Fluorescence and Phosphorescence

The normal, unexcited condition of an atom or molecule is called its ground state (S0). When a photon of light collides with a chlorophyll molecule, one of its outermost electrons is knocked up into a higher-energy orbit, a condition called the excited state; chlorophyll has three such excited states relevant here - a first singlet state (S1), a second singlet state (S2), and a first triplet state (T1). Which excited state an electron reaches depends on which colour of light struck the molecule: red light (lower photon energy) excites an electron only as far as the first singlet state S1, an inherently unstable condition with a half-life of about 10^-9 seconds, while blue light (shorter wavelength, higher photon energy) excites an electron all the way to the second singlet state S2, an even less stable condition with a half-life of under 10^-12 seconds. Because both S1 and S2 are unstable, the excited electron in each case falls back down toward the ground state S0, releasing its extra energy by one of several possible routes. In fluorescence, an electron in the first singlet state S1 drops directly back to the ground state S0, releasing its excess energy immediately, as light in the red region of the spectrum - the pathway is simply S1 to S0, and because the emission happens essentially instantly upon absorption, fluorescence is described as the immediate emission of previously absorbed light. In phosphorescence, the route is longer: an electron in the second singlet state S2 first drops to the first singlet state S1, losing part of its excess energy as heat rather than light; from S1 it drops further still into the metastable first triplet state T1, which has a comparatively long half-life of about 10^-3 seconds; only from this triplet state does the electron finally fall to …