Abstract
Abstract Nozzle expansion of pure SF 6 and SF 6 seeded in carrier gases e.g. Ar, He and N 2 , is used to study relaxation of the various degrees of freedom. The diode laser absorption spectrum of the Q branch of the ν 3 mode of SF 6 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 SF 6 are determined from these measurements. Seeded-beam data prove argon to be a better refrigerant than He or N 2 . Cooling in seeded beams at moderate stagnation pressures is not substantially enhanced even at very high dilution.
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