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

The metastable Xe(6 3 P 2 ) and resonance Xe(6 3 P 1 ) particle number densities have been studied in the positive column of a pulsed glow discharge in a mixture composed of He at pressure p He = 2.5 Torr and Xe at p xe = 0.05 Torr. Time-resolved measurements were performed in the active discharge phase and in the afterglow with a laser absorption technique. The non-stationary balance equation system for populations of excited states, electron density and temperature has been solved to describe the decay of the Xe(6 3 P 2 ) state density during 1000 µs and that of Xe(6 3 P 1 ) state during 40 µs in the afterglow. If only cooling of electrons in elastic collisions is considered then a fast decrease of the electron temperature to that of the neutral gas within 10 µs and a slow decay of the excited atoms, which does not correspond to the experimentally observed decay curves, are obtained. The population and decay of excited states are to a large extend influenced by the intermixing process due to electron collisions, with a rate coefficient exponentially depending on the electron temperature in the range from 300 to 1000 K. Additional heating of slow electrons in collisions with fast ones appearing in chemo-ionization and superelastic collisions maintains the electron 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 of electron temperature in the afterglow was analysed taking the heating mechanism of slow electrons into account. The experimental curves of the decay of metastable and resonance atoms at discharge currents of 20-200 mA were interpreted.