Femtosecond Study of Geminate Electron-Hole Recombination in Neat Alkanes

摘要

The fate of an electron upon ejection into a liquid has been of great interest. In nonpolar liquids, where there is only weak stabilization of the parent cation and the electron, the dynamics are thought to be controlled by the mutual diffusion of the electron-cation pairs in a Coulombic field. If the electron thermalizes before it reaches the Onsager radius, the distance where the Coulombic binding energy equals the thermal energy of the electron, there is a high probability of geminate recombination. The rate of geminate recombination is determined by the electron's excess kinetic energy and scattering cross section in the liquid. Electron scattering is related to the packing and shape of the solvent molecules, which also determine the electron mobility. The main focus of this letter is to study the influence of these solvent properties on the rate of geminate recombination. In this letter we present preliminary results on the first femtosecond study of geminate recombination of electrons in nonpolar liquids. The experiments reported here are done in two neat solvents, n-octane and iso-octane (2,2,4-trimethylpentane). These liquids highlight the very strong dependence of electron motions on the shapes of the constituent molecules, which in turn determines the structure and disorder in the liquid. This strong dependence on structure is revealed by the observation that iso-octane has a mobility nearly 200-fold greater than n-octane. It should be noted that their densities are the same (within 1.5%). In this study we ask how this large structural effect influences the recombination dynamics in these two isomeric liquids. Keywords: Isomers, Reprints. (aw)