Relaxation in pure and seeded supersonic jets of SF₆

Abstract

Nozzle expansion of pure SF₆ and SF₆ seeded in carrier gases e.g. Ar, He and N₂, is used to study relaxation of the various degrees of freedom. The diode laser absorption spectrum of the Q branch of the ν3 mode of SF₆ is used to derive the rotational temperature (T_rot). Time-of-flight studies determine the translational temperature (T_tg) while the vibrational temperature (T_vib) is estimated from the energy balance equation. Rotational and translational temperatures follow each other closely while the vibrational temperature lags behind considerably. The effective specific heat ratio (γ) and the collisional effectiveness parameter (e) for SF₆ are determined from these measurements. Seeded-beam data prove argon to be a better refrigerant than He or N₂. Cooling in seeded beams at moderate stagnation pressures is not substantially enhanced even at very high dilution.