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

Abstract

In this paper, we report measured Lorentz self-broadening and self-induced pressure-shift coefficients of 12CH 3D in the ν 2 fundamental band ( ν 0 ≈ 2200 cm −1). The multispectrum fitting technique allowed us to analyze simultaneously seven self-broadened absorption spectra. All spectra were recorded at the McMath-Pierce Fourier transform spectrometer of the National Solar Observatory (NSO) on Kitt Peak, AZ with an unapodized resolution of 0.0056 cm −1. Low-pressure (0.98–2.95 Torr) as well as high-pressure (17.5–303 Torr) spectra of 12C-enriched CH 3D were recorded at room temperature to determine the pressure-broadening coefficients of 408 ν 2 transitions with quantum numbers as high as J″ = 21 and K = 18, where K″ = K′ ≡ K (for a parallel band). The measured self-broadening coefficients range from 0.0349 to 0.0896 cm −1 atm −1 at 296 K. All the measured pressure-shifts are negative. The reported pressure-induced self-shift coefficients vary from about −0.004 to −0.008 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 3.6%. A semiclassical theory based upon the Robert–Bonamy formalism of interacting linear molecules has been used to calculate these self-broadening and self-induced pressure-shift coefficients. In addition to the electrostatic interactions involving the octopole and hexadecapole moments of CH 3D, the intermolecular potential includes also an atom–atom Lennard–Jones model. For low K ( K ⩽ 3) with | m| ⩽ 8 the theoretical results of the broadening coefficients are in overall good agreement (3.0%) with the experimental data. For transitions with K approaching | m|, they are generally significantly underestimated (8.8%). The theoretical self-induced pressure shifts, whose vibrational contribution is derived from results in the Q Q-branch, are generally smaller in magnitude than the experimental data in the Q P-, and Q R-branches (15.2%).