MOLECULAR BEAM SPECTRUM OF THE PERTURBED C-H STRETCHING REGION OF FLUOROFORM
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Date
1988
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Ohio State University
Abstract
The highly perturbed C-H stretching fundamental band $\nu_{1}$ of $CHF_{3}$ has thus far defied analysis even as its overtones received attention in studies of intramolecular vibrational relaxation. The spectrum of the fundamental region remains complex at Doppler-limited resolution, and we have obtained a Doppler free spectrum in an adiabatically cooled molecular beam at 4 K using a scanning color center laser with bolometric detection of the optically excited molecules. This has enabled us to make definite assignments of the observed lines. Under these conditions, the central Q-branch, which dominates the spectrum at room temperature, almost vanishes, while the bulk of the intensity is equally shared by two sidebands with Q-branches symmetrically displaced by $\pm 5 cm^{-1}$ from the suppressed central Q-branch. This peculiar structure is due to a close anharmonic resonance between the fundamental $\nu_{1}$ (species $A_{1}$) and the combinations $\nu_{4}^{\pm}-\nu_{5}^{\pm}-\nu_{6}^{2} [A_{1}-A_{2}]$ located only $1.45 cm^{-1}$ above $\nu_{1}$. whose origin is at $3033.52 cm^{-1}$. The coupling constant of the resonance is $k_{1456}/2\sqrt{2} = 3.67 cm^{-1}$. Additional severe perturbations by the E components of $\nu_{4}+\nu_{5}+\nu_{6}$ as well as by $\nu_{2}+\nu_{4}+\nu_{6}$ have been identified in the spectrum. Since the $\nu_{1}$ vibration is known to be in global Fermi resonance with the overtone $2\nu_{4}^{\circ}$ , we have also recorded the $2680 - 2615 cm^{-1}$ region, containing the $2\nu_{4}^{\circ}$ and $2\nu_{4}^{2}$ bands under Doppler-limited conditions using a difference frequency laser spectrometer. This has yielded much Improved spectroscopic constants, in particular for the strongly perturbed $2\nu_{4}^{-2}$ state.
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Author Institution: Molecular Spectroscopy Division, National Bureau of Standards; Department of Physics, Pennsylvania State University