Angular differential cross sections for the excitation of $1^1$S helium to the $2^1$S and $2^1$P states by 25- to 100-keV-proton impact

Angular differential cross sections for the proton-impact excitation of ground-state helium ${(1\mathrm{}}^1$S) to the $2^1$S and $2^1$P states have been measured for the first time in the energy range 25 to 100 keV with use of the energy-loss technique. The data indicate that, for very small scattering angles, at 25 keV the $2^1$S differential cross section is greater than the $2^1$P differential cross section. For impact energies greater than 50 keV, the $2^1$P differential cross section clearly dominates over the $2^1$S cross section in the very small scattering angle region. The present data have been numerically summed and integrated to compare with previous absolute experimental measurements on related processes. These are in very good agreement with the present results. An eight-state impact-parameter calculation incorporating the electron-capture channel was performed and resulted in the best agreement with the experimentally determined differential cross sections.