ABSORPTION OF $^{12}CH_{3}D$ AT $6-10 \mu m$. FREQUENCIES AND INTENSITIES

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1986

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Ohio State University

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The three bands $\nu_{3}, \nu_{5}, \nu_{6}$ of $^{12}CH_{3}D$ contributing to the absorption at $6-10 \mu m$ were analyzed as a triad, i.e., by accounting for the Coriolis couplings by a diagonalization method. High resolution spectra $(\delta\sigma = 0.005 cm^{-1})$ recorded by the vacuum FT spectrometer of our laboratory gave us the whole absorption without any interference with the strong water absorption (in the region of $\nu_{5}$). The 3240 assigned transitions $(J^{\prime} \le 18)$, of which about one thousand perturbation allowed transitions, were used for a new refinement of the ground state energies ($\sigma = 0.0003 cm^{-1}$ for 2641 GS differences), and for a fit of the triad upper state energies ($\sigma = 0.007 cm^{-1}$ for the 3240 data). The line strengths were calculated according to the strict selection rules on J and symmetry species, but without any restriction on $\triangle K$. The effective dipole moment matrix elements required for the transitions within the triad $\nu_{3}, \nu_{5},\nu_{6}$ were previously $^{(1)}$ calculated. Values for the dipole moment derivatives $\delta \mu (s= 3,5,6)$ were deduced from recent values of the band strengths ${^{(2,3)}} S_{3}$ ands $S_{4}$ of $^{12}CH_{4}$, assuming the $8 \mu/5 q_{8}$, in terms of internal coordinates $\Gamma_{i}$, to be unchanged by H/D substitution. The line strengths calculated using these values are in surprisingly good agreement with the experimental intensity data presently available for $\nu_{6}$ and $\nu_{3}$. Nevertheless, some systematic discrepancies at high J require an intensity analysis involving Herman-Wallis-type corrections. Such an analysis is in progress in collaboration with F, Cappellani and R. Restelli (Ispra, Italy). The energy and intensity parameters obtained have been used to produce a list of about 6000 transitions $(J\leqslant 18)$ predicted with line strengths $\geqslant 4.10^{-25} cm.molecule^{-1}$ (at 300 K) between 874 and $1695 cm^{-1}$.

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$^{1}$ G. TARRAGO, O.N. ULENIKOV and G. POUSSIGUE, J. Physique, 45, 1429-1447 (1984) $^{2}$ M. DANG-NHU, A.S. PINE and A.G. ROBIETTE, J. Mol. Spectrosc., 77, 57-68 (1979) $^{3}$ D.E. JENNINGS and A.G. ROBIETTE, J Mol. Spectrosc., 94, 369-379 (1982). Address: Laboratoire d'Infrarouge (Associ\'{e} au CNRS), Universit\'e de Paris-Sud, Batiment 350, 91405 ORSAY CEDEX, France.
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