Ultraviolet photoionization in CO2 TEA lasers

Abstract

The factors affecting spark ultraviolet (UV) emission and transmission, photoionization, and photoelectron loss mechanisms in CO2 TEA laser gas mixtures have been investigated and compared with the results of other workers. We found two different sources of UV radiation: N2 and an unidentified emitter, postulated to be associated with ionized electrode material; C (from CO and CO2) appeared to have little effect. UV emission was directly proportional to the amount of stored electrical energy in the spark-discharge circuit and to the cube of the peak current produced by the discharge of this energy. The uniformity of preionization produced by two kinds of spark-free space and notched surface guided was examined and the free-space spark was found to be superior. Photoionization was found to be due to the presence of low ionization potential (IP) alkenes in unseeded and fresh gas mixtures and also to NO and NO2 in sealed devices—these were formed as a result of spark plasma reactions. The nature of the photoionization process in NO and C3H6 (an alkene) was investigated; both were ionized in a single-step process. The absorption coefficient for ionizing radiation in CO2 was measured in a gas mixture containing 9% N2 and a N2 free mixture and values of 1.3 and 1.9 cm−1 atm−1 were obtained. It is thought that this may be due to changes in the spark UV emission spectrum when N2 is present. The loss of photoelectrons by third body attachment to NO, NO2, and O2 was monitored and only the attachment to O2 was found to be significant. However, the oxides of N can play a dominant role in the subsequent negative ion reactions on a millisecond timescale.