Thermalization of low energy electrons in liquid methylcyclohexane studied by the photoassisted ion pair separation technique

A spectral dependence of the charge carrier quantum yield in liquid methylcyclohexane was investigated on photoexcitation of geminate electrons at wavelengths between 425 and 867 nm. The electrons were produced by two photon ionization of the anthracene admixture with the UV laser pulse (wavelength 308 nm, duration about 20 ns) and, then, were excited by the dye laser pulse delayed for a time of 25 ns with respect to the UV one. The increase of photocurrent caused by an additional dye laser pulse action was observed as temperature decreased below 230 K. It has been found that the spectrum of the photocurrent enhancement in the range of dye laser radiation wavelengths from 425 to 867 nm is close to that of the trapped electron optical absorption. The increase of photocurrent has been attributed to photogeneration of "hot" quasi-free electrons arising from a photon absorption by geminate trapped electrons. Basing on the diffusion model of photostimulated ion pair dissociation, the thermalization length of 4.2-4.7 nm was determined for quasi-free electrons with an initial kinetic energy within the range from about 0.9 to 2.4 eV. The energy dissipation rate and effective mean free path were evaluated for the quasi-free electrons with the energy of about I eV in hydrocarbon matrix. (C) 2001 Published by Elsevier Science B.V.