Electron-impact vibrational relaxation in high-temperature nitrogen

Abstract

Vibrational relaxation process of N2 molecules by electron-impact is examined for the future planetary entry environments. Multiple-quantum transitions from excited states to higher/lower states are considered for the electronic ground state of the nitrogen molecule N2(X1 S+). Vibrational excitation and de-excitation rate coefficients obtained by computational quantum chemistry are incorporated into the diffusion model to evaluate the time variations of Vibrational number densities of each energy state and total vibrational energy. Results show a nonBoltzmann distribution of number densities at the earlier stage of relaxation, which in turn suppresses the equilibration process, but affects little on the time variation of total vibrational energy. An approximate rate equation and a corresponding relaxation time from the excited states, compatible with the system of flow conservation equations, are derived for the first time. The relaxation time from the excited states indicates the weak dependency of the initial vibrational temperature, and is shorter than the previously obtained relaxation time in which only excitation from the ground state was considered. The empirical curve-fit formulas for the improved e-V relaxation time is obtained. The rate equation and the relaxation time, suited for the numerical simulation of the highly ionized planetary entry flowfields, are suggested.