James K. Rice

Triplet states of acetylene by electron impact

Abstract Low-energy electron-impact spectroscopy has revealed two previously unknown low-lying triplet states in acetylene at 5.2 eV and 6.1 eV. The basis for this identification and the disparity in the electron energy-loss and optical absorption spectra are discussed.

Erratum: Electron Scattering by H2 with and without Vibrational Excitation. I. Quantum‐Mechanical Theory

A quantum‐mechanical model of elastic and inelastic electron scattering by a homonuclear diatomic molecule in its electronic ground state is presented. The model should be especially useful in the intermediate energy range (about 10–100 eV). It is applied to the calculation of differential and integral cross sections for elastic scattering and for excitation of the first, second, and third vibrational states of molecular hydrogen for impact energies in the 1–912‐eV range. The theory assumes plane waves for the scattering electron wavefunctions; it includes electron exchange effects by use of the Born–Ochkur–Rudge approximation, and it incorporates an electron–H2 interaction potential containing a semiempirical polarization potential and a static potential which includes a semiempirical quadrupole interaction. These potentials are adjusted to agree with available ab initio calculations of these potentials. The effects on the cross sections of electron exchange and the scattering by the various potential te...

Angular Dependence of Low‐Energy Electron‐Impact Excitation Cross Section of the Lowest Triplet States of H2

The differential cross sections for the electron-impact excitation of the lowest triplet states of molecular hydrogen (b3Sigmau+,a3Sigmag+) have been calculated from threshold to 85 eV impact energy using the Ochkur–Rudge theory. For the X1Sigmag+ --> b3Sigmau+ transition, the relative differential cross sections were measured with a low-energy, high-resolution electron-impact spectrometer from 10° to 80° scattering angle and impact energies of 25, 35, 40, 50, and 60 eV. Theory and experiment are in good agreement for the shape of the differential cross section for energies of 35 eV and above. However, at 25 eV, the theory continues to predict a rather well-developed maximum in the cross section at around 40° while the experimental cross sections are more isotropic. An appreciable contribution to the inelastic scattering in the energy loss region from 11 to 14 eV due to excitation to the a3Sigmag+ and/or c3Piu states is definitely established from the observed angular distributions. A quantitative evaluation of the individual angular behavior of the excitations in this region, however, would require a resolution higher than the presently available one of 0.030 eV.

The importance of polarization for electron scattering in the intermediate energy region

Abstract The angular dependence of electron scattering from the helium atom and the hydrogen molecule for small scattering angles in the 34 – 100 eV impact energy range is explained in terms of the first Born approximation and the polarized Born approximation. The theoretical results compare favorably with the experimental data for both elastic and inelastic scattering. New experimental and theoretical results are presented.

Electron scattering by H2 with and without vibrational excitation. III. Experimental and theoretical study of inelastic scattering

The ratios of the differential cross sections (DCSs) for excitation of the first, second, and third vibrational states of H2 in its ground electronic state to the elastic DCS have been measured as a function of scattering angle in the 10°–80° range and impact energy in the 7–81.6-eV range. From these ratios the DCSs corresponding to transitions from the ground to the first two vibrationally excited levels (fundamental and first overtone bands) were obtained by utilizing the elastic cross sections determined in the previous paper (II). In addition, the DCS for excitation of the second overtone band was determined for an impact energy of 10 eV. By angular extrapolation and integration of the DCSs the integral cross sections for the vibrational excitations were also determined. In addition, all these cross sections have been calculated using a quantum-mechanical method based on potential scattering in a plane wave scattering approximation which is described in Part I of this series. The present experimental and theoretical cross sections and previous measurements and calculations are compared. The calculated DCS ratios and the DCSs themselves for the fundamental excitation are in good agreement with experiment at 7 and 10 eV; however, at higher energies the calculated DCSs are generally larger than the experimental ones, at some angles by as much as a factor of 10. The calculated ratio of the DCS for the fundamental excitation to the elastic DCS shows a minimum as a function of angle, in qualitative agreement with the experimental results in the 13.6–81.6-eV energy range. The experimental DCSs for vibrational excitation also show a deep minimum. For excitation of the first overtone vibration, the experimental ratios are an order of magnitude larger than the calculated ones at low energy but in better agreement for the magnitude at higher energy. This discrepancy at low energies is explained in terms of resonance scattering. Our experiments are in good agreement with those of others in the few (low energy) cases where comparison is possible.

Differences in the Angular Dependencies of Spin- and Symmetry-Forbidden Excitation Cross Sections by Low-Energy Electron Impact Spectroscopy

Optically forbidden electronic transitions can be produced by low-energy electron impact. Recent experimental investigations of helium (1-3) have shown that the differential scattering cross sections for forbidden excitations are generally enhanced relative to those for allowed ones at low incident energies and large scattering angles. n nWe have now observed marked differences in the angular and energy dependencies of differential cross sections for various kinds of forbidden (spin, symmetry, or both) transitions in helium at low incident energies. Such differences may well provide a basis for determining the nature of optically forbidden transitions detected by electron-impact spectroscopy in other atoms and molecules.