Vibrational Excitation ofH2by Low-Energy Electrons

Abstract

The $v=0\mathrm{}\to 1$ vibrational cross section for ${\mathrm{H}}_2$ derived from swarm experiments is shown to be a good approximation to the sum of the cross sections for vibrational transitions that take place with and without simultaneous rotational excitation of the molecule. Experiments are described that prove that this summed cross section does not depend on whether the ${\mathrm{H}}_2$ molecules are in the $J=0\mathrm{}$ or the $J=1\mathrm{}$ rotational state, in agreement with a recent theoretical calculation. Reasons are given for the disagreement between the two published swarm-derived vibrational cross sections. However, the discrepancies between the threshold behavior of the cross sections determined by the swarm measurements, beam experiments, and theory have not yet been explained.