Electron thermalization in nanosecond pulse-ionized dry and humid air

The process of electron thermalization in dry and humid air has been studied by monitoring the relaxation of the microwave conductivity following pulsed ionization. The energy exchange rate coefficient K/sub u/ for dry air is found to decrease from 8 x 10-11 cm3 s-1 immediately following the pulse to approximately 1.3 x 10-11 cm3 s-1 at close to thermal energies. Small amounts of water vapor result in a marked increase in the rate of electron thermalization. The thermalization time tau/sub th/ = t(T/sub e/ = 1.1 T) at 294 K is given by tau/sub th/ approx. =(1.3 x 105+2.2 x 108 F/sub H/2O)-1 s Torr with F/sub H/2O the fractional water content. The electron--ion recombination rate constant is found to increase from 3.1 x 10-7 cm3 s-1 at 2 Torr to 1.9 x 10-6 cm3 s-1 at 50 Torr for dry air and is approximately 1 x 10-6 cm3 s-1 for 1.1% H2O at 2 Torr total pressure