Cross section for production of the metastable H(2S) state in proton collisions with atomic hydrogen. [Energy range 6 to 25 keV]

Cross sections for production of the metastable H(2S) state in collisions of 6--25-keV protons with atomic hydrogen have been measured in a modulated crossed-beam experiment. The metastable states excited through the process H/sup +/ + H ..-->.. H/sup +/ + H(2S) (direct excitation) and H/sup +/ + H ..-->.. H(2S) + H/sup +/ (charge transfer) were quenched in a weak electric field applied at the collision region. The emitted Lyman-..cap alpha.. radiation was detected in a direction perpendicular to the plane of the crossed beams. The processes were separable since the direct excitation signals saturated well before the charge transfer signals did as the quench field was increased. The present values were normalized to the absolute total cross section for H(2P) production measured by Kondow et al. The charge-transfer cross sections are in satisfactory agreement with the measurements of Morgan et al. and Bayfield except at 6 keV, where their values also begin to differ. The direct-excitation cross section differs in shape from the only other measurement of Morgan et al., but agrees in absolute magnitude when normalization uncertainties are included. The coupled-state calculations of Cheshire et al. using a pseudostate expansion show good agreement with the present results, indicatingmore » the importance of the inclusion of molecular features at small internuclear separations.« less