Phosphine spectrum at 4-5 μm: Analysis and line-by-line simulation of 2ν2, ν2 + ν4, 2ν4, ν1, and ν3 bands
The five absorption bands of PH3 at 4-5 μm, i.e., 2ν2, ν2 + ν4, 2ν4, ν1, ν3, were simultaneously analyzed from FT spectra recorded with an apodized resolution of 0.0054 cm-1. A theoretical model suited to the strong vibro-rotational couplings among the five bands was used. More than 4400 transitions (J <= 16) pertaining to the five bands were assigned in the range 1885-2445 cm-1, but only 3766 relatively unblended transitions were finally retained for the adjustment of the upper state energy parameters. A set of 57 energy parameters was required to reproduce the experimental wavenumbers with an overall standard deviation of 0.009 cm-1, consistent with the estimated upper limit of uncertainty when taking into account all factors of experimental errors. Absolute intensities of about 1600 lines were also measured, mainly in 2ν2, ν2 + ν4, and 2ν4 bands. The fitting of these data was achieved by using a restricted set of 53 energy parameters which yields the best compromise for both spectral position and intensity analyses. An estimate was then obtained for the four dipole moment derivatives involved in 2ν2, ν2 + ν4, and 2ν4, allowing us to reproduce the intensity measurements with an overall standard deviation of 10.9%, consistent with the experimental uncertainties. Absolute bandstrengths relative to 2ν2, ν2 + ν4, and 2ν4 were thus derived for the first time. Last, a line-by-line simulation of the five bands (J <= 16) was carried out in a form suitable for planetary applications. PH3, which is observed in Jupiter and Saturn, is an important molecule for studies of their atmospheric structures. The 5-μm bands are of special importance because the atmospheres are sounded deep at these wavelengths.Laboratoire Associé aux Universités Paris-Sud et P. et M. Curie.