COLLISIONAL TRANSFER OF EXCITATION IN HELIUM

Abstract

Relative apparent electron impact excitation cross sections have been determined for 27 levels of helium over a broad pressure range (0.02 - 0.91 Torr). Model calculations have been carried out where the data were fit to a set of coupled steady state equations which take into account primary and secondary electron excitation, radiative transfer, re-absorption of resonance radiation, and bimolecular and termolecular collisional processes. We find that collision Induced transitions of the type $n^{1}P \rightarrow n^{1}D$ are much more Important as populating mechanisms for $n^{1}D$ levels than had been previously believed and that the magnitude of the $4^{1}D$ apparent cross sections cannot be rationalised in terms of a model which assumes that transitions between all $(2L + 1) \times (2S + 1)$ sublevels are equally probable. A termolecular process, believed to involve formation of $He_{2}^{+}$, becomes an important loss mechanism for the longer lived n = 3 and 4 levels at pressures above 0.5 Torr. Rate constants for this process are estimated to be $(6.6 \pm 1.0) \times 10^{-27} cm^{6}$/sec and $(1.9 \pm 0.4) \times 10^{-27} cm^{6}$/sec for $3^{1}P$ and $4^{1}P$, respectively, the magnitude of the $3^{3}D$ and $4^{3}D$ apparent cross sections imply a large $4^{3}F$ population. This indicates that the 4F level is singlet-triplet mixed.