Why is Fluorescence usually emitted at longer wavelength than the exciting light?
Most molecules in the ground state are in the lowest vibrational energy level. A photon can be absorbed if it provides at least enough energy to raise the ground state to the lowest vibrational level of the excited state. A slightly more energetic photon might excite the molecule to a vibrationally excited level of the excited state, but this will lose excess vibrational energy in picoseconds. Photons of still higher energy might raise the molecule to higher electronically excited states, but in almost all cases these upper states either cause decomposition or very rapidly lose energy giving rise to the usual excited singlet state. If a molecule is in an isolated environment, such as in a gas phase, then the fluorescence is emitted from the lowest vibrational level of the excited state to the ground state and its vibrationally excited levels. This emission has a spectrum which is a mirror image of the absorption spectrum. Where emission is excited by a narrow spectral souce such as a l
