Far infrared absorption and rotational motion of CH3F and of CH3F-argon mixtures at high pressures to 1000 bar.

FIR-absorption spectra of methyl fluoride (CH3F) as a pure fluid and as a one mole percent component in mixtures with dense argon and CF4 have been measured. A frequency range between 10 cm -1 and 200 cubic meter -1 was covered. Temperatures of -10,69 and 125 degree C were used. Total sample pressures extended to 1000 bar. PVT-data permit the discussion of the spectra as functions of density. Measurements were made with a modified Fourier-transform IR-spectrometer. An optical absorption cell for use to 1000 bar and 200 degree C is described. The molar absorptivity at 69 degree C has a pronounced maximum at 40 cm -1 at all temepratures and densities which indicates hindered rotation. This absorption decreases with growing density and an increasing absorption develops above 70 cm -1 caused by intermolecular libration. The suppression of the CH3F-rotation in the dense argon is much weaker than in the pure polar fluid. The intermolecular mean squared torque increases linearly with density with the pure fluids CH3F and CHCl2F and about four times as strongly as with the compounds diluted in argon. Spectral fourth moments of the power spectra and time correlation functions for pure fluids and mixtures are compared to show the influence of dipole interaction on rotation and libration. Spectra obtained wit CH3F in high pressure CF4 are analogous to those with argon as solvent