H¬2-pressure broadening and frequency shifts of methane in the v2+v3 band measured in the temperature range between 80 and 370 K

We have made the first measurements of H2-broadened line shape parameters of CH4 transitions in the v2+v3 band located at 2.2 μm to support accurate atmospheric opacity calculations of cold giant planets, hot Jupiters, and low mass brown dwarfs, for which H2 is the dominant atmospheric constituent. For this reason, a series of spectra of pure CH4 and CH4-H2 mixtures were obtained in the Octad (4100-4600 cm-1) region for a wide temperature range between 80 and 370 K using a Bruker 125HR high-resolution Fourier transform spectrometer (FTS) at JPL. Three custom-designed gas absorption cells vacuum coupled to the FTS were used to obtain the spectra. All spectra were fit simultaneously using a multispectrum non-linear least-squares fitting software (Labfit), which adopts a speed-dependent Voigt line shape using full line mixing taken into account through a relaxation matrix operation. H2 pressure broadened half-width, and H2 pressure-induced shift coefficients were determined in 12 J-manifolds covering P(4) – R(6), including the Q-branch. The temperature dependences of the width and shift coefficients were determined based upon the power law and the linear models. Collisional line mixing coefficients for CH4- H2 were retrieved for ten transition pairs across the v2+v3 band along with their temperature dependence on the power-law model analogous to that of pressure broadening. The measured H2-widths at 296 K vary from 0.05 – 0.08 cm-1/atm, which are slightly greater than the Air-widths by ~8%. They also exhibited the well-known A/E/F rotational symmetry species dependence, as discussed in earlier literature. The H2 pressure-induced frequency shifts at 295 K were all measured negative and ranged between -0.005 and -0.015 cm-1/atm. Temperature dependences were observed to be 0.58 (i.e., very close to the theoretical value, 0.5) for the H2-broadening, which is substantially lower than that for Air-broadening, i.e., 0.82. We also measured the temperature dependence for the H2-shifts slightly smaller than that of Air-shifts for the corresponding transitions of CH4 in the same band, v2+v3. For the same transitions, speed dependence parameters for the CH4-H2 transitions were found to be smaller than for CH4-Air-broadening. For the collisional line mixing parameters, we present the off-diagonal relaxation matrix elements (ORMEs) for the line coupling pairs as well as Rosenkranz line mixing coefficients calculated from the retrieved ORMEs. Finally, we have compiled all the retrieved line parameters in an electronic format and reported as electronic supplemental documents to facilitate the analyses of planetary and exoplanetary atmospheres using ground-, air- and space-based observations (e.g., Keck I and II, SOFIA, JWST, etc.).