Investigation of resonance and metastable atoms in the afterglow of aHe-Xe positive column plasma

Abstract

The metastable Xe(6 3P2) and resonance Xe(6 3P1) particle number densities have been studied in the positive column of apulsed glow discharge in a mixture composed of He at pressure pHe = 2.5 Torr and Xe at pxe = 0.05 Torr. Time-resolved measurementswere performed in the active discharge phase and in the afterglow with a laserabsorption technique. The non-stationary balance equation system forpopulations of excited states, electron density and temperature has beensolved to describe the decay of the Xe(6 3P2) state density during1000 µs and that of Xe(6 3P1) state during 40 µs in theafterglow. If only cooling of electrons in elastic collisions is consideredthen a fast decrease of the electron temperature to that of the neutral gaswithin 10 µs and a slow decay of the excited atoms, which does notcorrespond to the experimentally observed decay curves, are obtained. Thepopulation and decay of excited states are to a large extend influenced by theintermixing process due to electron collisions, with a rate coefficientexponentially depending on the electron temperature in the range from 300 to1000 K. Additional heating of slow electrons in collisions with fast onesappearing in chemo-ionization and superelastic collisions maintains theelectron temperature at a level of about 1000 K over hundreds of microseconds.This leads to a faster decay of the metastable atoms. The temporal course ofelectron temperature in the afterglow was analysed taking the heatingmechanism of slow electrons into account. The experimental curves of the decayof metastable and resonance atoms at discharge currents of 20-200 mA wereinterpreted.