Formation of Fast Excited H Atoms. II. Charge-Transfer Neutralization of H+ on Molecular Gases

An experimental study is made of the cross sections for forming fast hydrogen atoms in the $3s$, $3p$, and $3d$ states by charge-transfer neutralizations of ${\mathrm{H}}^{+}$ as it traverses molecular targets. The formation of excited hydrogen is detected by a quantitative measurement of collisionally induced Balmer-$\ensuremath{\alpha}$ emission; the contributions from the $3s$, $3p$, and $3d$ levels are separated by a method that utilizes the different lifetime of the excited states. Proton-impact energies range from 75 to 700 keV; targets include ${\mathrm{H}}_{2}$, ${\mathrm{N}}_{2}$, NO, ${\mathrm{O}}_{2}$, CO, C${\mathrm{O}}_{2}$, C${\mathrm{H}}_{4}$, ${\mathrm{C}}_{2}$${\mathrm{H}}_{4}$, ${\mathrm{C}}_{2}$${\mathrm{H}}_{6}$, and ${\mathrm{C}}_{3}$${\mathrm{H}}_{3}$. Cross sections decrease rapidly with impact energy; the $3s$ cross section was always largest followed by the $3p$ and $3d$. There was no convincing evidence for a general additive rule whereby cross sections could be assigned to the individual constituent atoms of the molecule and then used to predict cross sections for complex molecules.