Photoinduced free carrier yields in high fields: Field dependent distribution functions

Free charge carrier generation which is photoinduced by excitation of charg e transfer complexes or by formation of exciplexes has often been interpret ed in terms of large "initial" separations between geminate carrier pairs. Such large separations are in contrast to the contact geometry of ground st ate charge transfer complexes and the relatively short range electron trans fers expected in exciplex formation. The large separations appear to be req uired by Onsager analysis of the electric field dependence of the free carr ier yield. Almost invariably the analysis is made under the assumption that the initial distribution of pair radii with respect to the applied field i s spherically symmetric. Here we propose the possibility that the angular d istribution function might be nonspherical and suggest a simple modificatio n of the Onsager analysis to include a field-dependent angular distribution of pair radii that is Boltzmann-weighted. The resulting electric field dep endence is shown to be much steeper than for the corresponding spherical di stribution and provides the opportunity to interpret experimental data in t erms of substantially smaller initial separations.