Helium emission spectra—II. Pressures to 0.91 torr

Abstract

Electron impact excitation is employed to study the pressure variation of the steady state luminescence from 27 levels of helium. Apparent cross sections are determined for these levels over the pressure range 0.025–0.91 torr, a much broader pressure range for observing collisional excitation transfer processes than has been previously investigated. Model calculations are carried out in which the data is fit to a set of coupled steady state equations. Primary and secondary electron excitation, radiative transfer, and bimolecular and termolecular collisional processes are accounted for in the analysis. We find that n 1 P→ n 1 D collisional excitation transfer is much more important as a populating mechanism for n 1 D levels than has been previously believed. The cross section for 4 1 P→4 1 D collisional transfer is estimated to be (1.3±0.3)×10 −14 cm 2. A termolecular process, believed to involve formation of He 2 +, becomes an important loss mechanism for the 3 1 P and 4 1 P levels at pressures above 0.5 torr. Rate constants for this process are estimated to be (6.6±1.0)×10 −27 cm 6/sec and (1.9±0.4)×10 −26 cm 6/sec for 3 1 P and 4 1 P, respectively. The magnitude of the 3 3 D and 4 3 D apparent cross sections require a large 4 3 F population, a result which leads us to conclude that the 4 F level is singlet-triplet mixed. There is evidence that collision induced transitions between all sublevels do not occur with equal probability. The most internally consistent results are arrived at by assuming Δ J=0 to be a favored collisional transfer process.