Integral, unified and Markovian theories of biexcitonic photoionization

The non-Markovian alternative to a conventional rate description of the bie xcitonic photoinduced processes in solutions was deduced from integral enco unter theory at sufficiently low concentration of excitations. This is the unified theory (UT) of photoionization followed by charge separation and re combination. The UT formalism allowed us to study the initial distribution of charged products of non-linear (biexcitonic) ionization as well as ion a ccumulation and recombination after delta-pulse excitation. The Markovian e quations were obtained from UT as a limiting case and their phenomenologica l rate constants were given the microscopic definitions, which related them to the charge transfer rates and encounter diffusion of reactants. In the case of short lived (singlet) excitations the non-Markovian kinetics was sh own to be more accurate and detailed than oversimplified Markovian kinetics . Only for long lived (triplet) excitations the non-Markovian and Markovian results are qualitatively similar, provided that the properly defined rate constants are used in the conventional rate equations. (C) 1999 Elsevier S cience B.V. All rights reserved.