Relaxation of the electron energy distribution function in the afterglow of a N2 microwave discharge including space-charge field effects.

The relaxation of the electron energy distribution function (EEDF) in the post-discharge of an omega/(2pi)=2.45 GHz microwave discharge in N2 has been investigated by solving the time-dependent Boltzmann equation, including a term taking into account electron losses by diffusion under the presence of a space-charge field. It is shown that although the high-energy tail of the EEDF is rapidly depleted in times of 10(-7) s (p=2 and 10 Torr), the electron density n(e)(t), the electron transport parameters, and the rate coefficients for some processes induced by electron impact, with energy thresholds typically smaller than approximately 2-3 eV, such as, e.g., stepwise excitation of N2(B (3)Pi(g)) and N2(C (3)Pi(u)) states from N2(A (3)Sigma(+)(u)) metastables and excitation of N2(X (1)Sigma(+)(g),v) levels, are only slowly modified in the time interval t approximately 10(-7)-3 x 10(-4) s due to the large characteristic times for ambipolar diffusion. As a result of modifications in n(e)(t), the change from ambipolar to free diffusion regimes occurs abruptly at t approximately 3 x 10(-4) and approximately 10(-3) s for p=2 and 10 Torr, respectively.