Comparison of Cu(111) in aqueous electrolytes and in ultrahigh vacuum: An optical second harmonic generation study

The rotational anisotropy of the second harmonic intensity has been used to provide information about the structural and electronic properties of a Cu(111) electrode surface in 0.01 M NaClO4. The experiments have involved measurement of the changes in the second harmonic (SH) intensity and relative phase of the SH response as the surface is biased at various potentials. Complementary measurements of the SH response from Cu(111) examined in ultrahigh vacuum (UHV) have also been performed and indicate that when this surface is in solution, it has very different optical (electronic) properties from the native surface at all applied potentials studied. Dosing of the sample in UHV with oxygen provides evidence for the existence of oxygen containing species on the surface in the electrochemical environment despite rigorous exclusion of molecular oxygen during both the surface preparation and the optical measurements. These studies demonstrate the utility of making comparative measurements of the surface in different environments, for which the SH technique is ideally suited.