Femtosecond dynamics of electron transfer, localization, and solvation processes at the ice-metal interface

The ultrafast dynamics of excess electrons in amorphous ice layers on single-crystal metal surfaces are investigated by femtosecond time- and angle-resolved two-photon-photoemission spectroscopy. Photoexcited electrons are injected from the metal Substrate into delocalized states of the conduction band of ice and localize in the ice layer within 100 fs. Subsequently, energetic stabilization of this localized species is observed on a time scale of similar to 1 ps, which is attributed to electron solvation by nonadiabatic Coupling to nuclear degrees of freedom of the surrounding polar molecular environment. Concomitant with this stabilization process, residual wave function overlap of the solvated electron with the metal substrate results in back-transfer by tunneling through the solvation shell. At such interfaces the correlation of electronic and molecular structure with the resulting solvation dynamics can be explored using different substrates as a template. Here we compare data on molecularly thin D2O ice layers grown on Cu(111) and Ru(001). On Ru(00 1) both the stabilization and back-transfer proceed about three times faster compared to Cu(111), which is attributed to different interfacial structures and the role of d-states, and projected band gaps in the electron transfer process.