Knowledge Bank

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 $n1P\to n1D$ are much more Important as populating mechanisms for $n1D$ levels than had been previously believed and that the magnitude of the $41D$ 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 $He2+$, 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 $(6.6\pm 1.0)\times 10-27cm6$/sec and $(1.9\pm 0.4)\times 10-27cm6$/sec for $31P$ and $41P$, respectively, the magnitude of the $33D$ and $43D$ apparent cross sections imply a large $43F$ population. This indicates that the 4F level is singlet-triplet mixed.