H2-pressure broadening and frequency shifts of methane in the v2+v3 band measured in the temperature range between 80 and 370 K. (November 2020)

• Record Type: Journal Article
• Authors: Sung, Keeyoon; Devi, V. Malathy; Benner, D. Chris; Drouin, Brian J.; Crawford, Timothy J.; Mantz, Arlan W.; Smith, Mary Ann H.
• Is Part Of: Journal of quantitative spectroscopy & radiative transfer. Volume 256(2020)
• Publication Date: 2020-11
• Subjects: H2-broadening -- Methane (CH4) -- Octad -- Pressure broadening -- Temperature dependence -- Relaxation matrix -- Line mixing; Spectrum analysis -- Periodicals; Radiation -- Periodicals; Analyse spectrale -- Périodiques; Rayonnement -- Périodiques; Radiation; Spectrum analysis; Periodicals; 543.0858
• DOI: 10.1016/j.jqsrt.2020.107264
• Languages: English
• ISSNs: 0022-4073

Abstract

Highlights: Obtained H2 -broadened CH4 spectra in the v 2 + v 3 band (2.2 µm) at 80 – 370 K. Fit 28 spectra simultaneously and achieved spectrum fitting residuals better than 1%. Retrieved H2 -broadened widths, H2 -pressue shifts, and their temperature dependences. Determined collisional line mixing coefficients through full line mixing operation. Showed J and A/E/F dependence of H2 -widths for CH4 in the v2+v3 band. Abstract: We have made the measurements of H2 -broadened line shape parameters of CH4 transitions in the v 2 + v 3 band located at 2.2 µm. These measurements support 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 Jet Propulsion Laboratory (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, which adopts a speed-dependent Voigt line shape using full line mixing taken into account through a relaxation matrix operation. H2 pressure broadened half-widths, and H2 pressure-induced shift coefficients were determined in 11 J-manifolds covering P(4) – R(6) plus 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. For the same transitions, speed dependence parameters for the CH4 H2 transitions were found to be smaller than for CH4 -air-broadening. We have compiled the retrieved line parameters in an electronic format and reported as supplemental document. This will facilitate the analyses of planetary and exoplanetary atmospheres using ground-, air- and space-based observations ( e.g., Keck I and II, SOFIA, JWST). Graphic abstract: Image, graphical abstract Figure. Spectrum fitting residuals in the R(6) manifold. (a) with no line mixing, (b) with line mixing, In the bottom panel the observed and calculated spectra where two pairs of transitions mix together, e.g., F1 and F2, and A1 and A2 are labeled. The vertical bars (in black at the top of the bottom panel) represent the locations of all transitions included in the calculations. The vertical bars in red, blue, and green indicate the line position of the 1st, 2nd and 3rd sets of five strongest lines each group, respectively. [ Colors available in the online version ].