Raman Cross-Section De-Enhancement Phenomenon in the Excitation Profile of the 1347 cm−1 Mode of Nitrobenzene

Abstract

Absolute values of the Raman scattering cross section of the totally symmetric 1347 cm−1 mode of nitrobenzene (NB) were measured over an excitation wavelength range of 260–520 nm. The data, when fitted to the Albrecht A-term preresonance approximation (AAT-PA) theory, showed the value of the enhancing state for the observed resonance to be 247 nm in wavelength units. This is considered to be in good agreement with the charge-transfer state of NB at about 250 nm. However, a deviation from the theory at around 350 nm, in the form of a de-enhancement in the Raman excitation profile (REP), was observed. This effect could not be adequately explained by invoking an interference phenomenon due to additive and subtractive combinations of the Raman tensors corresponding to the enhancing state and an electronically allowed but dipole-forbidden state, as has been attempted by others for benzene and some metal/ligand complexes. This is primarily because an alternative enhancement, either preceding or following this de-enhancement, has not been observed, which is expected from the formalism based on interference between the polarizability tensors representing the enhancing state and the relevent forbidden state. An explanation based on a competitive intersystem transition process in the vicinity of a forbidden state is presented. The forbidden n, Π* state of NB at about 350 nm is known to undergo rapid triplet-state conversion with high quantum yield. The explanation was supported by the anomalous results on the depolarization ratio of the Raman mode at the vicinity of this forbidden band.