Energy Transfer Kinetics of Vibrationally Excited Molecules

Abstract

Vibrational and electronic excitation of nitric oxide is studied experimentally using optical pumping by a CO laser. A mixture of nitric oxide and argon diluent is vibrationally excited in an optical absorption cell, by resonance absorption of CO laser radiation operating on a single line, in near resonance with one of NO(v=0 → v=1) fundamental band absorption transitions. Higher NO vibrational levels, not directly accessible to laser excitation, are populated by collisional vibration-vibration (V-V) energy exchange processes. Steady-state vibrational level populations and translational-rotational temperature in the cell are measured by Fourier transform infrared emission spectroscopy. At steady state, vibrational levels up to v~10 are populated. Steady-state vibrational temperatures up to Tv=3000-4500 K are maintained at low translational-rotational temperatures of T=330-440 K. At these conditions, UV emission (NO β and γ bands) is detected from the optically pumped cell, both at steady state and during pulsed laser excitation using a mechanical chopper to interrupt the laser beam. UV emission delay time is measured during pulsed excitation, relative to the laser pulse rise time. The results provide insight into kinetics of vibration-to-electronic (V-E) energy transfer in nitric oxide. Similar optical pumping technique can be used to study kinetics of dissociation of diatomic molecules at the conditions of extreme vibrational and electronic disequilibrium.