Spectral line parameters including line shapes in the 2ν3 Q branch of 12CH4

In this study, we report the first experimental measurements of spectral line shape parameters (self- and air-broadened Lorentz half-widths, pressure-shifts, and line mixing (via off-diagonal relaxation matrix elements) coefficients and their temperature dependences, where appropriate) for transitions in the 2ν3 Q branch manifolds, Q(11)–Q(1) of methane (12CH4), in the 5996.5–6007-cm-1 region. The analysis included 23 high-resolution, high signal-to-noise laboratory absorption spectra recorded with the Bruker IFS125HR Fourier transform spectrometer (FTS) at JPL. The experimental data were obtained using 12C-enriched 12CH4 and dilute mixtures of 12CH4 in dry air in the 130–296 K range using a room-temperature long path absorption cell and, two custom-built coolable cells. In the analysis, an interactive multispectrum fitting software was employed where all the 23 spectra (11 self-broadened and 12 air-broadened) were fit simultaneously. By carefully applying reasonable constraints to the parameters for severely blended lines, we were able to determine a self-consistent set of broadening, shift and line mixing (relaxation matrix coefficients) parameters for CH4–CH4 and CH4–air collisions. In the majority of cases, a quadratic speed dependence parameter common for all transitions in each Q(J) manifold was determined. However, temperature dependences of the Q branch line mixing parameter could not be determined from the present data. Since no other experimental line shape measurements have been reported for this Q-branch, the present results are compared to available values in the HITRAN2012 database.