Relaxation of the electron energy distribution function in the afterglow of a N-2 microwave discharge including space-charge field effects - art. no.046404

The relaxation of the electron energy distribution function (EEDF) in the p ost-discharge of an omega/(2 pi) =2.45 GHz microwave discharge in N-2 has b een investigated by solving the time-dependent Boltzmann equation, includin g a term taking into account electron losses by diffusion under the presenc e of a space-charge held. 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 e lectron density n(e)(t), the electron transport parameters, and the rate co efficients for some processes induced by electron impact, with energy thres holds typically smaller than similar to2-3 eV, such as, e.g., stepwise exci tation of N-2(B (3)Pi (g)) and N-2(C (3)Pi (u)) states from N-2(A (3)Sigma (+)(u)) metastables and excitation of N-2(X (1)Sigma (+)(g), v) levels, are only slowly modified in the time interval t similar to 10(-7)-3x10(-4) s d ue to the large characteristic times for ambipolar diffusion. As a result o f modifications in n(e)(t), the change from ambipolar to free diffusion reg imes occurs abruptly at t similar to 3x10(-4) and similar to 10(-3) s for p =2 and 10 Torr, respectively.