Spark photoionization of CO2 laser gases

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Abstract
An experimental investigation at atmospheric pressure of volume photoionization produced by bare sparks in He, CO2, and N2 gas mixtures containing tripropylamine is reported. Measurements were made using a probe and a pair of contoured collector electrodes with an applied dc potential. The variation of photoelectron density with gas mixture, tripropylamine addition, and uv source (spark) energy was investigated. Single‐step photoionization of tripropylamine vapor was found to be the dominant ionization mechanism in this experiment. The addition of moderate amounts of tripropylamine vapor to gas mixtures containing CO2 resulted in an increase of photoelectron density by more than an order of magnitude. The effects of gas mixture variation and tripropylamine addition on photoelectron temporal decay was studied and decay rates measured. The addition of tripropylamine to the gas mixtures was found to significantly increase electron decay rates. Measurements with a LiF window and a fused‐quartz window were used to estimate the photoionization contributions from different portions of the spark spectrum. Short‐wavelength radiation below the LiF cutoff appeared to be much more efficient for photoionization of the tripropylamine than that above the LiF cutoff. Radiation above the fused‐quartz cutoff made no measurable contribution to the photoionization.

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