Formation ofH(2p)andH(2s)in Collisions of Protons and Hydrogen Atoms with Hydrogen Molecules

Abstract

Cross sections for emission of Lyman-$\alpha$ radiation owing to formation of $\mathrm{H}\mathrm{}\left(2p\right)\mathrm{}$ and $\mathrm{H}\mathrm{}\left(2s\right)\mathrm{}$ in collisions of 5-25-keV protons and hydrogen atoms with molecular hydrogen have been determined. The intensity of Lyman-$\alpha$ emitted spontaneously from $\mathrm{H}\mathrm{}\left(2p\right)\mathrm{}$ was measured at 54.7° or 125.3° with respect to the projectile beam in an essentially field-free collision chamber with an oxygen-filtered photometer calibrated by reference to previous results for ${\mathrm{H}}^{+}$ on Ar. The narrow bandwidth of the oxygen filter allowed separation of the Doppler-shifted Lyman-$\alpha$, emitted by $\mathrm{H}\mathrm{}\left(2p\right)\mathrm{}$ formed in electron capture by fast incident protons or collisional excitation of fast incoming hydrogen atoms, and the virtually unshifted Lyman-$\alpha$ emitted by the far slower $\mathrm{H}\mathrm{}\left(2p\right)\mathrm{}$ produced in dissociative excitation of ${\mathrm{H}}_2$. The increase in intensity of Lyman-$\alpha$ when an electric field, oriented in the direction of the beam, was applied within the collision chamber has been used to derive separate cross sections for formation of $\mathrm{H}\mathrm{}\left(2s\right)\mathrm{}$ owing to projectile and dissociative excitation. The experimental configuration was designed to minimize the effect of polarization of the light emitted from $\mathrm{H}\mathrm{}\left(2p\right)\mathrm{}$ and $\mathrm{H}\mathrm{}\left(2s\right)\mathrm{}$, and, after small corrections for cascade effects, the data yield cross sections for population of these states. The total cross section for formation of $\mathrm{H}\mathrm{}(n=2\mathrm{})\mathrm{}$ via projectile excitation exceeds that for dissociative excitation in either ${\mathrm{H}}^{+}$ or H impact. ${\mathrm{H}}^{+}$ is generally more efficient than H in the production of Lyman-$\alpha$ at low energies in projectile excitation and at all energies in dissociative excitation, and the cross section for formation of $\mathrm{H}\mathrm{}\left(2p\right)\mathrm{}$ usually exceeds that for $\mathrm{H}\mathrm{}\left(2s\right)\mathrm{}$. Scaling relationships from limiting high-energy-scattering theory are found to have only moderate success in relating our results to previous measurements involving excitation of H to the $n=3\mathrm{} \mathrm{and} 4$ states in projectile excitation and as well as in dissociative excitation by other heavy particles.