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The time behaviour of electron transport in RF fields in gases

A theoretical study of the time behaviour of the electron transport in an RF field, E exp(i omega t), has been executed for the electron nonconservative case in molecular gases from the Boltzmann equation. The velocity distribution is expanded by Legendre polynomials in velocity space and Fourier series in time. In this study, the collisional relaxation times for electron energy and momentum are introduced in order to understand the time-modulation of the distribution. The authors have investigated the influence of the reduced angular frequency of the field, omega /N, both on the energy distribution and on the swarm parameters over a wide range 2.827*10-12<or= omega /N<or=2.827*10-7 cm3 s-1 in CH4. The appearance of the spike-like component in the RF energy distribution is pointed out at low energy less than the vibrational threshold. It is noted that the remarkable characteristic is proper for a Ramsauer molecular gas, which has a large vibrational cross section comparable to the elastic momentum transfer cross section at low energy. It is also emphasised that a maximum in a swarm parameter, such as the ionisation rate, appears by changing omega /N at a constant value of the effective reduced field strength (E/ square root 2)/N.