INSITU 2ND HARMONIC-GENERATION STUDIES OF CHEMISORPTION AT WELL-ORDERED PT(111) ELECTRODES IN PERCHLORIC-ACID SOLUTIONS

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.