Is an emission spectrum really independent of excitation wavelength?

  1. 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.


    Dave
     
    Last edited: Sep 6, 2013
  2. jcsd
  3. Ygggdrasil

    Ygggdrasil 1,572
    Science Advisor

    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
    (free version: http://usna.edu/Users/physics/mungan/Publications/Pub-Nature.php)
     
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