The delayed absorption of microwaves due to electron thermalization in nanosecond pulse irradiated N2, He, and Ar at atmospheric pressure

Abstract

The absorption of microwaves in pulse irradiated C₂H₆ (ethane), N₂, He, and Ar at atmospheric pressure has been investigated on a nanosecond timescale. In the case of C₂H₆, a time of less than 1 nsec following the pulse is required for the absorption signal to reach a time independent plateau value. For N₂, He, and Ar, a considerable delay in the development of the absorption signal is observed. This increase in absorption coefficient over 20, 50, and 300 nsec in the case of N₂, He, and Ar, respectively, is attributed to a time dependence of the electron collision frequency resulting from post‐pulse thermalization of the electron energy. Using momentum transfer collision frequency data from swarm studies, the time dependence of the electron energy has been derived and is discussed in terms of the energy exchange rate coefficient K_u, defined by −d?/dt=K_u [?−(3/2) kT]N. For N₂, K_u is found to be almost independent of electron energy over the range kT_e=0.03–0.7 eV and has a mean value of 1.55×10⁻¹¹ cm³ sec⁻¹. For He, K_u increases from 0.25×10⁻¹¹ cm³ sec⁻¹ at kT_e?0.03 eV to 1.3×10⁻¹¹ cm³ sec⁻¹ at kT_e?1.5 eV. The results are compared with energy exchange parameters from other sources. A further growth of absorption signal over several hundred nanoseconds in He is attributed to Penning ionization of impurities.