Measurements and theoretical calculations of N 2-broadening and N 2-shift coefficients in the ν2 band of CH 3D

Abstract

In this paper, we report measured Lorentz N 2-broadening and N 2-induced pressure-shift coefficients of CH 3D in the ν 2 fundamental band using a multispectrum fitting technique. These measurements were made by analyzing 11 laboratory absorption spectra recorded at 0.0056 cm −1 resolution using the McMath-Pierce Fourier transform spectrometer located at the National Solar Observatory on Kitt Peak, Arizona. The spectra were obtained using two absorption cells with path lengths of 10.2 and 25 cm. The total sample pressures ranged from 0.98 to 402.25 Torr with CH 3D volume mixing ratios of 0.01 in nitrogen. We have been able to determine the N 2 pressure-broadening coefficients of 368 ν 2 transitions with quantum numbers as high as J″ = 20 and K = 16, where K″ = K′ ≡ K (for a parallel band). The measured N 2-broadening coefficients range from 0.0248 to 0.0742 cm −1 atm −1 at 296 K. All the measured pressure-shifts are negative. The reported N 2-induced pressure-shift coefficients vary from about −0.0003 to −0.0094 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 4.7%. The N 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 in good overall agreement with the experimental data (8.7%). The N 2-pressure shifts whose vibrational contribution is derived from parameters fitted in the Q Q-branch of self-induced shifts of CH 3D, are also in reasonable agreement with the scattered experimental data (20% in most cases).