Metastable-hydrogen-atom scattering by crossed beams: Total cross sections for H*(2s)-Ar, Xe, and CCl4 at thermal energies

The collision velocity dependence of total (elastic plus radiative quenching) cross sections for metastable 2s state H atoms with Ar, Xe, and CCl4 has been measured in a crossed-beam experiment, in the velocity range between 4 and 40 km/s. Cross sections are obtained in well-controlled velocity and angular resolution conditions, allowing a meaningful comparison with theoretical calculations. The calculated cross sections, based on estimated potentials, are in good agreement with the experimental results and indicate that collisions in the present velocity range essentially probe the long-range van der Waals portion of both the H*(2s) and H*(2p) potentials with the target. Simple considerations indicate that previous low-resolution experiments underestimated the elastic component.