Identification of the origins of photoionisation in CO₂ TEA lasers

Abstract

In sealed CO₂ spark preionised lasers the preionisation is largely due to photoionisation of NO and NO₂, in seeded TEA lasers it originates from the low ionisation potential additive used. Unseeded and flowing gas lasers can still be successfully preionised but the source of this preionisation has remained a mystery; previous attempts to isolate and identify low I.P. gaseous impurities have failed. We have now identified these, using a combination of cryogenic impurity concentration and mass spectroscopy and found them to be a complex mixture of hydrocarbons (C₂−C₇). Of these hydrocarbons, the alkenes are found to be predominantly responsible for the photoionisation and are present in concentrations of ∼ 0.5 ppm. Deliberate addition of one of these alkenes, propene, to a uv preionised CO₂ TEA laser was found to enhance the lasers performance at high energy loadings.