Time-dependence of the electron energy distribution function in the nitrogen afterglow

In this paper, we present an investigation of the time-relaxation of the electron energy distribution function (EEDF) in the nitrogen afterglow of an ω/2π=433 MHz flowing discharge at p=3.3 torr, in a tube with inner radius R=1.9 cm. We solve the time-dependent Boltzmann equation, including the term for creation of new electrons in associative/Penning reactions, coupled to a system of rate balance equations for the heavy-particles. The EEDFs are also obtained experimentally, from second derivatives of digitized probe characteristics measured using a triple probe technique, and compared with the calculations. It is shown that an equilibrium between the vibrational distribution function of ground-state molecules N₂(X¹Σ_g⁺,v) and low-energy electrons is rapidly established, in times ∼10^-7 s. In these early instants of the postdischarge, a dip is formed in the EEDF around 4 eV. The EEDF finally reaches a quasi-stationary state for t≳10^-6 s, although the electron density still continues to decrease beyond this instant. Collisions of highly excited N₂(X¹Σ_g⁺,v≳35) molecules with N(⁴S) atoms are in the origin of a maximum in the electron density occurring downstream from the discharge at ≃2×10^-2 s. These reactions create locally the metastable states N₂(A³Σ_u⁺) and N₂(a´¹Σ_u^-), which in turn ionize the gas in associative/Penning processes. Slow electrons remain for very long times in the postdischarge and can be involved in electron stepwise processes with energy thresholds smaller than ∼2-3 eV.