Absorption and Emission Photons of energy hn excite electrons to the excited state. (S0 to S1, S0 to S2, etc.) Electrons in the excited state have a number of possible dexcitation paths: Resonance fluorescence (No Stoke's shift) Regular Fluorescence (Shifted S1 to S0) Phosphorescence (Shifted T1 to S0) Non-radiative deexcitation Time scales Resonance fluorescence: ~ 10 - 15 s Regular Fluorescence: ~ 10 - 9 s Phosphorescence: ~ 10 - 3 s Non-radiative deexcitation: Varies