Long-distance photoassisted electron transfer

The probabilities of one-photon- and two-photon-assisted electron transfer from a donor to an acceptor through the energy gap in semiconductors are calculated for a two-band model. We show that one-photon-assisted transfer of electrons in the energy gap of semiconductors is possible for a distance of 10–20 bonds (i.e., 15–30 Å). The efficiency of two-photon-assisted electron transfer decreases much slower with the distance that may lead to a very large (∼1000 Å) photocharge separation under high light intensities. We suggest the use of ‘‘localized-donor– semiconductor–localized-acceptor–semiconductor,’’ delocalized semiconductor layered structures (for example, quantum wells), or a similar kind of superlattice structure for detection of this effect. We propose to employ a thin semiconductor layer with a specific nonhomogeneous spatial distribution of donor and acceptor impurities as the elements of ir-to-visible image conversion and optical memory. We also discuss two new modifications of four-photon parametric spectroscopy for detection of long-distance photoassisted electron-transfer states in the proposed structures. We show that a polarization technique may allow one to extract information about donor-acceptor transitions on the nonresonant background. One- and two-photon-assisted long-distance electron transfer and four-photon parametric processes on donor-acceptor transitions can serve as examples of a new nonlinear optics: the nonlinear optics of photoassisted electron transfer.