Formation ofH(2p)andH(2s)in Collisions of 1-25-keV Hydrogen Atoms with the Rare Gases

Cross sections have been measured for the emission of Lyman-$\alpha$ radiation in collisions of 1-25-keV hydrogen atoms with He, Ne, Ar, Kr, and Xe owing to excitation of the fast atom to the $2p$ and $2s$ states. The intensity of Lyman-$\alpha$ emitted spontaneously from collisionally excited $\mathrm{H}\mathrm{}\left(2s\right)\mathrm{}$, was measured at 54.7 or 125.3° with respect to the H beam in an essentially field-free collision chamber with an oxygen-filtered photometer calibrated by reference to previous results for ${\mathrm{H}}^{+}$ on Ar. After a small correction for cascade effects, the data yield the cross sections for population of the $2p$ state in these collisions. An electric field, oriented in the direction of the beam, was applied within the collision chamber to quench collisionally excited $\mathrm{H}\mathrm{}\left(2s\right)\mathrm{}$, and the resulting increase in Lyman-$\alpha$ radiation has been used to derive the cross sections for formation of the $2s$ state. The viewing geometry and the electric field orientation were designed to minimize the effects of polarization of the light emitted from $\mathrm{H}\mathrm{}\left(2p\right)\mathrm{}$ and $\mathrm{H}\mathrm{}\left(2s\right)\mathrm{}$, and of changes in polarization when the quenching electric field was applied that might otherwise lead to large errors in the $2s$ cross section. The cross sections increase in magnitude and degree of structure with increasing atomic number of the target. Except for collisions with Ne, excitation of $\mathrm{H}\mathrm{}\left(2p\right)\mathrm{}$ is always more probable than excitation of $\mathrm{H}\mathrm{}\left(2s\right)\mathrm{}$. A relationship between the emission and stripping cross sections is presented, the relative probabilities of excitation and stripping being rather independent of target. Our results are compared with previous measurements made at higher energies and with theoretical estimates of these cross sections. The present extension of previous results to lower energy has revealed new structure in the energy dependence of the cross sections for the heavier targets. Existing theoretical treatments of these collisions fail to yield either the magnitude or energy dependence of the cross sections at energies below 5 keV.