Raman antiresonance: De-enhancement of Raman intensity by forbidden electronic transitions

Abstract

The intensities of the Raman lines of the transition metal complexes Co(en)33+, PdCl4=, PdBr4=, and PdI4=, have been investigated as a function of excitation energy in the vicinity of vibronically allowed ligand field electronic transitions. Whereas resonance enhancement usually results in local maxima in Raman intensities at the energies of allowed electronic transitions, we observe minima at the energies of these electronically forbidden transitions. These minima are ascribable to interference between the weak scattering from the forbidden electronic states and strong preresonance scattering from higher energy allowed electronic states. The excitation profiles can be reproduced with a three-state scattering model, using reasonable parameters.