Knowledge Bank

In a recent tunable difference-frequency laser $studya$ of collisional line shapes in the Ar- and $N2$-broadened P- and R-branch manifolds of the $\nu 3$ band of $CH4$, we observed the effects of Dicke narrowing and line mixing, analyzed in the Rosenkranz (linear pressure) regime with proper sum and selection $rulesb$ imposed on the mixing coefficients. The spectral traces recorded at pressures up to 67 kPa were individually fit with appropriate linear constraints on the parameters not well determined at some of the pressures. Previously, Benner $etalc$ had shown that simultaneous multispectrum fits of FTIR spectra of $CH4$ alleviated the requirement for such constraints, but they obtained noticeable deviations between observed and calculated spectra, implying a nonlinear pressure dependence of some of the parameters, thought to result from line mixing beyond the Rosenkranz regime. However, contrary to results presented here last year by Benner $etald$, we have found that multispectrum fitting directly for the relaxation matrix still does not adequately fit the data or yield reliable off-diagonal coupling elements. Indeed, systematic residuals well above the experimental noise level are observed for the R0, R1, R2 and P2 lines not subject to line mixing at the pressures studied. We will show that a simple model incorporating speed-dependent broadening and shifting parameters significantly improves the fits for these uncoupled lines as well as for the coupled lines in higher J manifolds within the Rosenkranz regime.