Dynamics of photo-injected electrons in amorphous and crystalline ice layers on metal surfaces

Uwe Bovensiepen, Fachbereich Physik, Freie Universität Berlin, Arnimallee 14, 14195 Berlin, Germany

Two-photon photoelectron spectroscopy (2PPE) is used to investigate femtosecond dynamics of electron transfer, localization and solvation in amorphous ice layers adsorbed on Cu(111). Electrons, optically excited in the metal, transfer into the conduction band of the ice layer and generate a feature e_CB in the angle-resolved 2PPE spectra. Electrons localize within the first 50 fs and form a state e_S at 2.9 eV above the Fermi energy. The binding energy of e_S increases by 270 meV/ps which is attributed to electron solvation. By separating e_S and e_CB the bottom of the conduction band in the ice layer is determined to occur 1.05 eV below the vacuum level. By reducing the water coverage below 2 bilayers (BL) an increase in the stabilization rate of solvated electrons is observed, which is attributed to structural changes of the solvation shell accompanying the percolation of ice islands at 2-3 BL.

D₂O layers adsorbed on Ru(001) exhibit significantly different electron dynamics for amorphous and crystalline layers. In amorphous layers, electron solvation (stabilization rate of 1 eV/ps) and the population decay occur much faster than for the Cu substrate, which is attributed to structural differences at the first bilayer and more efficient backtransfer of solvated electrons to the Ru substrate. In crystalline ice layers electron solvation is not observed, however electrons are trapped in extremely long-lived defect states.