Photoelectron-initiated avalanches can be used to enhance ionization and dissociation, monitor surface properties, and test models of low-pressure glow discharges. In this work we explore the effects that discharge power and frequency have on electron avalanching and find that lower-powered discharges exhibit inherently larger gain. Transient changes in both current (optogalvanic effect) and optical emission intensity are enhanced three- to fivefold relative to higher-powered discharges. Monte Carlo simulations show that sheath thickness, and not voltage, is the primary parameter that determines the extent of avalanching and current gain. A self-consistent single-beam fluid model shows that optogalvanic oscillations are produced by overcompensation of the plasma potential in releasing excess negative charge produced by photoemission at the cathode. The beam model is in good qualitative but only fair quantitative agreement with experimental observations because of implicit assumptions about electron scattering. Multibeam and hybrid particle-fluid codes should provide a better quantitative description. For materials-processing applications these results imply that photoinitiated avalanches are best used in enhancing low-powered discharges. Similarly, reactive surfaces are most sensitively monitored in situ using optogalvanic detection of photoemitted electrons in weak discharges. We also find the photoelectric yield sensitive to discharge frequency when photon energies near the excitation threshold are used. This effect is attributed to surface charging that shifts the photoelectric threshold energy.
All Science Journal Classification (ASJC) codes
- Atomic and Molecular Physics, and Optics