NASA - National Aeronautics and Space Administration

Pressure-broadened line shape parameters of transitions in the ν3 band of HDO (ν0 =3707.4 cm-1) were measured from spectra of HDO and mixtures of HDO and CO2 for application to accurate retrievals of HDO abundances and D/H ratios for CO2-rich planetary atmospheres of Mars and Venus. A few calculated line lists have recently been published on HDO-CO2 line shapes and their temperature dependences, but the present study represents the first laboratory measurements of those parameters in the ν3 band; Measurements for nearly 100 transitions in the ν3 band have been made. Room temperature measurements of self-broadened width and shift coefficients for all of these transitions, line mixing via offdiagonal relaxation matrix element coefficients and quadratic speed dependence parameter were measured for the majority of these transitions. All these measurements were made by simultaneously fitting eleven high-resolution spectra of HDO and HDO-CO2 mixtures at various temperatures and pressures recorded with the Bruker Fourier transform spectrometer at the Jet Propulsion Laboratory. Two specially built coolable absorption cells with path lengths of 20.38 cm and 20.941 m were used to contain the sample mixtures. Multispectrum nonlinear least squares fitting algorithm was employed in the analysis.

Calculations using the Modified Complex Robert-Bonamy formalism (MCRB) were made for the halfwidth coefficients, their temperature dependences and pressure shift coefficients for the HDO–CO2 and HDO–HDO collision systems. The calculations were made for all ν3 band transitions in the 1100–4100 cm-1 region on the HITRAN2012 database. Present measurements are compared with the MCRB calculations and other literature values.