Temperature Dependence of Collision-Broadened Lines of Phosphine

As an extension of a previous study at room temperature, high-resolution 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 2ν2, ν2 + ν4, and 2ν4 which contribute, along with the strong bands ν1 and ν3, to the absorption in the observational window of Jupiter near 4-5 μm. About 800 lines were analyzed by means of a nonlinear least-squares fit procedure in order to extract their individual spectral parameters. The retrieved values of the linewidths, along with those previously determined from the spectra at room temperature, were used to derive the temperature dependence of both H2 and He broadening of PH3 lines. The broadening coefficients were shown to fit closely the usual "exponential" relation γ(T) = γ0(T0) · (T0/T)n. Careful inspection of the values obtained for the n exponent shows that, for a single value of J, n is nearly K independent within the series of lines of a given 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 trend of n to decrease as J values increase. However, in the range of J values investigated here (J ≤ 13), the magnitude of variation of n remains small. Accordingly, for the purpose of planetary 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 H2 broadening and He broadening, respectively.