Excitation and ionization-excitation of helium by fast electron and proton impact: extreme ultraviolet (EUV) cross section measurements

Cross sections have been measured in the extreme ultraviolet for the excitation of helium, He I states (lsnp) 1Po, and the ionization-excitation of helium, He II states (np) 2Po, due to fast proton and electron impact for n = 2-5. Protons with velocities from 1.4-7.5 a.u. and electrons with velocities from 3.8-8.5 a.u. were used to excite helium gas in a differentially pumped target cell. A 2 MV Van de Graaff accelerator and an electron gun mounted in the target chamber provided the projectile beams, and a grazing incidence monochromator was used to measure the EUV radiation emitted by the target. A unique application of the Bethe approximation has been used to put the excitation results on an absolute scale. Cross sections for ionization-excitation from electron impact are substantially larger than those from proton impact above a velocity of 4 a.u., the difference reaching a maximum at approximately 6.5-7.0 a.u. for all principle quantum numbers. Cross section ratios for electron and proton impact, σ+(np)/σ+ (lsnp) are compared as a function of projectile velocity and are compared with the ratio of double to single ionization σ++/σ+ as a limiting value for large n