Photodynamics in superfluid helium: Femtosecond laser-induced ionization, charge recombination, and preparation of molecular Rydberg states A subpicosecond pump-probe laser study of ionization and geminate charge recombination kinetics in alkane liquidsThe efficiency of geminate recombination after bimolecular ionization of an excited electron donor ͑or acceptor͒ is studied. For ions starting from inside or outside the recombination layer, the recombination efficiency has the opposite viscosity dependence. As a result the latter becomes nonmonotonous, provided the kinetic controlled ionization gives way to a diffusional one, creating the more remote ions the higher the solvent viscosity. This effect, first discovered experimentally, receives its explanation here, which is conceivable only on the basis of the Integral Encounter Theory of remote electron transfer in the liquid state.

In a previous article (J. Chem. Phys. 2002, 116, 2472), the nontrivial viscosity dependence of the geminate recombination efficiency was theoretically studied and briefly shown experimentally. Now, the experiments performed are reported in detail. Furthermore, the multistage kinetics of the fluorescence quenching by electron transfer to impurities is subjected to exhaustive experimental study and well fitted theoretically using integral encounter theory (IET) of the bimolecular reactions in solution. The quantum yields of free ions and triplet excitations are calculated and their nontrivial viscosity dependence is specified as well as the asymptotics of the bimolecular recombination of both ions and triplets. The annihilation of triplets results in a restoration of singlet excitations responsible for the delayed fluorescence.

A quantum-classical approximation, capable of describing the evolution of open quantum systems well beyond the applicability limits of Redfield theory is suggested. The theory is based on the short lifetime of the quantum correlations between the quantum and the classical subsystem, caused by energy dispersion ͑per degree of freedom͒ of the canonical bath. The resulting quantum-classical approximation has the form of two auxiliary differential equations and fully accounts both for the arbitrary long memory of the heat reservoir and detailed balance. These equations allow direct solution in the time domain without constructing/diagonalizing Liouville space operators, and, in combination with molecular dynamics techniques to simulate bath dynamics, may be applied to quantum subsystems with a fairly large number of levels. A simple example of a two-level system, coupled to a single correlation time canonical bath, was considered to demonstrate different regimes of approaching the canonical equilibrium state.

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