High-resolution mid-IR molecular absorption spectroscopy of collisionally cooled hydrofluorocarbon vapours

Abstract

Rotationally and vibrationally cooled (below 100 K) high resolution (up to 0.004 cm⁻¹) mid-IR gas-phase spectra of three hydrofluorocarbons [difluoromethane (CH₂F₂), trifluoromethane (CHF₃) and 1,1,1-trifluoroethane (CH₃CF₃)] have been obtained using a Bruker IFS 120HR Fourier transform spectrometer and a collisional cooling (or ‘diffusive trapping’) technique at the Laboratory Spectroscopy Facility of the Rutherford Appleton Laboratory. To demonstrate the applicability of this technique to the simplification of dense molecular absorption features, the collisionally cooled spectra are compared with higher resolution (up to 0.0016 cm⁻¹) measurements of the gases in conventionally cooled gas cells at 296 and 200 K. The suppression of thermally populated vibrational states in the collisionally cooled spectra was particularly evident. The advantages of this technique over supersonic jet spectroscopy are discussed. Spectral features arising from the formation of hydrofluorocarbon molecular clusters were also observed.