Reflectance anisotropy spectroscopy (RAS) is a powerful optical probe that works on a polarization contrast basis. It can be operated in any environment, ranging from ultrahigh vacuum to vapor phases and liquids. The measured optical anisotropies are caused by several symmetry breaking effects and are exclusively assigned to the surface for otherwise bulk isotropic materials. In this work, we present a systematic study comprising in situ RAS-transient to assess the surface thermodynamics of the chloride adsorption on Cu(110) upon systematic variations of the applied electrode potentials in comparison to cyclic voltammetry (CV). Numerical time-derivatives of the measured RAS-transients are shown to be exclusively associated with electrical currents of those electrochemical reactions, which change the properties of the electrode surface. The recorded transient line-shapes track the Frumkin type isotherm properties related to chloride coverage. Both connections are theoretically discussed. Owing to the surface and interface specificity, RAS is shown to exhibit a high surface sensitivity. In particular, processes taking place in parallel, namely, the hydrogen evolution reaction (HER) as well as the copper dissolution as Cu+ and Cu2+, do not contribute to the RAS response.

In situ spectroscopic ellipsometry is combined with cyclic voltammetry to discover and quantify potential-dependent surface adsorbates and the electronic charge on copper single crystals in HCl solution. In comparison with electrochemical scanning tunneling microscopy, it is demonstrated that ellipsometry provides not only an extremely high finger print sensitivity to sub-monolayer surface modifications but that it is furthermore possible to determine qualified values with an appropriate optical model. As a critical bench mark we use the amount of adsorbed Cl– at the Cu(111) surface. In this context, we found clear optical evidence for a densified water layer at the Cu(111) surface. Particular attention is drawn to the potential range of the hydrogen evolution reaction and the catalytic efficiency of the relatively stable Cu(111) and the more open corrugated (110) surface. With the introduced ellipsometric method, we disclose a steplike increase of the surface electron excess and a decreasing lateral surface electron mobility at the onset of the hydrogen evolution reaction. Both are explained by protonation of the surface or the adsorbed water layer and demonstrate an unexpected inhibiting effect to the hydrogen evolution reaction.

While surface states (SSs) have been widely exploited under ultrahigh vacuum conditions, their appearance and role in metal–electrolyte interfaces are still a controversial debate. The existence of SSs, similar to those shown here, permits control and tunability of electronic properties of functional metal–electrolyte interfaces. Resonant excitations among them could enhance, for example, photocatalytic reactions or permit further investigations of the surface chemistry of such reactions with surface resonance Raman spectroscopy. On the one hand, SSs and other properties are readily adjustable via an applied electrical potential, that is, by promoting changes in the chemical potential favoring adsorption of ionic species. On the other hand, the presence of the electrolyte induces additional scattering and screening effects, so that the electron charge distributions can differ considerably in the presence of high electric fields as compared to the respective surfaces in vacuum. Hence, a straightforward comparison is challenging. In this work, we report on a systematic study, by means of electrochemical impedance spectroscopy (EIS) jointly with in situ reflectance anisotropy spectroscopy (RAS), which aimed to assess the evolution of surface properties and SSs occurring at Cu(110) in contact with an HCl solution. Thereafter, by modeling the RAS response and in comparison with electrochemical scanning tunneling microscopy measurements, specific surface structures have been identified and ascribed to the optical response. In a specific potential range, three additional resonances are detected in RAS that can be explained by two-dimensional confined SSs. This work renders both in situ RAS and EIS as useful tools to study and tune SSs in a systematic way by applied electrical potentials, thereby stabilizing thermodynamically preferred surface structures.