Knowledge Bank

The Raman spectra of degenerate fundamental bands of $C3v$ molecules consist of two sets of transitions, one with $\Delta K=+1$ and the other with $\Delta K=-2$ (taking $\ell as+1$ and letting K take both $+$ and $-$ values). From these two sets one may determine $A0$ uncoupled from the other molecular parameters. We have tested this method experimentally for several of the methyl halides, and we have found that both sets of transitions are in fact observable for one or more bands of each of them. From a least squares analysis of ground state combination differences from $\nu 4$ and $\nu 5$ of $CH3Br$ we have obtained a preliminary value of $Ao=5$,1799 $\pm$ $0.0015cm-1$, a value at least one order of magnitude better than what was previously available for this molecule. (In the fit all the J dependent parameters known from microwave and ultra-high resolution infrared spectra were constrained at their known values.)