Dynamics of electron transfer at polar molecule–metal interfaces: the role of thermally activated tunnelling

Heterogeneous electron transfer (ET) across interfaces is frequently discussed on the basis of Marcus theory taking into account the rearrangements of the solvent along a nuclear coordinate q. The ET 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 ET versus direct ET due to the strong electronic coupling to a metal substrate. As a model system, we investigate the ultrafast dynamics of ET 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 ET rate is independentof temperature within the first 500 fs after excitation, which demonstrates that for this system interfacial ET occurs in the strong-coupling limit and that thermally assisted tunnelling plays a negligible role.