TEMPERATURE-DEPENDENCE OF COLLISION-BROADENED LINES OF PHOSPHINE

As an extension of a previous study at room temperature, high-resoluti on FT spectra of phosphine have been obtained under lower temperature conditions, namely near 235 and 175 K. The spectral range investigated , 1950-2150 cm-1, covers mainly the three weak transitions 2nu2, nu2 nu4, and 2nu4 which contribute, along with the strong bands nu1 and n u3, to the absorption in the observational window of Jupiter near 4-5 mum. About 800 lines were analyzed by means of a nonlinear least-squar es fit procedure in order to extract their individual spectral paramet ers. The retrieved values of the linewidths, along with those previous ly determined from the spectra at room temperature, were used to deriv e the temperature dependence of both H-2 and He broadening of PH3 line s. The broadening coefficients were shown to fit closely the usual ''e xponential'' relation gamma(T) = gamma0(T0).(T0/T)n. Careful inspectio n of the values obtained for the n exponent shows that, for a single v alue of J, n is nearly K independent within the series of lines of a g iven rotational type. Also, for a given J it does not seem to exhibit any noticeable variation with the type of rotational transitions under study. Only when considering the dependence of the K-averaged values of n as a function of J is a slight variation observed, with a clear t rend of n to decrease as J values increase. However, in the range of J values investigated here (J less-than-or-equal-to 13), the magnitude of variation of n remains small. Accordingly, for the purpose of plane tary spectra modeling, it seems quite reasonable to adopt a mean value of the n exponent for any J and K. From the present measurements, we propose the values n = 0.73 and n = 0.30 for H-2 broadening and He bro adening, respectively. (c) 1994 Academic Press, Inc.