COLLISIONAL TRANSFER OF EXCITATION IN HELIUM
Loading...
Date
1975
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Ohio State University
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.
Description
Present address of P. H. Wine: Institute for Chemical Sciences, University of Texas at Dallas, Richardson, Texas, 75080.""
Author Institution: Department of Chemistry, Florida State University
Author Institution: Department of Chemistry, Florida State University