Can fluorescence ever be emitted at shorter wavelength than the exciting light?
The simple answer to this is ‘yes’. Such emission is known as ‘Anti-Stokes’ fluorescence and is most commonly seen when absorption and emission spectra overlap substantially. The process involves coupling of vibrational energy from the sample to the electronic excitation, and is rather similar to the effects that give rise to Anti-Stokes Raman spectra. Anti-Stokes fluorescence is not much used for practical applications however, mainly on account of relatively low efficiency.
