Ionization, charge separation, charge recombination, and electron transfer in large systems

Ionization of an isolated large molecule is discussed as a prototype of charge separation, in which the electron is removed in the absence of external environmental effects. The high density of electronic Rydberg states and the large distance over which the electron is transported suggests a picture where the motion of the electron is taken to be incoherent with strong damping due to coupling to the rovibrational modes. The theory predicts a delay between activation and ionization, which has already been observed in both large molecules and in clusters. The relevant physical parameters include the size and constitution of the molecule or cluster, its initial temperature, and the wavelength of the photoexcitation. Particular attention is given to the competition of the delayed ionization with other processes such as dissociation. The incoherent limit discussed here, where the molecule acts as its own solvent, is qualitatively different from the well-known approach based on the work of Marcus and further developments where the motion of the electron is coherent. The implications of the incoherent model can be extended to a broad range of experiments involving charge separation or charge recombination and their competition.