Knowledge Bank

The frequencies of l26 lines in the $\upsilon 2$ band of $H2$S have been determined to an average accuracy of $\pm 0.0014cm-1$. Approximately 60% of the frequencies ware measured using a tunable diode laser and the rest with a $0.04cm-1$ resolution grating spectrometer. The maximum values of J, $Ka$, and $Kc$ observed were 10, 7, and 9 respectively, The $\upsilon 2$ band measurements were combined with the available ground state microwave frequencies to obtain molecular constants for the ground and $\upsilon 2$ states. Rotational constants for Watson's AS and NS reduced $Hamiltonians2$ in both the $Tr$ and $IIIr$ coordinate representations were found. The best fits had standard deviations or 0.15 MHz $(5.0\times 10-6cm-1)$ for the microweve lines and $0.0018cm-1$ for the $\upsilon 2$ lines, Fits using the $AS-Ir,NS-Ir$, and $NS-IIIr$ forms of of the Hamiltonian all had very similar standard deviations. Fite using the $AS-IIIr$ form, though, had standard deviations approximately two orders of magnitude larger than the other fits when operators of the some power were $include1$ in all four Hamiltonian forms. The extremely poor behavior of the $AS=IIIr$ form is a result of its very slow convergence, and can he understood in terms of the unitary transformations that lead to this Hamiltonian can be predicted using a molecular force field for $H2S$ derived solely from vibrational band centre $frequencies.3$