Electron Capture into the $n=2\mathrm{}$ States of Hydrogen by Fast Proton Impact on Gases

Cross sections for electron capture into the $2s$ and $2p$ states of atomic hydrogen have been measured for proton impact on He, Ne, Ar, ${\mathrm{N}}_2$, and ${\mathrm{O}}_2$. In addition, capture into the $2s$ state has been measured for proton impact on ${\mathrm{H}}_2$. Generally, these measurements have been carried out in the energy range from 20 to 130 keV. Absolute calibration of the helium-filled iodine Lyman-alpha counter was obtained by a normalization procedure involving previous $2p$ capture measurements for impact on He, Ne, and Ar, after an investigation of the detector response for electron impact on ${\mathrm{H}}_2$. Calibration for the $2s$ capture was established by determining the $2s$ cross section relative to the $2p$ cross section for 20- and 30-keV impact on Ne. The polarization effect in the Stark-quenched $2s$ radiation was minimized by observing the Stark-induced radiation at 54.7° to the electric field direction. The $2s$ cross sections relative to the previously measured $3s$ cross section show a remarkable (about 10%) agreement with the $n^{-3\mathrm{}}$ law (predicted at high energies) above 20 keV for all gases, excluding He. It appears that the $n^{-3\mathrm{}}$ behavior sets in at about 100 keV for He. In general the $2s$ cross section tends to be somewhat higher than predicted by the $n^{-3\mathrm{}}$ law for impact on He in the range from 25 to 100 keV. It appears that the $2s$ cross sections will agree with the coupled-state calculation of Sin Fai Lam for impact on He above 100 keV. The $2p$ cross sections are lower than those predicted by theory for impact on He in this energy range. The $2p$ cross sections relative to the previously measured $3p$ cross sections appear to roughly follow an $n^{-3\mathrm{}}$ behavior at the higher energies, again with the possible exception of helium. Comparisons are made with the results of other experimental investigations.