Formation of Excited States in a Helium Target by the Impact of 0.15- to 1.0-MeV Protons and Deuterons. I. Experimental

An optical technique has been used to determine the absolute cross sections for the excitation of various $n^{1}S$, $n^{1}P$, and $n^{1}D$ states of neutral helium by the impact of 0.15- to 1-MeV protons on a low-pressure gaseous helium target. The emission resulting from the radiative decay of collisionally excited atoms is analyzed using an optical spectrometer, and detected by a low-noise photomultiplier. The absolute photon-detection efficiency of the system is determined with the aid of a tungsten-strip-filament lamp of known emissive power. It is shown how the rate of emission of photons may be related to the cross section for exciting the parent level in terms of known transition probabilities. Cross sections for the formation of various singlet excited states of helium by proton bombardment are presented as a function of projectile energy. Deuterons were used to extend the effective velocity range of the data in selected cases. The cross sections for the formation of the ${}_{}{}^1{}_{}{}^{}S$ and ${}_{}{}^1{}_{}{}^{}D$ states by excitation from the ground state show a rapid decrease with increasing projectile velocity. The ${}_{}{}^1{}_{}{}^{}P$-state excitation-cross sections peak at about 120 keV and fall off slowly with further increase of projectile energy. Data are also presented for the emission of the He II (4 → 3) transition induced by proton impact. Systematic discrepancies between previously published sets of data are identified as being caused by differences in the calibration of detector sensitivity. In a companion paper the present data are compared with theoretical predictions.