  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.
