We report in situ time-resolved surface x-ray scattering measurements of the underpotential deposition of Cu_2+ on Pt(111) in the presence of Cl~- in HClO_4 solution. Cronoamperometric (current vs. time) measurements indicate that after a potential step, the electrode-position current decays to 1/e of its initial value in at most 0.12 seconds.In contrast, our simulataneous time-resolved surface x-ray scattering data reveal that the overlayer requires on the order of two seconds to develop long-range periodic order. These results demonstrate that the kinetics of surface ordering can be significantly different from the kinetics of charge-transfer and illustrate the power of time-resolved surface x-ray scattering for in situ studies of electrodeposition.
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