We employ a surface hopping trajectory method to study the rapid nonadiabatic relaxation after an excess electron is injected in unperturbed fluid helium. Several distinctively different relaxation processes, characterized by their relative importance at different times during the relaxation to a localized equilibrium state are detailed. These processes include: Short time nonadiabatic leakage from cavity to cavity, exploring the fluctuating unperturbed solvent structure. This relaxation involves slow drifting of the occupied state through a continuum of levels. This is followed by rapid diabatic expansion of a particular solvent cavity once the electronndash;solvent forces begin to take effect on the solvent atoms in a particular region of the fluid. We also study the importance of nonadiabatic hang up trajectories in which the excess electron gets caught in the first excited state of a bistable well potential offered by a pair of closely coupled cavities in the solvent. We study the density dependence of the time scales and relative importance of these different processes and their influence on the transient absorption spectrum after electron injection into an unperturbed fluid. Though the dynamical properties of excess electrons under the conditions considered here have never been studied before, the behavior is remarkably similar to that observed in both experimental and theoretical studies of electron hydration dynamics, indicating that the processes we describe in this paper may be very general relaxation mechanisms for localization and trapping in fluids.
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