Non-Totally Symmetric Modes and Herzberg-Teller Scattering

9 Non-Totally Symmetric Modes and Herzberg-Teller Scattering 

The classic cases of the HT mechanism concern coupling between two electronic states of different symmetry or between the different components of two degenerate states of the same symmetry. An important example of the first case occurs when electric dipole transitions to one of the two states are forbidden (e.g. the Laporte-forbidden d—d and f-f transitions). In this case, the forbidden transition may acquire absorption intensity by HT mixing with an allowed transition via a non-totally symmetric mode of appropriate symmetry (the irreducible representation of the active mode must be contained in the direct product of the irreducible representations for the two states coupled by the HT mechanism). 

In the second case above, since the states have the same symmetry, both are required to give electric-dipole-allowed transitions. The most important examples of this type, in the context of resonance Raman scattering, are the much studied metal-lo-porphyrin molecules which constitute the active sites of the haem proteins, notably haemoglobin and cytochrome c (Section 4.8). The visible and near ultraviolet absorption spectra of these systems show two -n - -n transitions of the porphyrin ring both are allowed with in-plane polarisation and excited-state symmetry. The lower 

The HT coupling mechanism was first considered in the context of the Raman scattering tensor hy Albrecht (9) whose approach was briefly discussed in Section 2.4. 

The first-order terms from Eq. (6) are given in the low temperature limit (ug = 0) for the case of a single non-totally symmetric harmonic mode  

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