Knowledge Bank

The highly perturbed C-H stretching fundamental band $\nu 1$ of $CHF3$ 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 $\pm 5cm-1$ from the suppressed central Q-branch. This peculiar structure is due to a close anharmonic resonance between the fundamental $\nu 1$ (species $A1$) and the combinations $\nu 4\pm -\nu 5\pm -\nu 62[A1-A2]$ located only $1.45cm-1$ above $\nu 1$. whose origin is at $3033.52cm-1$. The coupling constant of the resonance is $k1456/22=3.67cm-1$. 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 $2680-2615cm-1$ region, containing the $2\nu 4\circ$ and $2\nu 42$ 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.