Mineo Kimura

Coupled diabatic configuration interaction treatment of the O2B’–X transition including computation of predissociation linewidths, optical f values, and generalized oscillator strengths

Ab initio CI (configuration interaction) calculations are carried out for O2 in various 3Σ−u excited states in order to study the avoided crossing that is responsible for the discrete B’–X bands found in the high‐energy region of the Schumann–Runge continuum. A diabatic transformation is carried out which allows for the description of nonadiabatic effects in this spectral range. A coupled vibrational treatment is employed in conjunction with the complex scaling technique in order to compute the predissociation linewidths of the B’ levels, and these results are found to be in good agreement with experiment, particularly for trends observed between different isotopomers of this system. Optical f values and generalized oscillator strengths (GOS) are calculated for a series of vibrational transitions and are also found to compare well with measured data. The observed finding that the GOS varies quite slowly with K2 for the 0–0 transition, but decreases rather sharply for the 1–0 species is reproduced in the c...

Elastic and vibrationally inelastic cross sections and energy loss spectra for electron collisions with GeH4

Absolute vibrationally elastic cross sections for e-GeH4 collisions have been determined for electrons of 1, 2, 2.5, 3, 5, 7.5, 10, 15, 20, 60 and 100 eV incidence energy over a scattering angular range of 10 degrees -130 degrees . The observed angular distributions correspond, at least qualitatively, to theoretical formulations using the continuum-multiple-scattering method, the parameter-free static-exchange-polarization approximation, and a multichannel Schwinger variational calculation. Vibrational excitation functions and the energy distribution of elastically scattered electrons reveal a shape resonance at about 2-2.5 eV. Calculations show that the resonance scattering state belongs to the t2 representation of the Td point group, which is also the representation of the lowest unoccupied molecular orbital (LUMO) of GeH4. Energy loss spectra recorded in the electronic threshold region of GeH4 reveal enhanced inelastic scattering at large angles in the loss range 7-9 eV characteristic of excitation to the lowest triplet state.

Elastic scattering and some vibrational excitation cross sections for electron collisions with Si2H6

Absolute elastic cross sections for e-disilane collisions have been measured by using incident kinetic energies of 2, 3, 4, 5, 7.5, 10, 15, 20, 40, and 100 eV and recorded over a scattering angular range of 10-130 degrees . These cross sections have been integrated by employing a nonlinear phase shift fitting procedure to generate a list of integral elastic and momentum transfer cross sections for the same energy range. The angular distributions have been found to agree reasonably well with a continuum multiple scattering calculation using only local potentials and also with a multichannel Schwinger variational calculation. Limited experiments on vibrationally inelastic scattering reveal the existence of a shape resonance with a peak at approximately 2 eV that is characteristic of trapping by a valence orbital with Si-H antibonding character analogous to that encountered in e-monosilane scattering.

Theoretical study of the electronic spectrum of ammonia: Generalized oscillator strength calculations for the A-X transition☆

Abstract A series of ab initio CI calculations is reported for the electronic spectrum of ammonia. The generalized oscillator strength for the A-X n→3s Rydberg transition is computed as a function of the square of the momentum transfer vector in electron impact studies and the results are found to be in good agreement with the corresponding measured data. Vertical and adiabatic transition energies are also reported for a series of low-energy Rydberg transitions involving 3p, 3d, 4s and 4p upper orbitals and these results are also found to compare well with the corresponding experimental findings. Optical φ values for the latter transitions have also been computed.

Electron degradation and yields on initial products. II. Subexcitation electrons in molecular nitrogen

Subexcitation electrons lose their kinetic energy through vibrational excitation, rotational excitation, and elastic collisions in molecular gases. Initial yields of vibrationally and rotationally excited states of nitrogen molecules are calculated by using the Spencer–Fano equation (SFE) and its simplification, the continuous‐slowing‐down approximation (CSDA), both in time‐independent and time‐dependent representations. One focus of the present study is a close comparison of the CSDA with the rigorous treatment of the SFE in the subexcitation domain. The present result reveals for the first time distinct energy regions in which either vibrational excitation or rotational excitation dominates. This recognition explains the different time dependence of the yields of vibrational and rotational excitation.

Electron degradation and yields of initial products. III. Dissociative attachment in carbon dioxide

We demonstrate the importance of subexcitation electrons in CO2 (with energies below 6 eV) by studying the yield of negative‐ion formation in the dissociative attachment process e−+CO2→CO+O−. We evaluate the electron degradation spectrum and the time dependence of the degradation process within the continuous‐slowing‐down approximation. Slowing down by vibrational and other excitation collisions and the O− production are competing processes. This explains why the O− yield is larger for subexcitation electrons with energies above 3.8 eV, which avoid the large energy loss by electronic excitation and can still pass through the resonance at about 4 eV. The attachment at 8 eV with a much larger resonance‐like cross section contributes only about 30% to the total O− yield in the degradation process.

Vibrational excitation of ethane by electron impact

Differential cross sections for electron impact vibrational excitation of ethane (C2H6) have been determined for incident energies from 3 to 20 eV over a scattering angular range of 20 to 130. In addition, vibrational excitation functions were determined for impact energies of 2 to 15 eV at scattering angles of 30, 60, 90 and 120. These measurements confirm the broad shape resonance at 7.5 eV but provide no evidence for a (second) resonance near 2.3 eV. A decomposition of the vibrational energy-loss spectrum shows strong resonance enhancement of selected vibrational modes, which may be explained by decay of a temporary negative ion (TNI) of symmetry species eu. A continuum multiple scattering calculation (CMS) reproduces these experimental results fairly well and indicates the simultaneous involvement of a TNI of species a2u.

Observation of the lowest triplet state in silane by electron energy loss spectroscopy

The electron energy loss spetra of silane was recorded at several impact energies and angles. It is concluded that the lowest triplet state of silane is formed from configuration mixing of the 4s Rydberg state with a valence state of the same symmetry.(AIP)

Electron energy distribution functions and thermalization times in methane and in argon–methane mixtures: An effect of vibrational excitation processes

Electron thermalization in methane and argon–methane mixtures is studied by using the Boltzmann equation. The presence of low‐lying vibrational excited states in methane significantly changes electron energy distribution functions and relaxation times. We found that (i) the mean electron energy just below the first vibrational excited state is reached faster by 1000 times when the vibrational states are taken into account, and (ii) electron energy distribution functions have distinct peaks at energy intervals equal to the vibrational threshold energies. Both these effects are due to large vibrational stopping cross section. The thermalization time in mixtures of argon–methane (without vibrational states) smoothly changes as the mixture composition varies, and no significant difference in the electron energy distribution function is observed. When the vibrational excited states are taken into account, thermalization is almost completely defined by CH4, even at very low fractional concentrations of CH4. The...

Electron degradation and yields of initial products. IV. Subexcitation electrons in molecular oxygen

Electron slowing‐down processes in molecular oxygen gas in the subexcitation domain (below the ionization threshold) are studied by using the Spencer–Fano (SF) equation and its simplification, the continuous‐slowing‐down approximation (CSDA), both in time‐dependent and time‐independent representations. Compared to the previously studied cases of N2 and CO2, O2 has the special features in its inelastic cross sections of (i) strong delta‐function‐like peaks in the vibrational excitation cross section below 1.3 eV and (ii) very low energy thresholds of electronic excitation channels. These features provide a stringent test for the CSDA. Indeed, our results clearly show for the first time that the CSDA fails even qualitatively to reproduce the electron degradation spectrum given by the exact SF method over the whole energy regime studied.