Multispectrum analysis of the ν9 band of 12C 2H 6: Positions, intensities, self- and N 2-broadened half-width coefficients

Abstract

Line positions, intensities, Lorentz self- and N 2-broadened half-width coefficients have been measured for PQ 3, PQ 2, PQ 1, RQ 0, RQ 1, RQ 2, and RQ 3 sub-band transitions in the ν 9 fundamental band of 12C 2H 6. A multispectrum nonlinear least-squares fitting technique was used to fit up to 17 high-resolution (∼0.00156 cm −1), room temperature absorption spectra of pure (99.99% chemical purity) natural sample of ethane and lean mixtures of the high-purity ethane diluted with N 2. A Bruker IFS 120HR Fourier transform spectrometer located at the Pacific Northwest National Laboratory (PNNL), in Richland, Washington was used to record the data. A standard Voigt line shape was assumed to fit all the data since no line mixing or other non Voigt line shapes were required to fit any of the spectra used in the analysis. Short spectral intervals (∼2–2.5 cm −1) of all 17 spectra covering a specific PQ or RQ sub-band were fit simultaneously. For the first time in an ethane band, pressure-broadened half-width coefficients were determined for the torsional-split components. However, for better reliability of the retrieved coefficients for the weaker components (transitions with large intensity ratios of 4:1 or 3:1 for most K levels between the strong and weak components), constraints were used such that the half-width coefficients of both torsional-split components for a given J were identical for a specific broadening gas. No pressure-induced shift coefficients were necessary to fit the spectra to their noise level. The present study revealed for the first time the dependence of self- and N 2-broadened half-width coefficients upon the J, K quantum numbers of the transitions in ethane. A number of transitions belonging to the ν 9+ ν 4− ν 4 and the ν 9+2 ν 4−2 ν 4 hot bands were also observed in the fitted regions and measurements were made when possible.