Near-threshold rotational and vibrational excitation of H2 by electron impact: Theory and experiment

A joint experimental-theoretical attack on low-energy e-H2 scattering is described. The cross sections calculated from a highly converged numerical solution of the nonrelativistic Schroedinger equation, using a parameter-free interaction potential, are first compared with results from swarm experiments, and are later used to improve the accuracy of the swarm analysis at energies above the first vibrational threshold. To provide further perspective, the theoretical results are compared with a variety of other experimental data. The theoretical results for the momentum-transfer and rotational-excitation cross sections are in excellent agreement with the results from swarm experiments, but there is an unresolved and significant difference in the threshold behavior of the vibrational-excitation cross sections. Both the theoretical and experimental approaches are subjected to close scrutiny in an attempt to uncover possible sources of error that could explain this difference. The failure to locate likely sources points to the need for further independent theoretical and experimental work to resolve a problem that has serious implications.