Measurements and theoretical calculations of N2-broadening and N2-shift coefficients in the ν2 band of CH3D

Abstract:

In this paper, we report measured Lorentz N2-broadening and N2-induced pressure-shift coefficients of CH3D in the m2 fundamentalband using a multispectrum fitting technique. These measurements were made by analyzing 11 laboratory absorption spectra recorded at0.0056 cm1 resolution using the McMath-Pierce Fourier transform spectrometer located at the National Solar Observatory on KittPeak, Arizona. The spectra were obtained using two absorption cells with path lengths of 10.2 and 25 cm. The total sample pressuresranged from 0.98 to 402.25 Torr with CH3D volume mixing ratios of 0.01 in nitrogen. We have been able to determine the N2 pres-sure-broadening coefficients of 368 m2 transitions with quantum numbers as high as J00 = 20 and K = 16, where K00 = K0 ” K (for a parallelband). The measured N2-broadening coefficients range from 0.0248 to 0.0742 cm1 atm1 at 296 K. All the measured pressure-shifts arenegative. The reported N2-induced pressure-shift coefficients vary from about 0.0003 to 0.0094 cm1 atm1. We have examined thedependence of the measured broadening and shift parameters on the J00, and K quantum numbers and also developed empirical expres-sions to describe the broadening coefficients in terms of m (m = J00, J00, and J00 + 1 in the QP-, QQ-, and QR-branch, respectively) and K.On average, the empirical expressions reproduce the measured broadening coefficients to within 4.7%. The N2-broadening and pressure-shift coefficients were calculated on the basis of a semiclassical model of interacting linear molecules performed by considering in addi-tion to the electrostatic contributions the atom–atom Lennard-Jones potential. The theoretical results of the broadening coefficients arein good overall agreement with the experimental data (8.7%). The N2-pressure shifts whose vibrational contribution is derived fromparameters fitted in the QQ-branch of self-induced shifts of CH3D, are also in reasonable agreement with the scattered experimental data(20% in most cases). 2005 Elsevier Inc. All rights reserved.

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