Perhaps the most promising candidate for a source of H2 in future hydrogen driven global energy economy is the electrolytic splitting of H2O. For the reductive half of this reaction, i.e. the hydrogen evolution reaction (HER), Pt is the current champion catalyst and its cost and scarcity limit widespread deployment of devices based on this chemistry. The HER on Pt has been studied for decades and, for much of that time, an enormous amount of effort has been expended looking for alternative catalysts with similar activity and stability, largely without success. Part of the challenge of such materials discovery efforts is that why Pt is such an excellent HER catalyst is not well understood: there is no proposed HER mechanism that is consistent with all experimental observations (e.g. that can explain the dependence on electrolyte, Pt surface structure and pH). Experimentally characterising HER mechanism is challenging: measurements under steady-state nonequilibrium conditions cannot generally distinguish intermediates from unwanted side products and elementary processes with timescales of femtoseconds to seconds are all important in understanding reactivity. We approach this problem by performing perturbation experiments in which we alter the chemical potential of Pt electrons using a femtosecond optical pulse and monitor interfacial structure using a combination of femtosecond resolved electrical and optical techniques as the electronic excitation dissipates. In this talk I will show how experiments detecting femtosecond induced photocurrents suggest that charge transfer along the Pt-H bond can be inhibited by interfacial water structure, how the results of interface-specific vibrational spectroscopy suggest that adsorbed hydrogen readily diffuses between different types of atop adsorption sites, and how this mobility appears to dramatically change at 0 V vs. RHE. Register in advance at: https://zoom.us/meeting/register/tJIpd-GgqDgrHNCGr9cfKTv8Zop4cXQSUI_S After registering, you will receive a confirmation email containing information about joining the seminar.