Infrared diode laser absorption study of free jets of NH₃

Abstract

Supersonic free-jets of pure NH₃ and 1:1 mixture of NH₃ in argon and SF₆ have been probed by IR diode laser absorption of the ν₂ band of NH₃. Rotational population distribution and condensation have been studied from the observed spectral features. The importance of background absorption has been recognized and taken into account in the analysis of the data. Integrated absorption, rather than peak absorption, has been given more weight in the analysis. Thermal distribution for the rotational level population has been found to prevail. Measured rotational temperature increases with increasing pressure, which has been attributed to condensation of NH₃ molecules. Fractional condensation of NH₃ in free-jets has been derived from the observed pressure dependence of the absorption line intensities. Model calculations have been carried out to s simulate the observed spectral features. The splitting of the absorption lines originating from the low-lying rotational levels and large spectral linewidth observed in free-jets have been explained in terms of condensation, large flow velocity and jet divergence.