Combination-Band Progressions Involving Solely Totally Symmetric Modes

8 Combination-Band Progressions Involving Solely Totally Symmetric Modes 

When a long overtone progression is observed for a totally symmetric mode in a resonance Raman spectrum, it is not uncommon to see secondary progressions in the frequency of this mode, based upon one quantum of another totally symmetric mode. The members of the secondary progression may be designated by Vx where 

These secondary progressions may be discussed in terms of the general two-dimensional Franck-Condon factors (Ox O2 Inx n2 (nx n2 Ivx V2 . The numerical subscripts to the vibrational quantum numbers refer to the two coordinates Qx, Q2 with fundamental frequencies j i, respectively. In the independent-mode approximation, these FC factors are directly factorisable, i.e. (Ox O2 nx n2 (nx n2 vx V2 = 

The two-dimensional FC approach brings out the point that, in general, a particular Raman band, attributable to a totally symmetric mode, obtains intensity enhancement by excitation in resonance with transitions to each of the combination levels I nx n2 of the excited state. This is true even for the individual fundamentals through terms of the type 

In the case of the linear-chain compounds mentioned earlier, the secondary progressions have been discussed qualitatively assuming that the FC mechanism operates (37). The appearance of ligand modes was attributed to relaxations in the steric inter- 

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