H(2s)formation inH+-H and H-H collisions

Abstract

Cross sections for metastable $\mathrm{H}\mathrm{}\left(2s\right)\mathrm{}$ formation by electron caputre in ${\mathrm{H}}^{+}-\mathrm{H}\mathrm{}\left(1s\right)\mathrm{}$ collisions and by excitation in $\mathrm{H}\mathrm{}\left(1s\right)\mathrm{}-\mathrm{H}\mathrm{}\left(1s\right)\mathrm{}$ collisions have been measured over the energy range 1.9-92 keV. A fast beam of ${\mathrm{H}}^{+}$ ions or $\mathrm{H}\mathrm{}\left(1s\right)\mathrm{}$ atoms was passed through a tungsten-tube-furnace target which contained thermally dissociated hydrogen. Fast metastable $\mathrm{H}\mathrm{}\left(2s\right)\mathrm{}$ atoms formed by collisions in the target were detected downstream using electric field quenching and Lyman-$\alpha$ photon-counting techniques. The present values are normalized at 24.5 keV to the average value of three previous independent measurements of the $\mathrm{H}\mathrm{}\left(2s\right)\mathrm{}$ formation cross section in ${\mathrm{H}}^{+}$-H collisions which agree to within 20%. Measured cross sections for both ${\mathrm{H}}^{+}$-H and H-H collisions contain only one maximum, in contrast with certain theoretical predictions. For ${\mathrm{H}}^{+}$-H the low-energy data are in good agreement with theoretical results based on a multistate molecular treatment of the collision. Above 75 keV the ${\mathrm{H}}^{+}$-H data agree with the Born approximation cross sections. Close-coupling pseudostate predictions lack overall detailed agreement with the present results, although the maximum in the cross section is reproduced well. High-energy ($E>\mathrm{}15\mathrm{}$ keV) coupled-state calculations using a scaled hydrogenic two-center expansion are in good accord with the data. For H-H collisions all theoretical treatments are in poor accord with the present experimental results. Above 40 keV the measured H-H cross section is inversely proportional to the impact energy. This $E^{-1\mathrm{}}$ energy dependence is in agreement with high-energy theoretical predictions. However, above 10 keV the Born approximation predicts structure in the cross section due to simultaneous excitation of the projectile and target which is not observed. The present results are compared with previous experimental determinations, and discrepancies are found to exist.