Knowledge Bank

The 0-0 band of the $\pi \ast \leftarrow \pi$ transition of thionaphthene at $2936\backslash AA$ was recorded using an 8-meter Czerny mount spectrometer with a resolving power of approximately 500,000 and a linear dispersion on the order of $1cm/\backslash AA$. The main features of the spectrum include Q and P sub-bands and two intense maxima. Using computer simulation techniques it has been possible to determine the excited state rotational constants to a high degree of accuracy and to demonstrate conclusively the hybrid character of the band. The latter yields the direction of the transition moment. Using ground state rotational constants of $A""=0.104335cm-1,B""=0.042772cm-1$ and $C""=0.030336cm-1$ as determined from assumed bond angles and lengths, the excited state values obtained from the best fit of computed with experimental contour are $A\prime =0.100182cm-1,B\prime =0.042157cm-1$ and $C\prime =0.029671cm-1$. The prominent peak approximately $3cm-1$ to the blue of the main peak cannot be fit by pure A-type computation and is found to result from a B-type contribution, while most of the rotational fine structure is due to an A-type band. Mixing of various amounts of A-type and B-type character yields the direction of the transition moment.