Matthew L. Lynch

In situ second harmonic generation studies of chemisorption at well-ordered Pt(111) electrodes in perchloric acid solutions

Abstract The technique of optical second harmonic generation (SHG) is used an in situ probe of chemisorption and structure at flame-annealed Pt(111) electrodes in perchloric acid solutions. Rotational anisotropy SHG measurements demonstrate that the single crystal surface order is preserved when the electrodes are transferred to solution with an iodine overlayer and cleaned through an iodine-CO exchange reaction and subsequent CO oxidation electrochemistry. The chemisorption of both iodine and CO produces an anisotropic response from the surface with a reduced average surface symmetry due to the presence of the adsorbed overlayer. The SHG signal from the well-ordered Pt(111) surface in perchloric acid after the removal of the CO also reveals the presence of an ordered chemisorbed overlayer at all electrode potentials negative of the butterfly peaks. Although the identity of the chemisorbed species cannot be determined unambiguously from the SHG measurements, comparisons with the potential dependent SHG response from Pt(100) and polycrystalline platinum electrodes relate the SHG signal in this potential range to an ordered overlayer of a hydrogen species. The SHG measurements also confirm the electrochemical evidence that these ordered structures are modified upon the disruption of the longer range domain structure of the well-ordered Pt(111) surface.

Surface phase transition of ordered iodine monolayers on a Pt(111) electrode as studied by normal incidence optical second harmonic generation

The spectroscopic study of electrochemical surfaces provides the modern electrochemist with the detailed information required for the interpretation of charge transfer processes at solution/ electrode interfaces. Fundamental surface reactions such as catalytic oxidation and thin film growth can depend critically upon the local adsorbate structure at the electrode surface. This adsorbate structure will vary with changes in the morphology of the substrate, the concentrations of solution phase species, and the electrostatic fields at the surface. Through a combination of in situ and ex situ spectroscopic measurements, a detailed molecular level understanding of the important adsorbate-surface interactions can be obtained [l]. Optical second harmonic generation (SHG) is a surface-selective spectroscopic technique that has been applied to the in situ study of a variety of metal electrode surfaces [2-41. In a surface SHG experiment, changes in the electronic surface structure of an electrode upon adsorption are correlated with changes in the elements of the surface nonlinear susceptibility tensor x as measured by the potential and polarization dependence of the SHG signal [5]. At single crystal surfaces, additional tensor elements of x may appear due to a reduction in surface symmetry upon adsorption or surface reconstruction [6-81. In a series of recent papers [9] we have reported the use of SHG at well-ordered Pt(ll1) electrodes. A reduction in the surface symmetry was observed in the presence of adsorbed monolayers of iodine, CO and hydrogen on the platinum surface. In this note we demonstrate how SHG measurements performed at an incident angle of 0” (normal incidence) can further elucidate the reduction of surface symmetry upon