Determining the optical properties of adsorbate covered surfaces by dichroism in VUV-photoemission

The dichroic effects of photoemission excited by polarised light depend on the angle of photon incidence. Measurements of the difference in the angle-resolved photoemission intensities from Cs 5p semi-corelevels excited by photons of opposite helicity, or of orthogonal linear polarisation are reported. It was thought that the dependence of dichroism on the angle of incidence cannot be described using equations omitting metal optics. A complete consideration of the vector potential of the exciting light is needed, including the phase shift between it's parallel and perpendicular components. The phase shift is produced by the complex index of refraction. Using the Fresnel equations for structureless media, one finds that the second and the third components of the Stokes vector both change their sign, if only the initial helicity or linear polarisation are reversed. The complex index of refraction of a Cs-covered W(110) surface is determined from measurements using the quantitative knowledge of the dichroism effects, that is, CDAD and LDAD.