Molecular beam spectrum of the highly perturbed C–H stretching region of fluoroform

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Abstract
Assignments of the C–H stretching fundamental of fluoroform, CHF3, have been obtained from a spectrum recorded near 3035 cm−1 at sub-Doppler resolution (∼10 MHz FWHM) and low effective temperature (∼4 K) in an adiabatically cooled molecular beam using bolometric detection of molecules excited by a color-center laser. This fundamental band is highly perturbed and has resisted analysis at higher temperatures, even at Doppler-limited resolution, whereas its overtones have been the subject of several studies of intramolecular vibrational relaxation. Under molecular beam 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 ∼±5 cm−1 from the suppressed central Q branch. This peculiar structure is due to a close anharmonic and Coriolis resonance between the fundamental ν1 (species A1) and the combinations ν±4+ν±5+ν±6 (A1+A2) located only 1.40 cm−1 above ν1, whose origin is at 3033.55 cm−1. The coupling constant of the resonance is k1456/2=3.68 cm−1. Additional severe perturbations by the E components of ν4+ν5+ν6 as well as by ν2+ν4+ν6 have been identified in the spectrum.