Knowledge Bank

The Doppler-limited spectrum of the $2v1+v3$ combination band of $SF6$ at 2489.5$cm-1$ has been recorded at T=150K and 295K using a tunable difference-frequency laser spectrometer. The rotational line structure in the very dense spectrum is almost completely resolved at the lower temperature because of the reduction of hot bands and Doppler width. The observed spectrum appears to be relatively unperturbed in that a second order spherical top Hamiltonian applied to an $\ell =1F10$ vibration accurately simulates the spectrum to moderate J ($\sim 30$) values. We find that the coriolis splitting parameters k and g are remarkably similar to those in the fundamental $v3$ band; whereas the anharmonic $\Delta $B-type terms, p and v, are significantly larger. This leads to a weak bandhead in the R Branch and eliminates the Q-branch subbandheads so prominent in $v3$. A least-squares fit of this band should enable us to iteratively extend the assignments and the model and to improve the rotational constants.