Resonance Raman scattering effective lifetime

Figure Al.6.15. Schematic diagram, showing the time-energy uncertainty principle operative in resonance Raman scattering. If the incident light is detuned from resonance by an amount Aco, the effective lifetime on the excited-state is i 1/Aco (adapted from [15]).

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). 

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