Excitation Profiles for Totally Symmetric Modes

A characteristic feature of scattering for totally symmetric modes is that contributions arise generally from all the vibrational levels of the resonant electronic state. The A-term scattering intensity depends on the square modulus of the scattering tensor, which is given by (resonant part only) 

The cross terms appearing in the double summation of Eq. (20) represent the interference between the contributions from different levels v, v . This may lead to an increase or decrease in scattering intensity compared with that resulting from the sum of individual resonances [first sum in Eq. (20)]. Destructive interference is possible because, when the Raman FC factors are individually non-zero, the closure sum rule ( 0 v v n = 0 for n 0) implies that, for some values of v, 01vXv n must be 

An interesting consequence of the long lifetimes for discrete resonance is that the molecule may perform numerous rotations before re-emission. This causes depolarisation of the resonance-scattered radiation. For totally-symmetric vibrational modes in the normal Raman effect the scattering is polarised since the lifetime of the intermediate state is very short compared with a rotational period ( 10 s). 

With large bandwidths, F hcu, the absorption profile is smooth, showing no vibronic structure, and the excitation profiles appear similar. The excitation profile of a fundamental will usually peak near the FC maximum of the absorption band if not, this may indicate more than one electronic transition under the same absorption band contour. An increase in the bandwidths with increasing vibrational quantum number in the excited state can lead to an excitation-profile peak red-shifted relative to the absorption band maximum (25). Even with smooth absorption bands the overtone profiles need not peak at the same energy as that of the fundamental and Eq. (20) predicts a progressive blue shift (each shift hoc) with increasing overtone number. This has been observed for the bands of Wolffram s red and the [FeBr4] ion (28,29) (Section 4). 

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