Electron Scattering by H2 with and without Vibrational Excitation. III. Experimental and Theoretical Study of Inelastic Scattering

Abstract

The ratios of the differential cross sections (DCS's) 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 DCS's 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 DCS's 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 DCS's themselves for the fundamental excitation are in good agreement with experiment at 7 and 10 eV; however, at higher energies the calculated DCS's 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 DCS's 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.