Knowledge Bank

Predictions that those normal modes which are most responsible for vibrationally induced intensity in allowed electronic transitions should show striking activity in Raman scattering are subjected to their first experimental test. The $v5(b2g)$ vibration of liquid pyrazine is known to mix an excited state with the $^{e} B_{3}u $ excited state of the 3200 {\AA} $(\Pi \ast ,n)$ electronic transition. Its pre-resonance Raman intensity has been compared with that of the vibronically-inactive mode $\nu 4(b2g)$. Assuming constant intensity for $\nu 4$ we have found that $\nu 5$ exhibits nearly ten times as much intensity for excitation with 4047 {\AA} radiation as for excitation with 6471 {\AA} radiation. The quantitative use of the data to test each of three theoretical $treatments1,2,3$ will be discussed.