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.
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