as well as to theoretical values using known enthalpies of formation.

11.
J Chem Phys ; 148(5): 054302, 2018 Feb 07.
Artigo em Inglês | MEDLINE | ID: mdl-29421906

RESUMO

The line intensity of photoelectron spectra when either the neutral or cationic species display a Renner-Teller coupling is derived and applied to the modeling of the photoelectron spectra of CNC, CCN, and HCCN. The rovibronic energy levels of these three radicals and of their cations are investigated starting from ab initio results. A model treating simultaneously the bending mode and the overall rotation is developed to deal with the quasilinearity problem in CNC+, CCN+, and HCCN and accounts for the large amplitude nature of their bending mode. This model is extended to treat the Renner-Teller coupling in CNC, CCN, and HCCN+. Based on the derived photoelectron line intensity, the photoelectron spectra of all three molecules are calculated and compared to the experimental ones.

12.
Mon Not R Astron Soc ; 470(4): 4075-4088, 2017 Oct 01.
Artigo em Inglês | MEDLINE | ID: mdl-29142332

RESUMO

We report the detection of linear and cyclic isomers of C3H and C3H2 towards various starless cores and review the corresponding chemical pathways involving neutral (C3Hx with x=1,2) and ionic (C3Hx+ with x = 1,2,3) isomers. We highlight the role of the branching ratio of electronic Dissociative Recombination (DR) reactions of C3H2+ and C3H3+ isomers showing that the statistical treatment of the relaxation of C3H* and C3H2* produced in these DR reactions may explain the relative c,l-C3H and c,l-C3H2 abundances. We have also introduced in the model the third isomer of C3H2 (HCCCH). The observed cyclic-to-linear C3H2 ratio vary from 110 ± 30 for molecular clouds with a total density around 1×104 molecules.cm-3 to 30 ± 10 for molecular clouds with a total density around 4×105 molecules.cm-3, a trend well reproduced with our updated model. The higher ratio for low molecular cloud densities is mainly determined by the importance of the H + l-C3H2 → H + c-C3H2 and H + t-C3H2 → H + c-C3H2 isomerization reactions.

13.
J Phys Chem Lett ; 8(17): 4038-4042, 2017 Sep 07.
Artigo em Inglês | MEDLINE | ID: mdl-28796511

RESUMO

The cyano radical is a ubiquitous molecule and was, for instance, one of the first species detected in astrophysical media such as comets or diffuse clouds. In photodissociation regions, the reaction rate of CN+ + CO → CN + CO+ is one of the critical parameters defining nitrile chemistry. The enthalpy of this charge transfer reaction is defined as the difference of ionization energies (EI) between CN and CO. Although EI(CO) is known accurately, the EI(CN) values are more dispersed and deduced indirectly from thermodynamic thresholds only, all above EI(CO), leading to the assumption that the reaction is fast even at low temperature. Using a combination of synchrotron radiation, electron/ion imaging coincidence techniques, and supporting ab initio calculations, we directly determine the first adiabatic ionization energy of CN at 13.956(7) eV, and we demonstrate that EI(CN) < EI(CO). The findings suggest a very slow reaction in the cold regions of interstellar media.

14.
J Chem Phys ; 147(1): 013908, 2017 Jul 07.
Artigo em Inglês | MEDLINE | ID: mdl-28688448

RESUMO

We present the photoelectron spectroscopy of four radical species, CHxCN (x = 0-2) and CNC, formed in a microwave discharge flow-tube reactor by consecutive H abstractions from CH3CN (CHxCN + F → CHx-1CN + HF (x = 1-3)). The spectra were obtained combining tunable vacuum ultraviolet synchrotron radiation with double imaging electron/ion coincidence techniques, which yielded mass-selected threshold photoelectron spectra. The results obtained for H2CCN complement existing ones while for the other radicals the data represent the first observation of their (single-photon) ionizing transitions. In the case of H2CCN, Franck-Condon calculations have been performed in order to assign the vibrational structure of the X+ 1A1←X 2B1 ionizing transition. A similar treatment for the HCCN, CCN, and CNC radicals appeared to be more complicated mainly because a Renner-Teller effect strongly affects the vibrational levels of the ground electronic state of the HCCN+, CCN, and CNC species. Nevertheless, the first adiabatic ionization energies of these radicals are reported and compared to our ab initio calculated values, leading to new values for enthalpies of formation (ΔfH2980(HCCN+(X2A'))=1517±12kJmol-1,ΔfH2980(CCN(X2Π))=682±13kJmol-1, and ΔfH2980(CNC(X2Πg))=676±12kJmol-1), which are of fundamental importance for astrochemistry.

15.
Astrophys J Lett ; 8512017 Dec 21.
Artigo em Inglês | MEDLINE | ID: mdl-29862006

RESUMO

We present the first detection of gas phase S2H in the Horsehead, a moderately UV-irradiated nebula. This confirms the presence of doubly sulfuretted species in the interstellar medium and opens a new challenge for sulfur chemistry. The observed S2H abundance is ~5×10-11, only a factor 4-6 lower than that of the widespread H2S molecule. H2S and S2H are efficiently formed on the UV-irradiated icy grain mantles. We performed ice irradiation experiments to determine the H2S and S2H photodesorption yields. The obtained values are ~1.2×10-3 and <1×10-5 molecules per incident photon for H2S and S2H, respectively. Our upper limit to the S2H photodesorption yield suggests that photo-desorption is not a competitive mechanism to release the S2H molecules to the gas phase. Other desorption mechanisms such as chemical desorption, cosmic-ray desorption and grain shattering can increase the gaseous S2H abundance to some extent. Alternatively, S2H can be formed via gas phase reactions involving gaseous H2S and the abundant ions S+ and SH+. The detection of S2H in this nebula could be therefore the result of the coexistence of an active grain surface chemistry and gaseous photo-chemistry.

16.
Phys Chem Chem Phys ; 18(42): 29286-29292, 2016 Oct 26.
Artigo em Inglês | MEDLINE | ID: mdl-27731878

RESUMO

The rate coefficients of the barrierless O(1D) + CH4 reaction are determined both theoretically and experimentally at 50-296 K. For the calculations, ring polymer molecular dynamics (RPMD) simulations are performed on the basis of a new neural network potential energy surface (PES) in the reactant asymptotic part. Only the reactant asymptotic part of the PES is constructed because of its barrierless and exothermic properties. Experimentally, the reaction rate coefficients are measured using a supersonic flow reactor. Pulsed laser photolysis of O3 molecules is used as the source of O(1D) atoms, which are detected directly through vacuum ultraviolet laser induced fluorescence at 115 nm. The branching ratio for H atom production is measured by comparing the H atom yields of the O(1D) + CH4 and O(1D) + H2 reactions. At T ≥ 75 K, good agreement between theoretical and experimental rate coefficients is found, while at 50 K, the larger difference is discussed in detail.

17.
J Phys Chem Lett ; 7(18): 3641-6, 2016 Sep 15.
Artigo em Inglês | MEDLINE | ID: mdl-27574866

RESUMO

Recent studies of neutral gas-phase reactions characterized by barriers show that certain complex forming processes involving light atoms are enhanced by quantum mechanical tunneling at low temperature. Here, we performed kinetic experiments on the activated C((3)P) + H2O reaction, observing a surprising reactivity increase below 100 K, an effect that is only partially reproduced when water is replaced by its deuterated analogue. Product measurements of H- and D-atom formation allowed us to quantify the contribution of complex stabilization to the total rate while confirming the lower tunneling efficiency of deuterium. This result, which is validated through statistical calculations of the intermediate complexes and transition states has important consequences for simulated interstellar water abundances and suggests that tunneling mechanisms could be ubiquitous in cold dense clouds.

18.
J Phys Chem A ; 120(16): 2504-13, 2016 04 28.
Artigo em Inglês | MEDLINE | ID: mdl-27046417

RESUMO

The gas-phase quenching reaction C((1)D) + N2 → C((3)P) + N2 has been investigated experimentally over the temperature range 50-296 K using a supersonic flow reactor. C((1)D) atoms were produced in situ by the pulsed multiphoton dissociation of CBr4 precursor molecules. Rate constants for this process were measured using a chemical tracer method whereby the C((1)D) + H2 → CH + H reaction was employed to follow C((1)D) decays by monitoring vacuum ultraviolet laser-induced fluorescence of the atomic hydrogen product at 121.567 nm. The deactivation rates are seen to increase at lower temperature, indicating the likely influence of the CNN intermediate complex lifetime on intersystem crossing for this system. We also performed electronic structure calculations of relevant C((3)P)-N2 and C((1)D)-N2 potential energy curves as well as triplet-singlet spin-orbit coupling terms using the explicitly correlated internally contracted multireference configuration interaction method (ic-MRCI-F12). The calculations were performed for the collinear and perpendicular approach of the C atom toward the N2 molecule, which allowed us to construct the approximate spherical (isotropic) potential model of C-N2(j = 0). The computed reduced dimensional potentials were used in quantum close coupling scattering calculations of the electronic quenching cross sections and rate constants. While the calculated potential energy curves form the basis for a good qualitative description of the reaction, the calculated rate constants are significantly smaller than the measured ones, and fail to reproduce the temperature dependence of the experimental results. Several possible reasons are provided to explain the origin of these differences.

19.
Mon Not R Astron Soc ; 456(4): 4101-4110, 2016 Mar 11.
Artigo em Inglês | MEDLINE | ID: mdl-27013768

RESUMO

We present the detection of two H2C3O isomers, propynal and cyclopropenone, toward various starless cores and molecular clouds, together with upper limits for the third isomer propadienone. We review the processes controlling the abundances of H2C3O isomers in interstellar media showing that the reactions involved are gas-phase ones. We show that the abundances of these species are controlled by kinetic rather than thermodynamic effects.

20.
J Phys Chem A ; 120(27): 4838-44, 2016 Jul 14.
Artigo em Inglês | MEDLINE | ID: mdl-26814664

RESUMO

The kinetics of the gas-phase quenching reactions O((1)D) + N2, O((1)D) + O2, and O((1)D) + Ar have been studied over the 50-296 K temperature range using the Laval nozzle method. O((1)D) atoms were created in situ by the pulsed photolysis of O3 precursor molecules at 266 nm. Rate constants for these processes were measured directly, following the decay of O((1)D) atoms through vacuum ultraviolet laser-induced fluorescence at 115.215 nm. For the O((1)D) + N2 and O((1)D) + O2 reactions, the quenching efficiencies are seen to increase as the temperature falls. For the O((1)D) + N2 system, this indicates the likely influence of the intermediate complex lifetime on the quenching rate through nonadiabatic processes. For the O((1)D) + O2 system, which is considerably more complex, this behavior could result from the interactions between several potential energy surfaces.

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