Pressure broadening and shift coefficients for H2, He and N2 in the 3ν2 band of 12CH3D retrieved by a multispectrum fitting technique

Improved determinations of CH3D abundances, as well as of the deuterium to hydrogen ratio (D/H) in the atmospheric methane of Saturn, Uranus, Neptune and Titan are reachable with a complete set of spectroscopic parameters for the 3 nu(2) band of (CH3D)-C-12 which take into account the dominant gases in these atmospheres (i.e. H-2, He and N-2) Line intensities and self-broadening coefficients derived from a previous analysis made by our group for 255 lines in the region of the 3 nu(2) band of (CH3D)-C-12 at 1.55 mu m have been recently reported. In this paper we present (CH3D)-C-12 line broadening coefficients and pressure-induced shifts by H-2, He and N-2. A set of Fourier transform spectra recorded at Kitt Peak National Observatory with pressures of 100, 250 and 500 Torr for H-2 and N-2, and 600 Torr for He, has been used for this study. Data reduction was performed using a non-linear least-squares fitting multispectrum program developed for this purpose. The broadening coefficients, averaged over K, range roughly between 0.0685 and 0.0569 cm(-1) atm(-1) for (CH3D)-C-12-H-2, between 0.0434 and 0.0335 cm(-1) atm(-1) for (CH3D)-C-12-He, and between 0.0665 and 0.0533 cm(-1) atm(-1) for (CH3D)-C-12-N-2, up to a maximum J of 14. The uncertainties are estimated to about 1.5% in the H-2 and N-2 broadening cases, and 3% in the He case. The pressure shift coefficients averaged over both J and K are found to be - 0.0067 cm(-1) atm(-1) for (CH3D)-C-12-H-2, + 0.0021 cm(-1) atm(-1) for (CH3D)-C-12-He, and -0.0119 cm(-1) atm(-1) for (CH3D)-C-12-N-2. We estimate an uncertainty in these average shift coefficients of 15% in the H-2 and N-2 broadening cases, and of 35% in the He case. Dependences in J and K for these parameters have also been investigated with empirical expressions. (C) 1999 Elsevier Science Ltd. All rights reserved.