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In Resonance Raman Spectroscopy we often want to avoid the fluorescence. But what is the problem with fluorescence . What we want is a shift between exciting line and emitted radiation and both can show the vibration energy difference??

also from "What is the difference between Raman scattering and fluorescence?" I come to know the difference between Raman and fluorescence is that of lifetime of the molecule in the excited state, but how does a molecule come to know that it is studied for Raman spectroscopy so its lifetime in excited state is small or it is studied for fluorescence.

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Let's put it clear first: for Raman scattering there is no excited state at all, the light just bounces of a molecule. If the photo has the right energy, it can bring the molecule to an excited state. Different things can happen to a molecule in this state - in most of the cases the energy will be dissipated through collisions, but in a rare case the molecule will fall back and emit a photon it will fluoresce.

The energy of emitted photon depends on the configuration of the surroundings of the molecule - different molecules will emit photons with somewhat different energies and lines in the emission spectrum will broaden. Raman scattering does not involve an intermediate state, so it doesn't produce the side effects associated with it.

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  • $\begingroup$ The question is about resonance Raman scattering, in which case one can't quite dismiss an intermediate state so quickly. Consider an isolated molecule. The interaction forms a mixed ("intermediate") state that can persist indefinitely. The real distinction is whether or not the mixed state can dephase. If it does then we call it fluorescence, if it does not we call it Raman scattering. An isolated molecule cannot dephase, so in a sense there is no distinction in that case, or perhaps no fluorescence. The line between the two descriptions is fuzzy in any system. $\endgroup$ – garyp Mar 2 '17 at 13:21
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In Raman scattering the photons from the source creates an induced dipole moment in the molecules by exciting them to a virtual energy state. The virtual level (Ev) can be described as a linear combination of real states. Ev=aE01+bE02+cE03+... where E01, E02 are real states which could be either vibrational or rotational or electronic. In general, For any given frequency, there can be a virtual state. So for Raman scattering to occur, the incident frequency is not important. We call Raman scattering Resonance Raman scattering when the virtual state coincides with one of the electronic state i.e when the laser energy is same as one of the electronic energy level.In this case Raman scattering is highly enhanced.

But fluorescence happens only when the incident frequency matches with one of the electronic energy level. Here the photon is completely absorbed and a new photon with different frequency is emitted.So in Resonance Raman spectroscopy fluorescence often creates problem.

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