Preresonance Raman scattering of overtones: The scattering of two overtones of benzene in the ultraviolet

Raman scattering of two vibrational overtones of benzene is found to undergo strong preresonance enhancement as the excitation frequency approaches the 1B2u state, the lowest lying, forbidden electronic transition at 265 nm. These preresonance active modes are the first overtone of the 606 cm−1 e2g fundamental, which is responsible for the vibronically induced 1B2u intensity, and the first overtone of the 848 cm−1 e1g fundamental, an out‐of‐plane CH bending mode. In order to treat this overtone preresonance Raman behavior theoretically, the molecular polarizability is consistently expanded to second order in nuclear displacements, and thus we consider all preresonance contributions of this order. The leading contributions of strictly dipole‐forbidden electronic transitions to the scattering cross‐section of overtones are exposed. Within this theoretical context it is seen how scattering cross‐section for each of these resonance active overtones is derived from different sources. The preresonance scattering of the 606 cm−1 overtone results from the vibronic activity of this fundamental in the 1B2u transition. On the other hand, the 848 cm−1 overtone derives its 1B2u resonance activity from the change in its fundamental frequency (to 585 cm−1) in this excited electronic state. Furthermore, the 848 cm−1 overtone activity is built upon signifficant background preresonance contributions from other higher lying electronic transitions.