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Abstract
The behavior of molecules at the electrochemical interface was probed using infrared visible sum frequency generation spectroscopy (SFG).
Potential dependant SFG spectra of the aqueous/alkanethiol interface in the OH stretching region are presented. Spectra have been mathematically fit to extract information used to elucidate the structure and conformation of H2O at these electrochemical interfaces and the existence of a 'flip-flop' model for water in these environments is investigated. Comparison to recent work on Ag(111)/aqueous interface and other works on hydrophobic/aqueous interfaces corroborates assignments of OH bands in the 3800-2800 cm -1 region.
SFG is used to probe the orientation of three pH accessible ionic states of p-amino benzoic acid (PABA) on an Ag(111) electrode surface as a function of electrochemical potential. Experimentally measured phase differences between resonant and non resonant SFG signals confirm features in electrochemical capacitance and cyclic voltammograms correspond to ordering and reorientation of the PABA molecule on the electrode surface as the surface potential is varied. The surface potential at which PABA reorients is quite different from the potential of zero charge (PZC) determined in neat electrolyte solutions. The measured reorientation potentials for zwitterionic PABA (ca. - 300 mV) and anionic PABA (-800 mV) reflect a substantial shift of the PZC of the electrode with the adsorbed dipolar organic species.
The dependence of single crystal Ag azimuthal rotation on resonant SFG signal line shapes is examined using broad band multiplex SFG. Detailed fitting analysis of acquired spectra is used to extract data as a function of azimuthal rotation angle. Magnitude of non-resonant background, relative phase of vibrational modes, and other parameters are reported for varying levels of background suppression achieved by adjusting temporal overlap of incident laser pulses. The extracted fitting parameters show varying degrees of symmetry depending on the selected crystal face, and transformation of resonant signal line shapes from peak, dip, to derivative-shaped features is shown to be associated with changing intensity of non-resonant background signal.
