Vibronic effects on the low-lying electronic excitations in CO2 induced by electron impact

Theoretical Study of Valence Shell Excitation by Electron Impact in CCl4

This paper reports a theoretical study of valence shell excitation in CCl₄ by high-energy electron impact. Generalized oscillator strengths are calculated for the molecule at the equation-of-motion coupled-cluster singles and doubles level. To elucidate the influence of nuclear dynamics on electron excitation cross-sections, the effects of molecular vibration are included in the calculation. Based on a comparison with recent experimental data, several reassignments of spectral features are made, and it is found that excitations from the Cl 3p nonbonding orbitals to σ* antibonding orbitals, 7a₁ and 8t₂, play dominant roles below the excitation energy of ∼9 eV. Furthermore, the calculations reveal that distortion of the molecular structure due to the asymmetric stretching vibration significantly affects the valence excitations at small momentum transfers, where contributions from dipole transitions are dominant. It indicates that vibrational effects have a considerable influence on Cl formation in the photolysis of CCl₄.

Electron-molecule collisions with explicit rovibrational resolution at MRCI level and using even tempered basis sets

A method for calculating the generalized oscillator strengths (GOSs) and differential cross section (DCS) with vibration and rotation resolution is presented. The importance of accounting for the rotational contribution is to be emphasized since it has not previously been considered in GOS calculations. Although largely neglected due to its small effect on various properties, the rotational resolution proved to be fundamental in the study of certain phenomena, such as the interference between rotational states in a molecule. As the general goal of this work is to obtain theoretical values comparable to high resolution experiments, special care was taken on the calculation of the electronic part of the scattering amplitude, particularly in what concerns the choice of the atomic basis set. Accordingly, even-tempered basis sets have proved to lead to good results. The helium atom was taken as a model system for this aspect of the problem. Then, GOS and DCS, for explicit vibrational and rotational transitions, were calculated for hydrogen and nitrogen molecules. For higher accuracy, a non-Franck-Condon approach was used to obtain transitions involving vibrational states. The resultant values have shown good agreement with the available experimental data.

Forward-backward asymmetry in electron impact ionization of CO

We experimentally investigate the molecular-orientation dependence of high-energy electron-impact ionization of CO. The direction of the molecular-axis with respect to the momentum transfer vector K is deduced from the angular correlation between the fragment ion and the scattered electron. The experimental results on the 3 ²Π ionization reveal that at small momentum transfer, the ionization probability near the threshold is higher when K points toward the C atom along the molecular axis than when it is in the opposite direction. Such a forward-backward asymmetry does not appear in single-photon ionization and requires non-dipole contributions. It is also shown that the {4 ²Σ⁺ + 5 ²Σ⁺ + 6 ²Σ⁺} ionization preferentially takes place in the vicinity of the molecular orientation parallel to K at small momentum transfer, while non-dipole contributions cause the decrease in the relative intensity of the parallel direction.

Generalized oscillator strengths of the low-lying valence-shell excitations of N2, O2, and C2H2 studied by fast electron and inelastic x-ray scattering

The generalized oscillator strengths of the low-lying valence-shell excitations of N₂, O₂, and C₂H₂ have been studied by the high-energy electron scattering, the high-resolution inelastic X-ray scattering, and the multireference single- and double-excitation configuration-interaction methods. Good agreement between the present electron-scattering results and the X-ray-scattering ones for the a^''1Σ_g ⁺v^'=0 and a^''1Σ_g ⁺v^'=1+b¹Π_uv^'=0 excitations of N₂ and the A^'3Δ_u excitation of O₂ is achieved in the small squared momentum transfer region, while obvious discrepancies among them are observed in the large squared momentum transfer region. This phenomenon indicates that the first Born approximation is satisfied in the small squared momentum transfer region, while it does not hold in the large squared momentum transfer region at an incident electron energy of 1500 eV, in view of the fact that the first Born approximation is satisfied in the X-ray scattering. In addition, the present calculation for the a^''1Σ_g ⁺ excitation shows that the traditional assigned v' = 0 and 1 of the a^″1Σ_g ⁺ excitation correspond to v' = 9 and 13 of the 2¹Σ_g ⁺ excitation and reproduces the X-ray-scattering results of the a^''1Σ_g ⁺v^'=0 excitation very well except the ones in the small squared momentum transfer region. We also report the generalized oscillator strengths of the à + B̃ excitations of C₂H₂, and its profile shows that the bending geometry has great influence on the transition feature.

Development of an electron-ion coincidence apparatus for molecular-frame electron energy loss spectroscopy studies

We report details of an electron-ion coincidence apparatus, which has been developed for molecular-frame electron energy loss spectroscopy studies. The apparatus is mainly composed of a pulsed electron gun, an energy-dispersive electron spectrometer, and an ion momentum imaging spectrometer. Molecular-orientation dependence of the high-energy electron scattering cross section can be examined by conducting measurements of vector correlation between the momenta of the scattered electron and fragment ion. Background due to false coincidences is significantly reduced by introducing a pulsed electron beam and pulsing scheme of ion extraction. The experimental setup has been tested by measuring the inner-shell excitation of N₂ at an incident electron energy of 1.5 keV and a scattering angle of 10.2°.

Stereodynamics of electron-induced dissociative ionization of N2 studied by (e, e+ion) spectroscopy

We report an (e, e+ion) spectroscopy study on electron-induced dissociative ionization of N₂. Vector correlation between the scattered electron and N⁺ ion has been measured for inner-valence ionization of N₂ at an incident electron energy of 1.4 keV and scattering angles of 2.2°, 4.2°, and 8.2°. By analyzing the experimental data, partial ion yield spectra have been obtained for transitions to the C ²Σ, F ²Σ, and 2σ ion states, showing that the individual transitions depend on the momentum transferred to the target, K, in different ways. The molecular-orientation dependence of the ionization cross section has subsequently been examined for the F ²Σ ionization. To account for the angular anisotropy of the scattering cross section, a compact analytical form has been developed. It is elucidated that for small K the F ²Σ ionization preferentially takes place when the molecule has its axis aligned parallel to the momentum transfer vector due to σ_u shape resonance, while the angular distribution drastically changes with K, indicating strong influences of non-dipole interaction on the ionization dynamics. It has been shown that the present method provides a powerful means to explore K-dependent stereodynamics in electron-induced dissociative ionization of molecules.

Ab Initio Studies on Spectroscopic Constants for the HAsO Molecule

The anharmonic force fields and spectroscopic constants of the electronic ground state (X̃¹A') for the HAsO molecule are reported employing the MP2, B3LYP, B3P86, and B3PW91 methods with cc-pVQZ and cc-pV5Z basis sets. The calculated molecular geometries, rotational constants, vibrational frequencies, and anharmonic constants of the HAsO molecule are compared with the experimental data. It is found that the best agreement between the calculated results and experiment data is at the B3LYP/cc-pV5Z theoretical level. The predicted cubic and quartic force fields, vibration-rotation interaction constants, quartic and sextic centrifugal distortion constants, and Coriolis coupling constants of the HAsO molecule at the B3LYP/cc-pV5Z theoretical level are expected to be reliable.

Vibrational effects on valence electron momentum distributions of CH2F2

We report an electron momentum spectroscopy study of vibrational effects on the electron momentum distributions for the outer valence orbitals of difluoromethane (CH2F2). The symmetric noncoplanar (e,2e) experiment has been performed at an incident electron energy of 1.2 keV. Furthermore, a theoretical calculation of the electron momentum distributions of the CH2F2 molecule has been carried out with vibrational effects being involved. It is shown from comparisons between experiment and theory that it is essential to take into account influences of the CH2 asymmetric stretching and CH2 rocking vibrational modes for a proper understanding of the electron momentum distribution of the 2b1 orbital having the CH-bonding character. The results of CH2F2and additional theoretical calculations for (CH3)2O and H2CO molecules strongly suggest that vibrational effects on electron momentum distributions tend to be appreciable for non-total symmetry molecular orbitals delocalized over some equivalent CH-bond sites.

Interplay between temperature-activated vibrations and nondipolar effects in the valence excitations of the CO₂ molecule

We report a study on the temperature dependence of the valence electron excitation spectrum of CO2 performed using nonresonant inelastic X-ray scattering spectroscopy. The excitation spectra were measured at the temperatures of 300 and 850 K with momentum-transfer values of 0.4-4.8 Å(-1), i.e., from the dipole limit to the higher-multipole regime, and were simulated using high-level coupled cluster calculations on the dipole and quadrupole level. The results demonstrate the emergence of dipole-forbidden excitations owing to temperature-induced bending mode activation and finite momentum transfer.