Rapid electron trapping studied by pump-probe photoconductivity: kinetic analysis

The use of ultrafast pump-probe conductivity (PPC) for studies of photoelectron dynamics in nonpolar molecular liquids has been based upon the perturbation of geminate recombination dynamics of trapped electrons by their laser photoexcitation into the conduction band. Such a method is unsuitable for the studies of electron trapping dynamics of quasifree electrons per se. We demonstrate that the PPC method can be extended to study such dynamics, provided that the time resolution of the conductivity setup is better than the ratio mu[e_qf]tau[e_qf]/mu[e_trap] of the mobility-lifetime product for the quasifree electron and the mobility of the trapped electron. For some liquids (e.g., supercritical CO2) this time is sufficiently long (> 100 ns) and the standard conductivity equipment can be used. Even if the time resolution cannot be increased (due to the adverse effect on the sensitivity), the trapping dynamics can be studied provided that the trapping competes with cross recombination of quasifree electrons with holes in the solvent bulk. Since the mobility of these quasifree electrons is very large (10-10^3 cm^2/Vs), this recombination is facile even when the density of ionization events is fairly low (< 1 mM). Perturbation of the geminate electron-hole dynamics is not required for this method to work.