Metastable hydrogen atom scattering by crossed molecular beams: Total cross-sections for H(2s)-Kr, O2, and Cl2 at thermal energies

The velocity dependence of total integral cross-sections for collisions of H(2s) atoms with Kr, O2 and Cl2 has been measured, in the velocity range from 4 to 40 km/s, in a crossed molecular beam apparatus. The detector acceptance angle was well below the minimum critical angle, allowing a comparison with quantum mechanical cross-section calculations. The interaction is modeled in terms of two intermolecular potentials of Sigma symmetry, correlating asymptotically with 2s and 2p states of hydrogen atom and the cross-sections have been calculated within a two-state resonance model. It has been found that the cross-sections and their velocity dependences are a probe of the long-range van der Waals portion of both the Sigma symmetry potentials.