Vibrational levels and anharmonicity in SF 6—I. Vibrational band analysis

Abstract

The vibrational spectrum of SF 6 has been recorded with a Fourier-transform i.r. spectrometer at a resolution of 0.05 cm −1 and pressure—path length products of up to 2 × 10 5 Torr-cm. Twenty-nine bands were observed. Rotational structure was resolved for 11 of these and polynomials were fitted to the observed frequencies to yield the scalar spectroscopic constants, including the band origins m and derived values of B′ B 0 and the Coriolis constants ζ. For 12 other unresolved bands accurate estimates of the origins could be made from the frequency of a sharp Q-branch edge. Three more bands (ν 3, 2ν 1 + ν 3, and 3ν 3) were not resolvable at our resolution but have been previously analyzed from Doppler-limited or sub-Doppler spectra. In addition, about 10 assignable hot bands were observed whose frequency shifts relative to the principal transitions could be accurately measured; two of these were sufficiently resolved for full scalar analyses. These frequencies were combined with results of several high-resolution Raman studies by other authors to yield the most complete data set on SF 6 vibrational levels yet obtained. Isotopic frequency shifts have also been measured. The effective Coriolis constants for combination and overtone bands of octahedral molecules are discussed.