The characteristics of luminescence, excited by long wavelength radiation, in various solid systems (ionic crystals, activated by rare earth elements; semiconducting crystals with absorbed layers of sensitizer dye; green leaves of plants) have been investigated. It was found that in all cases studied the emission arises as a result of cooperative processes, owing to interactions of two or more excited states. The role of cooperative excitation in the phenomenon of sensitization of various photochemical and photophysical processes is pointed out.
P. P. Feofilov and V. V. Ovsyankin, "Cooperative Luminescence of Solids," Appl. Opt. 6, 1828-1833 (1967)