Resonances and their effects above and below the electron-impact ionization threshold of H2

Abstract

Two types of spectra obtained with low energy bombarding electrons are presented which have an energy resolution of 40 meV (FWHM). The energy loss spectrum obtained in the forward direction (10 degrees ) with E_i=25 eV electrons shows a series of unresolved peaks that agree well with the Rydberg states D,D',D",... and B,B',B",... and especially with photoionization data. Complementing this, the threshold excitation spectrum of H₂ shows a simpler structure which agrees well with the following series of triplet states c³ Pi _u, e³ Sigma _u⁺, g³ Sigma _g⁺, d³ Pi _u, k³ Pi _u and n³ Pi _u. It was observed that most of these states have a very large and sharp excitation cross section at threshold which has its origin in several series of resonances lying within 30 meV of these triplets. The angular behaviour of the threshold excitation spectra is discussed and absolute differential cross sections are reported for the c³ Pi _u (v=1) states as a typical example.