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Is an emission spectrum really independent of excitation wavelength?

by dentedduck
Tags: emission, energy, fluorescence, stoke's shift
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Sep6-13, 08:13 PM
P: 12
I've often read that the emission spectrum of a fluorescent molecule is independent of the wavelength used for the excitation. But what happens in the case of a small Stoke's shift where the excitation and emission wavelengths overlap?

If I use a narrow band excitation with a wavelength in the overlap region then the energy of the excitation light would be lower than the highest energy photons in the emission. Wouldn't that break conservation of energy? I would expect the bandwidth of the emission to be limited in that case.

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Even though the emitted photons have more energy than the absorbed photons, this does not break the conservation of energy. Rather, what actually occurs is a phenomenon called "fluorescent cooling." In essence, the laser picks off the highest energy particles from the ground state population, and relaxation from the excited state puts them (on average) into a lower energy level of the ground state.

See, for example, Epstein et al. 1995 Observation of laser-induced fluorescent cooling of a solid. Nature: 377 500. doi:10.1038/377500a0
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