Ultrafast electron dynamics in amorphous and crystalline D2O layers on Ru(0 0 1)

Femtosecond dynamics of excess electrons photo-injected into amorphous and crystalline D2O layers on Ru(0 0 1) have been investigated by time-resolved two-photon photoelectron spectroscopy. In the crystalline case, excited electrons are transferred into delocalized states considered as image potential states in the conduction band of ice and relax back to the metal on an ultrafast time scale. The life time of the n = 1 image potential state is <5 fs. In the amorphous case, spectral features arise from delocalized and localized electronic states. Relaxation of delocalized electrons back to the metal is as fast as in the crystalline case. The binding energy of localized electrons, however, is found to increase as a function of time delay by 1 eV/ps, which is attributed to the formation of solvated electrons. Such energetic stabilization starting at the bottom of the conduction band is clearly absent in crystalline layers. This pronounced correlation of electronic structure and electron dynamics with molecular structure is associated with the presence of localized states near the bottom of the conduction band in amorphous ice. Such localized states are absent for perfect periodic crystalline structures but prevail in amorphous systems where they serve as precursor sites for electron solvation.