The function for the excitation of the helium 31P level by electron impact has been determined optically. Light intensity of the 31P → 21S transition (5016 A) was detected by a sensitive photomultiplier; its output was measured by a microammeter.
The intensity of the 5016 A line was proportional to electron beam current for all currents used and proportional to helium pressure for less than 3 × 10-4 mm. Above 3 × 10-4 mm pressure imprisonment of resonance radiation enhanced the light intensity.
The excitation function has a peak value of 2.4 × 10-18 cm2; this occurs at an electron energy of 100 ev. Absolute determination of the cross section was possible through the calibration of the light detection system by a tungsten ribbon standard lamp. The experimental curve is corrected by removing cascade components from it.
ROBERT M. ST. JOHN, CHARLES J. BRONCO, and R. G. FOWLER, "Excitation Function of Helium 31P for Electron Collisions," J. Opt. Soc. Am. 50, 28-31 (1960)