Room-temperature broadening and pressure-shift coefficients in the ν2 band of CH 3D–O 2: Measurements and semi-classical calculations

Abstract

We report measured Lorentz O 2-broadening and O 2-induced pressure-shift coefficients of CH 3D in the ν 2 fundamental band. Using a multispectrum fitting technique we have analyzed 11 laboratory absorption spectra recorded at 0.011 cm −1 resolution using the McMath–Pierce Fourier transform spectrometer, Kitt Peak, Arizona. Two absorption cells with path lengths of 10.2 and 25 cm were used to record the spectra. The total sample pressures ranged from 0.98 to 339.85 Torr with CH 3D volume mixing ratios of 0.012 in oxygen. We report measurements for O 2 pressure-broadening coefficients of 320 ν 2 transitions with quantum numbers as high as J″ = 17 and K = 14, where K″ = K′ ≡ K (for a parallel band). The measured O 2-broadening coefficients range from 0.0153 to 0.0645 cm −1 atm −1 at 296 K. All the measured pressure-shifts are negative. The reported O 2-induced pressure-shift coefficients vary from about −0.0017 to −0.0068 cm −1 atm −1. We have examined the dependence of the measured broadening and shift parameters on the J″, and K quantum numbers and also developed empirical expressions to describe the broadening coefficients in terms of m ( m = − J″, J″, and J″ + 1 in the Q P-, Q Q-, and Q R-branch, respectively) and K. On average, the empirical expressions reproduce the measured broadening coefficients to within 4.4%. The O 2-broadening and pressure shift coefficients were calculated on the basis of a semiclassical model of interacting linear molecules performed by considering in addition to the electrostatic contributions the atom–atom Lennard-Jones potential. The theoretical results of the broadening coefficients are generally larger than the experimental data. Using for the trajectory model an isotropic Lennard-Jones potential derived from molecular parameters instead of the spherical average of the atom–atom model, a better agreement is obtained with these data, especially for | m| ⩽ 12 values (11.3% for the first calculation and 8.1% for the second calculation). The O 2-pressure shifts whose vibrational contribution are either derived from parameters fitted in the Q Q-branch of self-induced shifts of CH 3D or those obtained from pressure shifts induced by Xe in the ν 3 band of CH 3D are in reasonable agreement with the scattered experimental data (17.0% for the first calculation and 18.7% for the second calculation).