Differentiation between resonance Raman scattering and single vibrational level fluorescence 5100 cm−1 above the 1B2u origin in benzene vapor by means of excitation profiles

We have measured the absolute Raman scattering cross section of the 992 cm−1 a1g ring breathing mode of benzene vapor in the 150 cm−1 region surrounding the 610150 vibronic transition to the 1B2u excited electronic state. We clearly observe emission directly from the resonant 610150 level, located ∼5100 cm−1 above the 1B2u origin. This Raman-like resonance emission is observed as an enhancement of the 992 cm−1 vibrational mode fundamental, as well as in the appearances of ν1 overtones and the progression 2ν6+nν1. Although the resonance emission from 1B2u vibronic levels has previously been denoted as ‘‘single vibrational level fluorescence’’ (SVLF), resonance fluorescence (RF), or hot luminescence (HL), we find by means of the analysis of the 992 cm−1 excitation profile that the resonance emission also contains a significant resonance Raman scattering (RRS) component. For excitation into the 610150 vibronic level of benzene vapor, the high density of background states within the molecule promotes processes such as dynamic intramolecular vibrational redistribution. These processes result in the intramolecular dephasing which determines the relative fractions of HL and RRS present in the emission from the ensemble of ‘‘isolated’’ molecules that give rise to the structured emission of vapor phase benzene. RRS derives from scattering from a superposition state while HL emission derives from an excited molecular eigenstate.