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.