Speed-dependent line mixing in the ν3 band Q branch of methane

Abstract

In 1992, we reported high-resolution spectra of the Q branch of the CH4ν3 band from the Doppler limit to pressures of 500 Torr for various buffer gases recorded with a tunable difference-frequency laser. For P ≤ 100 Torr, the individual transitions were well enough resolved to be fit with simple Dicke-narrowed lineshapes, but at higher pressures, line mixing among the severely blended lines prevented further analysis. Subsequently, using a simultaneous multispectrum analysis, we were able to fit the full range of pressures quite satisfactorily with Dicke-narrowed first-order line-mixing profiles. We also tested scaled model relaxation W-matrix mixing and hypergeometric speed-dependent broadening, but found only minimal improvement not worth the computational complications and increased processing time. Since then, fast quadratic speed-dependence algorithms have been developed, and simplified W-matrix methods involving selected-coupled-doublets have been demonstrated, recently for this same Q branch. Therefore, we have reexamined these newer developments on our higher resolution spectra and found that the improved efficiency is tempered by a strong competition between Dicke narrowing and speed dependence and by a deteriorated fit using only selected-coupled-doublets. A detailed comparative study is reported here, along with some new models proposed and tested for truncating the W-matrix and for a hybrid combination of first-order mixing with selected W-matrix elements.