Optogalvanic effect sign change in a hollow cathode discharge plasma

The use of diode lasers in research and technology is rapidly expanding due to their high efficiency, low cost and simplicity. Low-power diode lasers in the visible have been successfully used for investigation of the optogalvanic profiles of Ne and He transitions in hollow cathode discharge plasma. In this work the spatial distribution of the optogalvanic signal (OGS) in Ne at 667.8 nm has been investigated in longitudinal and transverse hollow cathode discharges. It was observed that while in the longitudinal configuration the OGS at the center of the tube changes its sign from positive to negative with pressure increase, in the transverse configuration the OGS is positive at all pressures studied. A simple model is proposed for explanation of the optogalvanic signal sign change. The ions in the discharge are assumed to be produced by electron excitation from the Ne ground state and the Ne metastabte states. The ion loss is assumed to be a result of diffusion out of the discharge volume and three-body recombination of ions. Consideration of the rate equations shows that when diffusion is the dominant loss, the OGS is positive; if recombination becomes the dominant loss, this could lead to negative optogalvanic effect. (C) 2000 Elsevier Science Ltd. All rights reserved.