Knowledge Bank

An extended investigation of the infrared spectrum of combination, overtone, and difference bands of gaseous $SF6$ has been undertaken to elucidate the anharmonicity constants and hence the nature of excited vibrational states. Spectra were obtained on an FTIR spectrometer at a resolution of $0.04cm-1$. Some 15 combination and difference bands of $\mathrm{<mrow\; data-mjx-texclass="ORD">31</mrow>}SF6$ have analyzed by fitting a polynomial expression to P-and R-branch manifold frequencies. These analyses yield the scalar spectroscopic constants together with values for $B\zeta ,\zeta$, and the centrifugal distortion constant D. Some of these bands $(\upsilon 2+\upsilon 4,forexample)$ exhibit more than one progression in each branch duo to the presence of $P-$ and $R+$ transitions together with the usual $P+$ and $R-$; these can be separately assigned and the effective Coriolis constants $\zeta$ determined. In addition to these bands, there are about 15 more that can not be analyzed at FTIR resolution, but which have sharp Q-branch edges from which the band origins can be estimated. All of these data have been combined to yield precise values for the 21 anharmonicity constants $Xiji$ most of these have been determined with accuracies of the order of $0.01to0.003cm-1$. The higher vibrational levels of $SF6$ will be discussed.