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

Abstract

The differential cross sections for the electron‐impact excitation of the lowest triplet states of molecular hydrogen ${\mathrm{(b}}^3{\Sigma }_u^{+}{\mathrm{,\; a}}^3{\Sigma }_g^{+})$ have been calculated from threshold to 85 eV impact energy using the Ochkur–Rudge theory. For the $X^1{\Sigma }_g^{+}{\mathrm{\hspace{0.17em}\to \hspace{0.17em}b}}^3{\Sigma }_u^{+}$ 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 $a^3{\Sigma }_g^{+}$ and/or $c^3{\Pi }_u$ 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.