Dynamics of electron transfer at polar molecule-metal interfaces: The role of thermally activated tunnelling Short Title: Dynamics of electron transfer at polar molecule-metal interfaces

Heterogeneous electron transfer across interfaces is frequently discussed on the basis of Marcus Theory taking into account rearrangements of the solvent along a nuclear coordinate q. The electron transfer process itself occurs via tunnelling through a barrier normal to the interface. The key point is not whether tunnelling occurs, but whether thermally activated solvent fluctuations initiate the tunnelling. Here, we discuss the role of thermally activated tunnelling in heterogeneous electron transfer versus direct electron transport due to strong electronic coupling to a metal substrate. As a model system we investigate the ultrafast dynamics of electron transfer at amorphous ice-metal interfaces (4-6 bilayers D2O/Cu(111) and Ru(001), respectively) by time-resolved two-photon photoelectron spectroscopy. We find that the electron transfer rate is independent of temperature within the first 500 fs after excitation, which demonstrates that for this system interfacial electron transfer occurs in the strong coupling limit and that thermally assisted tunnelling plays a negligible role. PACS: 73.90.+f, 78.47.+p, 79.60.Dp, 31.70.Hq 1 University of Minnesota, Department of Chemistry, Mineapolis, Minnesota 55455, USA 2 Author to whom any correspondence should be addressed.