The Raman scattering of two vibrational overtones of benzene is found to undergo strong preresonance enhancement as the excitation frequency approaches the state, the lowest lying, forbidden electronic transition at 265 am. These preresonance active modes are the first overtone of the 606 cm-l eZS fundamental, which is responsible for the vibronidy induced 'Bzu intensity, and the first overtone of the 848cm-'el, 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 dipoleforbidden electronic transitions to fhe 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-' overtone results from the vibronic activity of this fundamental in the 'BZU transition. On the other hand, the 848crn-' overtone derives its lB2, resonance activity from the change in its fundamental frequency (to 585 cm-') in this excited electronic state. Furthermore, the 848 an-' overtone activity is built upon significant background preresonance contributions from other higher lying electronic transitions.
INTRODUCIlON
Recent experience has shown how the preresonance and resonance Raman scattering behavior of molecules can provide much detailed information concerning excited electronic states. Most of the experimental and theoretical studies of the resonance Raman effect have naturally dealt with the scattering of fundamentals since, generally, overtone (and combination) scattering is simply a weaker phenomenon. Sources of fundamental Raman scattering cross-section are well characterized. *
Theory also shows how the origins of overtone (and combination) preresonance Raman scattering are significantly different from those of fundamental scat-teri~~g.*-~ The leading contribution to the scattering cross-section of a totally symmetric fundamental is derived from an excited electronic state potential energy surface which is displaced along appropriate nuclear coordinates relative to the ground state. This origin of fundamental scattering has been called 'A-term' activity.
However, a leading source of preresonance overtone scattering may be derived from both a change in force constant in an excited state potential energy surface as well as a displacement of that surface along the appropriate nuclear coordinate relative to the ground state potential surface. For each of these sources to be strongly active in the preresonance region, the excited electronic state should be dipole coupled to the ground state. Nontotally symmetric fundamentals derive their t Supported by grants from the National Science Foundation, the