Excitation of helium by protons in the 1–150 keV region

Abstract

Absolute cross sections have been determined for excitation of helium from the ground state to n 1 S, n 1 D( n = 3, 4, 5, 6) and n 1 P ( n = 3, 4, 5) states by the impact of protons in the 1–150 keV energy region. Low target gas pressures were used so as to avoid secondary processes. The cross sections show a similar dependence on impact energy for different levels of a spectral series except at low energies. Generally more then one maximum is found. Our experimental results are compared with those of other groups, which have measured up to 1000 keV, and with Born and distortion calculations. For 1P levels the Born approximation starts to hold at about 300 keV. For 3 1P the distortion approximation agrees well with experiment above 30 keV. For 1S and 1D levels there is a large discrepancy between the experimental absolute cross sections of different groups. In the comparison with the Born approximation, when we consider the relative energy dependence of the cross section, the experimental cross sections become larger than the Born ones below an impact energy of about 1000 keV. The polarization of the collision induced radiation has been determined for 1P− 1S and 1D− 1P transitions. A maximum value of the polarization is found at an impact energy for which the excitation cross section also displays a maximum. The experimental polarization differs from theory.