Negative glow and positive column discharges for helium/metal-vapour lasers-a comparison

The comparative abilities of the negative glow and positive column regions of one and the same helium/zinc discharge to tolerate an increasing density of the metal vapour have been investigated. Spatially resolved profiles of sidelight emission from particular upper laser levels of Zn II were recorded over a wide range of discharge conditions. Evidence that the helium ion production rate can be maintained in the negative glow at pressures of zinc vapour as high as 0.05-0.5 Torr is the undiminished intensity of Zn II levels known to be populated by heavy-body collision processes. The same transitions, excited in the positive column, show a rapid decline in their intensity at zinc vapour pressures above 0.005 Torr. The difference in behaviour is ascribed to the existence, independent of metal vapour density, of a significant tail of high energy electrons in the negative glow electron energy distribution. The factors which determine the optimum partial pressure in the two cases are compared.