If we observe isolated chlorophyl solution with UV light, chlorophyl is seen as red. Explanation suggests that when a specific atom of chlorophyl absorbs UV light, the atom gets excited and in a short time electrons still goes from excited state to ground state. But why it is not emits same wavelength light, and instead emits lower energy light - red ?
There are three types of re-emission when photons interact with matter:
- Emitting the same wavelength, e.g., simple reflection, resonance fluorescence and stimulated emission.
- Emitting longer wavelength due to losses in absorption of initial photon, e.g., fluorescence and phosphorescence and Compton scattering.
- Emitting shorter wavelength than that incident. This occurs in two photon or multiphoton fluorescence. It requires the molecule stay in a metastable state long enough for a second photon to raise it to a higher yet state, requiring intense light. AFAIK, there is some little literature on UV-incident multiphoton fluorescence, but I don't know if chlorophyll has been investigated.