NEAR-THRESHOLD PHOTOIONIZATION SPECTRA IN NONPOLAR LIQUIDS - A GEMINATE PAIR BASED MODEL

A three-step model for the photoproduction of fully separated charge in amorphous molecular liquids is examined from the point of view of the commonly found power law (in excess energy above threshold) for photoconductivity spectra in such systems. The three steps involve (1) absorption of light; (2) production of a geminate charge pair; and (3) geminate pair escape. Onsager's theory of geminate pair diffusion in step 3, and a simple ballistic model to describe the process of electron thermalization in step 2, are used to derive analytic functions for the dependence of the geminate pair escape probability on photon energy. These functions are shown to fit power laws surprisingly well. They are fit to photoconductivity spectra from a variety of sources to give two parameters: an ionization potential, which corresponds very closely to the ionization potential obtained from the empirical power law fits, and a length parameter, which is taken to be related to the thermalization length, or an average geminate pair radius. The latter proves to be unexpectedly sensitive to the nature of the ionizing photodonor state, pointing either to a weakness in the simple model for step 2 or to actual important vicinal effects by the photodonor as the electron emerges from it and the geminate pair is created.