Effects of Lasering upon the Electron Gas and Excited-State Populations in Xenon Discharges

Abstract

Spatially resolved electron density measurements are reported for dc-excited xenon laser discharges over a pressure range of 12.5 to 25.5 mTorr. Due to electrophoretic effects within the closed capillary tube, anode-directed gradients in electron density are established which affect local population inversions. The influence of lasering upon these discharges is investigated experimentally by using a 3.51-μ (5d33−6p22) laser whose optical cavity Q is periodically spoiled by a mechanical chopping wheel. It is observed that lasering noticeably alters the state of the electron gas and the populations of excited xenon atoms. The discharge current is decreased due to lasering by up to 0.02%, and spatially resolved microwave cavity measurements reveal that lasering may either increase or decrease the electron density by approximately 109 cm−3 depending upon local discharge conditions. Laser-induced changes in the microwave cavity Q and the electron drift velocity indicate that lasering also affects the mean electron energy. Ionization via electron impact from the upper laser level and the xenon metastables is suggested as the dominant physical process responsible. Confirming spectroscopic observation of laser-induced changes in the spontaneous radiation are presented. It is demonstrated that excitation from the xenon metastables populated the 6p and 6p′ levels, degrading the laser population inversion and significantly decreasing the 3.51-μ laser output power. Radiative and collisional mechanisms responsible for the propagation of the laser-induced perturbation from the 5d33 and 6p22 levels to other excited levels are also discussed. It is suggested that laser media in general can be analyzed and improved upon by studying the laser-induced changes in excitation and relaxation processes.