Optical pumping studies of vibrational energy transfer in high-pressure diatomic gases

Abstract

Spontaneous Raman scattering is used to experimentally determine the vibrational distribution functions of diatomic species in N2/CO and N2/CO/O2 gas mixtures optically pumped by a CO laser in the pressure range 410–760 torr. In N2/CO mixtures, as many as 38 vibrational levels of CO are observed, in addition to six levels of N2. The CO vibrational distribution function is highly non-Boltzmann, exhibiting the well-known Treanor plateau. In N2/CO/O2 mixtures, up to 13 vibrational levels of O2 are observed, which also exhibit a highly non-Boltzmann distribution. Experimental data are compared to predictions of a master equation kinetic model, which incorporates absorption of the laser radiation, species, and quantum state-specific vibration–vibration and vibration–translation energy exchange, as well as diffusion of vibrationally excited species out of the laser-excited volume. It is shown for the first time that modest power continuous wave lasers can be used to establish highly excited steady-state vibrational distributions of all three major diatomic species in CO-seeded atmospheric pressure dry air. This has implications for the energy-efficient creation of low-temperature, high-pressure air plasmas, in which the principal free electron loss mechanism is known to be three-body attachment to molecular oxygen.