Quantitative vibrational sum-frequency generation spectroscopy of thin layer electrochemistry: CO on a Pt electrode

Simultaneous electrochemical and vibrational sum-frequency generation (SFG) spectroscopy measurements are made of oxidation of CO on a polycrystalline Pt electrode with both CO-free and CO-saturated electrolytes. A femtosecond broad-band multiplex SFG apparatus permits rapid spectral acquisitions (5 s−1) synchronized with the potential sweep at a rate of 5 mV/s. The electrochemical cell has an ideal configuration, where the thickness of the supporting electrolyte (here 25 μm) was controlled by a spacer. The SFG lineshape is analyzed with a method that extracts the amplitude, peak frequency and Lorentzian linewidth of the CO stretch resonance as well as the nonresonant contribution from the Pt electrode. With a CO-free electrolyte, it is shown that the loss of SFG signal from CO quantitatively tracks the CO surface coverage, as determined by the integrated current or charge. The CO peak shift undergoes Stark tuning at a rate of 27.5 ± 1 cm−1 V−1 until an electrode potential slightly below the onset of CO oxidation. As CO is oxidized, the peak frequency dips and then recovers, while the peak linewidth broadens and then narrows. These effects are more pronounced with CO-saturated electrolyte. The relationships between the CO peak shift and linewidth and the microscopic dynamics of CO at the electrochemical surface are discussed briefly.