The reactivity of activated complexes in these reactions can be quantified by the Stern-Volmer relation.
In one reaction with this type of catalysis, a gamma-hydrogen is abstracted from a ketone, producing a 1,4-diradical that may cyclize to form a cyclobutane. In that reaction of this type, the Norrish reaction, the stereochemistry of the product is determined by the degree of intersystem crossing. Porter et al. won the 1967 Nobel Prize for developing a technique employing this type of reaction that was the first to provide sub-millisecond spectroscopic resolution. The reactivity of activated complexes in these reactions can be quantified by the Stern-Volmer relation. The transition states of these reactions are called exciplexes. These reactions are often conducted in media like penta-1,3-diene, which is a (*) triplet quencher. The synthesis of xenon fluorides and the formation of halogen radicals from atmospheric CFCs both occur via one of these reactions. For 10 points, identify these reactions, in which electron excitation provides the activation energy. 
In one reaction with this type of catalysis, a gamma-hydrogen is abstracted from a ketone, producing a 1,4-diradical that may cyclize to form a cyclobutane.
light-catalyzed reactions [or photon-catalyzed reactions; or photochemical reactions; or obvious equivalents that mention "light" or "photons"]