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The first measurement of the photoelectron spectrum of the silylidyne free radical, SiH, is reported between 7 and 10.5 eV. Two main photoionizing transitions involving the neutral ground state, X+1Σ+ ← X2Π and a+3Π ← X2Π, are assigned by using ab initio calculations. The corresponding adiabatic ionization energies are derived, IEad(X+1Σ+) = 7.934(5) eV and IEad(a+3Π) = 10.205(5) eV, in good agreement with our calculated values and the previous determination by Berkowitz et al. [J. Chem. Phys. 86, 1235 (1987)] from a photoionization mass spectrometric study. The photoion yield of SiH recorded in this work exhibits a dense autoionization landscape similar to that observed in the case of the CH free radical [Gans et al., J. Chem. Phys. 144, 204307 (2016)].

We report a synchrotron radiation vacuum ultraviolet photoionization study of the hydroperoxyl radical (HO

A fresh perspective on the interaction of electron and nuclear motions in photon induced dynamical processes can be provided by the coupling of photoelectron angular distributions and cation vibrational states in the photoionization of chiral molecules using circularly polarized radiation. The chiral contributions, manifesting as a forward-backward asymmetry in the photoemission, can be assessed using Photoelectron Circular Dichroism (PECD), which has revealed an enhanced vibrational influence exerted on the outgoing photoelectron. In this paper, we investigate the PECD of a rigid chiral monoterpene, 3-carene, using single-photon vacuum ultraviolet ionization by polarized synchrotron radiation and selecting energies from the ionization threshold up to 19.0 eV. By judicious choice of these photon energies, two factors that influence PECD asymmetry values, electron kinetic energy and ion vibrational level, can be effectively isolated, allowing a clear demonstration of the very marked vibrational effects. A slow photoelectron spectrum is used to examine the vibrational structure of the isolated outermost valence (HOMO) photoelectron band, and peak assignments are made with the aid of a Franck-Condon simulation. Together, these provide an estimate of the adiabatic ionization energy as 8.385 eV. The reported chiral asymmetry from the randomly oriented 3-carene enantiomers reaches a maximum of over 21%. Theoretical PECD calculations, made both for the fixed equilibrium molecular geometry and also modeling selected normal mode vibration effects, are presented to provide further insight.

The C

We report the vacuum-ultraviolet threshold-photoelectron spectrum of HC

International audience; The threshold photoelectron spectrum of methylene (CH2), produced by consecutive H atom abstractions on methane, has been recorded using synchrotron radiation. The experimental spectrum spans the region of the X+ 2Πu ← X 3B1 ionizing transition. It is modeled starting from ab initio bending potentials and using the bending approach introduced by Coudert et al. [J. Chem. Phys. 148, 054302 (2018)] accounting for the quasilinearity of CH2 and the strong Renner-Teller interaction in CH+2. This first calculation yields a theoretical threshold photoelectron spectrum which is in moderate agreement with the experimental one. A more accurate approach treating the three vibrational modes is developed for computing the threshold photoelectron spectrum of triatomic C2v molecules. This new treatment is tested modeling the already measured threshold photoelectron spectrum of the X+ 2Πu ← X 1A1 ionizing transition of the water molecule. The threshold photoelectron spectrum of CH2 computed with the new approach compares more favorably with the experimental spectrum and yields an adiabatic ionization potential of 10.386(6) eV.

The double imaging photoelectron photoion coincidence technique has been applied to investigate the dissociation of state-selected O

International audience; 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.

We present the photoelectron spectroscopy of four radical species, CH

We report on photoionization studies of Xe.CF4 and Kr.CF4 van-der-Waals complexes produced in a supersonic expansion and detected using synchrotron radiation and photoelectron-photoion coincidence techniques. The ionization potential of CF4 is larger than those of the Xe and Kr atoms and the ground state of the Rg.CF4 (+) ion correlates with Rg(+) ((2)P3/2) + CF4. The onset of the Rg.CF4 (+) signals was found to be only ∼0.2 eV below the Rg ionization potential. In agreement with experiment, complementary ab initio calculations show that vertical transitions originating from the potential minimum of the ground state of Rg.CF4 terminate at a part of the potential energy surfaces of Rg.CF4 (+), which are approximately 0.05 eV below the Rg(+) ((2)P3/2) + CF4 dissociation limit. In contrast to the neutral complexes, which are most stable in the face geometry, for the Rg.CF4 (+) ions, the calculations show that the minimum of the potential energy surface is in the vertex geometry. Experiments which have been performed only with Xe.CF4 revealed no Xe.CF4 (+) signal above the first ionization threshold of Xe, suggesting that the Rg.CF4 (+) ions are not stable above the first dissociation limit.

We present a microwave discharge flow tube coupled with a double imaging electron/ion coincidence device and vacuum ultraviolet (VUV) synchrotron radiation. The system has been applied to the study of the photoelectron spectroscopy of the well-known radicals OH and OD. The coincidence imaging scheme provides a high selectivity and yields the spectra of the pure radicals, removing the ever-present contributions from excess reactants, background, or secondary products, and therefore obviating the need for a prior knowledge of all possible byproducts. The photoelectron spectra encompassing the X(3)Σ(-) ground state of the OH(+) and OD(+) cations have been extracted and the vibrational constants compared satisfactorily to existing literature values. Future advantages of this approach include measurement of high resolution VUV spectroscopy of radicals, their absolute photoionization cross section, and species/isomer identification in chemical reactions as a function of time.

The inner-shell photoionization of unoriented camphor molecules by circularly polarized light has been investigated from threshold to a photoelectron kinetic energy of approximately 65 eV. Photoelectron spectra of the carbonyl C 1s orbital, recorded at the magic angle of 54.7 degrees with respect to the light propagation direction, show an asymmetry of up to 6% on change of either the photon helicity or molecular enantiomer. These observations reveal a circular dichroism in the angle resolved emission with an asymmetry between forward and backward scattering (i.e., 0 degrees and 180 degrees to the light beam) which can exceed 12%. Since the initial state is an atomiclike spherically symmetric orbital, this strongly suggests that the asymmetry is caused by final-state effects dependent on the chiral geometry of the molecule. These findings are confirmed by electron multiple scattering calculations of the photoionization dynamics in the electric-dipole approximation.

Polycyclic aromatic hydrocarbons (PAHs) are key species encountered in a large variety of environments such as the Interstellar Medium (ISM) and in combustion media. Their UV spectroscopy and photodynamics in neutral and cationic forms are important to investigate in order to learn about their structure, formation mechanisms, and reactivity. Here, we report an experimental photoelectron-photoion coincidence study of a prototypical PAH molecule, coronene, and its small clusters, in a molecular beam using the vacuum ultraviolet (VUV) photons provided by the SOLEIL synchrotron facility. Mass-selected high resolution threshold photoelectron (TPES) and total ion yield spectra were obtained and analyzed in detail. Intense series of autoionizing resonances have been characterized as originating from the monomer, dimer, and trimer neutral species, which may be used as spectral fingerprints for their detection in the ISM by VUV absorption spectroscopy. Finally, a full description of the electronic structure of the monomer cation was made and discussed in detail in relation to previous spectroscopic optical absorption data. Tentative vibrational assignments in the near-threshold TPES spectrum of the monomer have been made with the support of a theoretical approach based on density functional theory.

International audience; We studied the single photon ionization of gas phase adenine and cytosine by means of vacuum ultraviolet synchrotron radiation coupled to a velocity map imaging electron∕ion coincidence spectrometer. Both in-vacuum temperature-controlled oven and aerosol thermodesorption were successfully applied to promote the intact neutral biological species into the gas phase. The photoion yields are consistent with previous measurements. In addition, we deduced the threshold photoelectron spectra and the slow photoelectron spectra for both species, where the close to zero kinetic energy photoelectrons and the corresponding photoions are measured in coincidence. The photoionization close and above the ionization energies are found to occur mainly via direct processes. Both vaporization techniques lead to similar electronic spectra for the two molecules, which consist of broadbands due to the complex electronic structure of the cationic species and to the possible contribution of several neutral tautomers for cytosine prior to ionization. Accurate ionization energies are measured for adenine and cytosine at, respectively, 8.267 ± 0.005 eV and 8.66 ± 0.01 eV, and we deduce precise thermochemical data for the adenine radical cation. Finally, we performed an evaluation and a comparison of the two vaporization techniques addressing the following criteria: measurement precision, thermal fragmentation, sensitivity, and sample consumption. The aerosol thermodesorption technique appears as a promising alternative to vaporize large thermolabile biological compounds, where extended thermal decomposition or low sensitivity could be encountered when using a simple oven vaporization technique.

The trifluoromethyl radical, CF(3)(●), is studied for the first time by means of threshold photoelectron spectroscopy (TPES). The radical is produced in the gas phase using the flash-pyrolysis technique from hexafluoroethane as a precursor. CF(3)(+) total ion yield and mass-selected TPES of the radical are recorded using a spectrometer based upon velocity map imaging and Wiley-McLaren time-of-flight coupled to the synchrotron radiation. The high resolution of the instrument and of the photons allows the observation of rich vibrational progressions in the TPES of CF(3)(●). By using Franck-Condon factors computed by Bowman and coworkers, we have been able to simulate the TPES. The initial vibrational temperature of the radical beam has been evaluated at 350 ± 70 K. The structures have been identified as transitions between (n(1),n(2)) and (n(1)(+),n(2)(+)) vibrational levels of CF(3) and CF(3)(+) with small excitation of the breathing mode, ν(1)(+) (,) and large excitation (n(2)(+) = 10-26) of the umbrella mode, ν(2)(+), in the cation. From the energy separation between the two resolved peaks of each band, a value of 994 ± 16 cm(-1) has been derived for the ν(1)(+) breathing frequency of CF(3)(+). For the high-lying n(2)(+) levels, the apparent ν(2)(+) umbrella spacing, 820 ± 14 cm(-1), is fairly constant. Taking into account the ν(2)(+) anharmonicity calculated by Bowman and coworkers, we have deduced ν(2)(+) = 809 ± 14 cm(-1), and semi-empirical estimations of the adiabatic ionization energy IE(ad.)(CF(3)(●)) are proposed in good agreement with most of previous works. A value of the vertical ionization potential, IE(vert.)(CF(3)(●)) = 11.02 eV, has been derived from the observation of a photoelectron spectrum recorded at a fixed photon energy of 12 eV.

The photoionization of enantiomerically pure epichlorohydrin (C(3)H(5)OCl) has been studied using linearly and circularly polarized vacuum ultraviolet synchrotron radiation. The threshold photoelectron spectrum was recorded and the first three bands assigned using molecular orbital calculations for the expected conformers, although uncertain experimental conformer populations and an anticipated breakdown in Koopmans' theorem leave some ambiguity. Measurements of the photoelectron circular dichroism (PECD) were obtained across a range of photon energies for each of these bands, using electron velocity map imaging to record the angular distributions, during which a record PECD chiral asymmetry factor of 32% was observed. A comparison with calculated PECD curves clarifies the assignment achieved using ionization energies alone and further suggests a likely relative population of the conformers. Threshold photoelectron-photoion coincidence methods were used to study the ionic fragmentation of epichlorohydrin. Fragment ion appearance energies show nonstatistical behavior with clear indications that the cationic epoxide ring is unstable and lower energy decay channels proceeding via ring breaking are generally open. Extensive neutral homochiral clusters of epichlorohydrin may be formed in supersonic molecular beam expansions seeded in Ar. Electron angular distribution measurements made in coincidence with dimer and trimer ions are used to effect an examination of the PECD associated with ionization of size-selected neutral cluster species, and these results differ clearly from PECD of the neutral monomer. The shifted ionization thresholds of the n-mers (n = 2, ..., 7) are shown to follow a simple linear relationship, but under intense beam expansion conditions the monomer deviates from this relationship, and the monomer electron spectra tail to below the expected monomer adiabatic ionization potential (IP). PECD measurements made in coincidence with monomer ions obtained under different beam expansion conditions were used to identify unambiguously a contribution from dissociative photoionization of larger clusters to the monomer parent mass ion yield above and below its adiabatic IP.

An electron imaging technique has been used to study the full angular distribution of valence photoelectrons produced from enantiomerically pure molecular beams of camphor when these are photoionized with circularly polarized light. In addition to the familiar beta parameter, this provides a new chiral term, taking the form of an additional cosine function in the angular distribution which consequently displays a forward-backward electron ejection asymmetry. Several ionization channels have been studied using synchrotron radiation in the 8.85-26 eV photon energy range. With alternating left and right circularly polarized radiations the photoelectron circular dichroism (PECD) in the angular distribution can be measured and shows some strong dynamical variations with the photon energy, depending in sign and intensity on the ionized orbital. For all orbitals the measured PECD has a quite perfect antisymmetry when switching between R and S enantiomers, as expected from theory. In the HOMO(-1) channel the PECD chiral asymmetry curves show a double maxima reaching nearly 10% close to threshold, and peaking again at approximately 20% some 11 eV above threshold. This is attributed to a resonance that is also visible in the beta parameter curve. Newly optimized CMS-Xalpha photoionization dynamics calculations are also presented. They are in reasonably good agreement with the experimental data, including in the very challenging threshold regions. These calculations show that PECD in such randomly oriented samples can be understood in the electric dipole approximation and that, unlike the case pertaining in core-shell ionization-where a highly localized achiral initial orbital means that the dichroism arises purely as a final state scattering effect-in valence shell ionization there is a significant additional influence contributed by the initial orbital density.