Absorption in the troughs of the far infrared spectra of NH 3 and mixtures of NH 3 and H 2

Abstract

The absorption coefficients in the troughs between the J-multiplets of the far infrared rotation-inversion spectrum of NH 3 and NH 3 broadened by H 2 were measured at 296 K in the region from about 65–230 cm −1. Wave numbers were selected in the troughs that are removed from nearby weak absorption lines, and measurements of the absorption versus the density of NH 3 and of H 2 verified that the absorption was due to the wings of distant rather than nearby spectral lines. These measurements were compared with computations of the absorption arising from binary collisions of NH 3–NH 3 and NH 3–H 2 for uncoupled Lorentz lines. Except at the lowest and highest frequencies, the measured continuum absorption in the troughs was less than the computed absorption, a discrepancy that increased with increasing frequency. An improved representation of the measured absorption was obtained by fitting the Rosenkrantz line shape (Orton et al., unpublished) with the Ben-Reuven (Lightman and Ben-Reuven. J Chem Phys 1969;50:351–3) formulation of the line coupling coefficient of the inversion J-doublets. The application of these results to the spectrum of Jupiter shows clear differences between the spectrum of the above model and the Lorentz line shape, with the largest differences occurring in the clearest troughs.