Microwave radiometric investigation of vibrational relaxation in low pressure nitrogen discharges

A novel technique based on the coupling between electrons and molecular vibration levels has been used to investigate the temporal buildup of vibrational energy during the active period of pulsed N₂ discharges. The radiometric technique monitors the decay of the electron average energy Ū_e in the afterglow of a discharge excited by voltage pulses of variable duration. Since the electron distribution function in the N₂ afterglow is often non‐Maxwellian, Ū_e is determined by first calculating the electron velocity distribution function from microwave measurements of the collisional broadening of the radiation temperature of electron cyclotron emission. If the quasi steady state value of ${Ū}_e\text{[⪞]\hspace{0.17em}0.5\hspace{0.17em}}\mathrm{eV}$ in the afterglow, the electrons are essentially in thermal equilibrium with the molecular vibrational levels. Thus, the measured effective electron temperature can be taken as a direct measure of the vibrational temperature achieved in the active period. Results of this first low pressure study yield an effective electron superelastic de‐excitation rate between 6 and 7 × 10⁻⁹ cm³/sec, a room temperature surface catalytic efficiency of ≈ 10⁻² for deactivation at Vycor walls with pressure in the neighborhood of 1 Torr, and a normalized effective collision frequency for vibrational excitation by electrons of ≃ 10⁻⁸ cm³/sec.