In situ time-resolved second-harmonic generation from Pt(111) microfacetted single-crystal platinum microspheres

In sim time-resolved second-harmonic generation (SHG) signals have been obtained from well-defined Pt(111) facets of micrometer dimensions formed spontaneously on the surface of Pt single-crystal microspheres grown by melting/cooling techniques in the ambient atmosphere, Plots of the intensity of the SHG signal, I_pp(2ω), versus the potential, E, where pp refers to p-input and p-output polarization, recorded while E was scanned linearly between two prescribed values at a rate of 50 mV/s in 0.1 M HClO₄ aqueous solutions were found to be linear in the range 0.05-0.6 versus RHE and, thus, in excellent agreement with the behavior believed to be characteristic of larger Pt(111) surfaces prepared by more conventional techniques. Measurements were also performed in which the input voltage to the potentiostat was repeatedly stepped (>500 000 times) between 0.1 and 0.7 V versus RHE at frequencies of 250 Hz, while accumulating I_pp(2ω) signals from a single Pt(111) microfacet. In situ I_pp(2ω) transients acquired with submicrosecond resolution revealed a damped oscillatory behavior both in the actual potential of the Pt single-crystal microspherical electrode, as measured with a high-impedance oscilloscope, and also in I_pp(2ω). These oscillations lasted for ca. 100 μs before the potential and I_pp(2ω) settled at constant values. Extensions of this overall strategy to the acquisition of time-resolved SHG measurements of well-defined surfaces in even shorter time domains are discussed.