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Abstract: The lowest small-amplitude vibration in acetaldehyde (CH3CHO) is the in-plane aldehyde scissors mode ν10 at 509cm−1. This mode lies about 175cm−1 above the top of the barrier to internal rotation of the methyl group and is relatively well separated from other small-amplitude vibrational states (the next fundamental occurring more than 250cm−1 higher). It thus provides an excellent example of an isolated small-amplitude fundamental (bright state) embedded in a bath of dark states. Since the bath states at these energies are not too dense, and since they arise purely from states of the large-amplitude torsional vibration of the methyl rotor, a detailed spectroscopic analysis of interactions between the bright state and the bath states should be possible. This paper represents the first step toward that goal. We have assigned several thousand transitions in the ν10 band (J ⩽28, K ⩽12), and have carried out a simultaneous fit of 2400 of these transitions (J ⩽15, K ⩽9) with over 8100 transitions to the torsional bath state levels. Three vibration–torsion interactions, which give rise to rather global level shifts of the order of 1cm−1 in the ν10 levels, have been identified and quantitatively fit. A number of vibration–torsion–rotation interactions, which give rise to localized (avoided-crossing) shifts in ν10 have also been determined. The present analysis indicates the need for reliable spectroscopic information on more of the torsional bath states in the immediate vicinity of the ν10 levels. Possible ways of obtaining such information in future studies are considered. [Copyright &y& Elsevier]

Abstract: Absolute intensities, self- and air-broadening coefficients, self- and air-induced shift coefficients and their temperature dependences have been determined for lines belonging to the P- and R-branches of the ν2 band of H12C14N centered near 712cm−1. Infrared spectra of HCN in the 14-μm region were obtained at high resolution (0.002–0.008cm−1) using two different Fourier transform spectrometers (FTS), the McMath-Pierce FTS at the National Solar Observatory on Kitt Peak and the Bruker IFS 120HR FTS at the Pacific Northwest National Laboratory. Spectra were recorded with 99.8% pure HCN as well as lean mixtures of HCN in air at various temperatures ranging between +26 and −60°C. A multispectrum nonlinear least squares technique was used to fit selected intervals of 36 spectra simultaneously to obtain the line positions, intensities, broadening, and shift parameters. The measured line intensities were analyzed to determine the vibrational band intensity and the Herman–Wallis coefficients. The measured self-broadening coefficients vary between 0.2 and 1.2cm−1atm−1 at 296K, and the air-broadening coefficients range from 0.08 to 0.14cm−1atm−1 at 296K. The temperature dependence exponents of self-broadening range from 1.46 to −0.12 while the corresponding exponents for air broadening vary between 0.58 and 0.86. The present measurements are the first known determination of negative values for the temperature dependence exponents of HCN-broadening coefficients. We were able to support our self-broadening measurements with appropriate theoretical calculations. Our present measurements are compared, where possible, with previous measurements for this and other HCN bands, as well as the parameters that are included in the 2000 and 2004 editions of the high-resolution transmission (HITRAN) database. [Copyright &y& Elsevier]

Abstract: A semiclassical theory based upon the Robert–Bonamy formalism has been developed to explain the experimental measurements of self-broadening, self-induced pressure shift coefficients in the ν1,ν2, 2ν2 bands of H12C14N and the 2ν1 band of H13C14N, as well as the temperature dependences of these parameters with special emphasis on the ν2 band. Our calculations include only electrostatic interactions and neglect the vibrational dependence of the isotropic part of the intermolecular potential, which probably has a weak contribution to the HCN self-shifts for the bands investigated in this study. The agreement between theory and measurements is good in the cases of self-broadening coefficients and their variation with temperature, as well as the self-shift coefficients determined at room temperature. However, the observed temperature dependence of self-shift coefficients in the ν2 band is different from that derived theoretically. [Copyright &y& Elsevier]