SF₆ condensation in supersonic nozzle expansions

The condensation of SF₆ in a dilute mixture with either Ar or He as a carrier gas when quickly expanded in a small, adjustable nozzle is investigated. Condensation is observed by monitoring the disappearance of individual rotational‐state lines in the SF₆ ν₄ absorption band. For this, a diode‐laser spectroscopy system is employed, the nozzle is two dimensional, and the flows are short‐duration pulses. The gas density at which condensation onset occurs is measured as a function of local gas temperature (30°–100° K), local expansion rate, and SF₆ mole fraction (0.18%–3.4%). The onset density depends strongly on the local temperature and expansion rate and not at all on the mole fraction. For the SF₆/Ar data, Ar co‐condensation appears to reduce the SF₆ onset density level at the lowest mole fractions, and SF₆ vibrational lag appears to increase the onset density level at the highest mole fractions. A simple ’’trimers‐critical’’ model for condensation onset is derived, and with this model reasonable correlation of much of the onset data is obtained. From this correlation, an effective dimer binding energy and trimer formation probability are extracted.