Masakazu Yamazaki

Vibrational effects on valence electron momentum distributions of ethylene

We report an electron momentum spectroscopy study of vibrational effects on the electron momentum distributions for the outer valence orbitals of ethylene (C(2)H(4)). The symmetric noncoplanar (e,2e) experiment has been conducted at an impact energy of 1.2 keV. Furthermore, a theoretical method of calculating electron momentum distributions for polyatomic molecules has been developed with vibrational effects being involved. It is shown from comparisons between experiment and theory that taking into account effects of the CH(2) asymmetric stretching and CH(2) rocking vibrational modes of C(2)H(4) is essential for a proper understanding of the electron momentum distribution of the 1b(3g) molecular orbital.

Photoelectron?photoion?photoion momentum spectroscopy as a direct probe of the core-hole localization in C 1s photoionization of C2H2

Angular distributions of C 1s photoelectrons, relative to a dissociation axis for C2H2, have been measured with a photoelectron?photoion?photoion coincidence technique. The photoelectron angular distribution (PAD) for three two-body fragmentations (symmetric, non-symmetric and proton-migration fragmentations) is completely different. The PADs for non-symmetric fragmentation provide direct evidence of a localized core-hole and preferential bond breaking following Auger decays. Moreover, the PAD for symmetric fragmentation has been interpreted as the interference between photoemissions from the two carbon atoms.

Classical trajectory calculations for collision-energy/electron-energy resolved two-dimensional Penning ionization electron spectra of N2, CO, and CH3CN with metastable He*(2 3S) atoms

Collision-energy/electron-energy resolved two-dimensional Penning ionization electron spectra (2D-PIES) of N2, CO, and CH3CN with metastable He*(2 3S) atoms are measured, and classical trajectory calculations with anisotropic entrance and exit potential energy surfaces are performed for these systems. Numerical qualities of the entrance potential surfaces are decisively important to understand the collisional ionization dynamics as well as to reproduce observed 2D-PIES, whereas the exit potential surfaces are less sensitive to the collisional ionization dynamics and the electron spectra except for special cases in which a deep potential well is relevant in the entrance potential surface. Ab initio calculations of both entrance and exit potentials as well as ionization widths are found to be reliable in obtaining their anisotropy and radial dependence with good quantitative accuracy.

N 1s photoelectron angular distributions from fixed-in-space NO2 molecules: Stereodynamics and symmetry considerations

Angular distributions of N 1s photoelectrons from fixed-in-space NO(2) molecules have been measured over the energy region of shape resonance and above. A multiple-coincidence velocity-map imaging technique for observation of molecular frame photoelectron angular distributions (MF-PADs) has been extended to nonlinear molecular targets. Density functional theory calculations have also been conducted to elucidate the photoionization dynamics and shape resonance in the N 1s photoionization of NO(2). Results show that the N 1s MF-PADs exhibit strong shape variation as a function of both photoelectron kinetic energy and symmetries of final states, whereas asymmetry parameters of laboratory frame PADs show a local minimum around the shape resonance region and increase monotonically as the photon energy increases. Over the shape resonance, the spatial shape of the photoelectron wave function with b(2)-symmetry closely resembles that of 5b(2)(∗) unoccupied molecular orbital of NO(2), although the MF-PAD pattern for b(2)-symmetry does not correspond directly to the 5b(2)(∗) orbital shape. At higher kinetic energy of 90 eV, MF-PADs become less structured, but still show a significant dependence on the symmetry of final states.

Site-specific photoemission dynamics of N2O molecules probed by fixed-molecule core-level photoelectron angular distributions

The site-specific 1s core-level photoionization of N2O molecules has been studied experimentally and theoretically at a time-dependent density functional level. The photoelectron angular distributions have been measured from the oriented molecules. The effects of site of ionization, of energy dependence and of configurations of molecular axis and polarization vector, have been discussed. The presence of two shape resonances is reflected in the particular behaviours observed in the photoelectron angular distribution, while their nature has been assessed in terms of the calculated photoelectron wavefunction. Comparison with isoelectronic CO2 molecules suggests that symmetry lowering does not play an important role, except for the resonant energy region.

An overlap expansion method for improving ab initio model potentials: Anisotropic intermolecular potentials of N2, CO, and C2H2 with He*(2 3S)

An overlap expansion method is proposed for improving ab initio model potentials. Correction terms are expanded in terms of overlap integrals between orbitals of the interacting system. The method is used to improve ab initio model potentials for N2+He*(2(3)S), CO+He*(2(3)S), and C2H2+He*(2(3)S). Physical meanings of the optimization are elucidated in terms of target orbitals. Correction terms are found to be dominated by the components of HOMO, LUMO, next-HOMO, and next-LUMO on the target molecule. The present overlap expansion method using a limited number of correction terms related to frontier orbitals provides an efficient and intuitive approach for construction of highly anisotropic intermolecular interaction potentials.

Trajectory calculations of two-dimensional Penning ionization electron spectra of N2 in collision with metastable He*23S atoms

Abstract Ionization cross-sections of N2 in collision with He * 2 3 S as functions of the collision energy and the ejected electron kinetic energy (two-dimensional Penning ionization electron spectra, 2D-PIES) have been evaluated by trajectory calculations based on quantum chemical potential surfaces of both entrance and exit channels as well as on the transition widths for producing X, A, and B states of N2+. The present approach using a Li atom for He* and an overlap approximation for Γ has given theoretical 2D-PIES in good agreement with the observation and a promise for its application to the study of dynamics in collisional ionization involving highly anisotropic target systems.

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.

Classical trajectory calculations of collision energy dependence of Penning ionization cross-sections for N2 and CO by He*23S; optimization of anisotropic model potentials

Abstract Anisotropic potential energy surfaces were optimized for classical trajectory calculations in order to reproduce collision energy dependence of partial Penning ionization cross-sections for N 2 and CO molecules with a He ∗ (2 3 S) atom. Ab initio model potentials using a Li(2 2 S) atom in place of a He ∗ (2 3 S) atom based on the similarity of valence electronic configurations were found to be successfully corrected by addition of negative terms including an exponential function combined with Legendre expansions. Optimized sets of parameters demonstrate that the most important correction terms are related to attractive interactions due to electron transfer from He ∗ (Li) to unoccupied molecular orbitals.

Two-dimensional Penning ionization electron spectroscopy of CO/He*(23S)

Abstract Two-dimensional Penning ionization electron spectrum (2D-PIES) of CO by collision with He*(2 3 S) was observed. Collision energy dependence of partial ionization cross-sections was reproduced theoretically by trajectory calculation with anisotropic potential energy surface. In order to obtain the potential energy surfaces for exit channel, ionic ground and excited states were calculated for CO + –He. By adding up the electron transition probabilities for the specific electron energy calculated by the difference between potential energies for the entrance and the exit channels at each trajectory step, the collision-energy-resolved Penning ionization electron spectrum (CERPIES) was obtained. The CERPIESs at various collision energies were then converted into calculated 2D-PIES.