Search

International audience; A new experimental method has been developed to record photoelectron spectra based on the well-established pulsed-field-ionization zero-kinetic-energy photoelectron spectroscopy technique and inspired by the data treatment employed in slow photoelectron spectroscopy. This method has been successfully applied to two well-known systems: the X+2Pg,1/2(v+=0) X1S+g (v=0)and the X+1S+(v+=2) X2P1/2(v=0)ionizing transitions of CO2 and NO, respectively. The first results highlight several advantages of our technique such as an improved signal-to-noise ratio without degrading the spectral resolution and a direct field-free energy determination. The data obtained for NO indicate that this method might be useful for studying field-induced autoionization processes.

The NH

International audience; The NH 2 radical is a key component in many astrophysical environments, both in its neutral and cationic forms, being involved in the formation of complex N-bearing species. To gain insight into the photochemical processes into which it operates and to model accurately the ensuing chemical networks, the knowledge of its photoionization efficiency is required, but no quantitative determination has been carried out so far. Combining a flow-tube H-abstraction radical source, a double imaging photoelectronphotoion spectrometer and a vacuum-ultraviolet synchrotron excitation, the absolute photoionization cross section of the amino radical has been measured in the present 1 work for the first time at two photon energies: σ NH 2 ion (12.7 eV) = 7.8 ± 2.2 Mb and σ NH 2 ion (13.2 eV) = 7.8 ± 2.0 Mb. These values have been employed to scale the total ion yield previously recorded by Gibson et al. (JCP, 83, pp 4319-4328 (1985)). The resulting cross section curve spanning the 11.1-15.7 eV energy range will help in refining the current astrophysical models.

The photoelectron spectroscopy of CH2NC (isocyanomethyl) radical species is investigated for the first time between 9.3 and 11.2 eV in the vicinity of the first photoionizing transition X+1A1 ← X 2B1. The experiment combines a microwave discharge flow-tube reactor to produce the radicals through the CH3NC + F → CH2NC + HF reaction, a VUV synchrotron radiation excitation, and a double imaging electron/ion coincidence spectrometer which allows the recording of mass-selected threshold photoelectron spectra. Assignment of the observed vibrational structure of the CH2NC+ cation is guided by ab initio calculations and Franck-Condon simulations. From the experimental spectrum, the first adiabatic ionization energy of the CH2NC radical is measured as 9.439(6) eV. Fundamental wavenumbers are determined for several vibrational modes of the cation [small nu, Greek, tilde]1+(CH2 symmetric stretch) = 2999(80) cm-1, [small nu, Greek, tilde]2+(NC stretch) = 1925(40) cm-1, [small nu, Greek, tilde]4+(H2C-N stretch) = 1193(40) cm-1, [small nu, Greek, tilde]6+(CNC out-of-plane bend) = 237(50) cm-1, and [small nu, Greek, tilde]8+(CH2 rock) = 1185(60) cm-1.

The photoelectron spectroscopy of CH

The C

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.

International audience; The vacuum-ultraviolet threshold photoelectron spectrum of methyl isocyanate CH3NCO has been recorded from 10.4 eV to 12 eV using synchrotron radiation and a coincidence technique allowing for a mass-discrimination of the photoelectron signal. A significant improvement is achieved over previous investigations as this experimental setup leads to a much more resolved spectrum. Ten sharp peaks and a broad feature spanning 1.2 eV were recorded. This spectrum consists of X̃+ 2A″←X̃ 1A' and Ã+ 2A'←X̃ 1A' ionizing transitions. For the former, the adiabatic ionization energy was determined experimentally to be 10.596(6) eV; for the latter, its value was estimated to be 10.759(50) eV. Seven sharp peaks could be assigned to vibrational modes of the cation X̃+ 2A″ and neutral X̃ 1A' ground electronic states involving only the NCO group atoms. Theoretical modeling of the threshold photoelectron spectrum has proven difficult as methyl isocyanate is a non-rigid molecule displaying large amplitude internal rotation of the methyl group and ∠CNC bending mode, leading to the quasi-symmetry. With the help of ab initio calculations, a theoretical model in which these two large amplitude motions are included in addition to the five small amplitude vibrational modes involving NCO group atoms is proposed. Comparison with the experimental spectrum shows that the broad feature and the strongest peak line positions are well accounted for; their intensities are also fairly well reproduced after adjusting a few parameters.

International audience; The hydroxyl free radical, OH, is one of the most important radicals in atmospheric and interstellar chemistry, and its cation plays a role in the reactions leading to H 2 O formation. Knowledge of the photoionization efficiency of the OH radical is crucial to properly model the water photochemical cycle of atmospheres and astrophysical objects. Using a gas-phase radical source based on a single H-abstraction reaction combined with a photoelectron/photoion imaging coincidence spectrometer coupled with synchrotron radiation, we recorded the OH + photoion yield over the 12.6-15 eV energy range, and we set it to an absolute cross section scale using an absolute point measurement performed at 13.8 eV: σ ion OH = 9.0 ± 2.7 Mb. The resulting cross section values differ by approximately a factor 2 from the recent measurement of Dodson et al. performed with a different radical source is somewhat greater than the combined uncertainties of the measurements. This finding underlines the need for further investigations of this cross section. The hydroxyl radical, OH, is one of the most important free radicals in atmospheric and astrophysical chemistry. It is involved in atmospheric cycles, e.g. as an oxidizing "detergent" on Earth, 1,2 but also in complex photophysical processes in a wide variety of astrophysi-cal media (planetary atmospheres, 3-5 comets, 6 interstel-lar clouds, 7,8 etc.). In these media, OH plays a key role in the water photochemical cycle. The OH + H 2 → H 2 O + H reaction is endothermic, 9 thus most of the reactions involving OH at low temperature occur with O, N, and C atoms and lead to O 2 , NO, and CO compounds rather than H 2 O. On the other hand, the cationic form OH + reacts fast with H 2 , 10 to produce H 2 O + which in turn reacts with H 2 to generate H 3 O +. H 2 O can then be formed through dissociative recombination of H 3 O +. 11 Several theoretical 12 and experimental 13-17 studies have been carried out on the lowest electronic states of neutral OH. However, absorption studies in the Vacuum UltraViolet (VUV) range (λ < 200 nm) are scarce and absolute measurements in this region are even more so. 18,19 For the ionization process, only three experimental works have reported the relative photoionization yield (or constant-ionic-state spectra) of the hydroxyl radical in the VUV range, to our knowledge. 20-23 Dehmer's work covered the photon energy range between 13.0 and 16.5 eV (≈ 95 − 75 nm), at a resolution of 1-3 meV (0.007 − 0.023 nm). 20 Autoionization features were observed in the ion yield and were assigned to a Rydberg series converging to the a + 1 ∆ state of the OH + ion (lo-cated 2.16 eV above the cationic ground state). Later, Cutler et al. carried on Dehmer's work and studied the photoionization of both OH and OD isotopologues between 13.1 and 18.2 eV (94.64-68.12 nm) at a resolution of 1 meV (0.007 nm). 21 They assigned two new Rydberg series converging to the OH + b + 1 Σ + and A + 3 Π ionic states. In 2018, Dodson et al. published the first experimental measurement of the absolute photoionization cross section for OH, where they deduced the cross section from the analysis of time-resolved radical-kinetics measurements of a multi-reaction network in which OH is produced in the reaction of O(1 D) with H 2 O. The absolute cross section of OH was determined relative to that of O(3 P). Their work was supported by new theoretical calculations of the OH cross section using equation-of-motion coupled-cluster Dyson orbitals and a Coulomb photoelectron wave function. 24 The absolute photoionization cross section of the OH radical is important for reliably describing the abundances of OH and OH + and their involvement in the photochemical networks of interstellar media. However, before the work of Dodson et al., modelers could only use the results from theoretical calculations, 12,25,26 or ignore the cross section completely. For instance, the photoion-ization of OH is not present in the Leiden Database 27 or the Meudon PDR code. 28 The new value is expected to find considerable applications in the modeling of atmospheric and interstellar chemistry. Given the importance of this cross section, a complementary experimental determination would also be valuable. We have recently determined the absolute photoion