The dependence of dichroism in VUV-photoemission on the optical properties of adsorbates: Cs-monolayer on W(110)

We report on the dependence of dichroic photoemission excited by polarised light on the angle of photon incidence. We measured the difference in the angle-resolved photoemission intensities from Cs-Sp semi-corelevels excited by photons of opposite helicity, known as CDAD and of orthogonal linear polarisation-LDAD. We found that the dependence of dichroism on the angle of incidence can not be described by the equations used in the photoemission model for free atomic states. A complex consideration of the parallel and perpendicular components of the vector potential of the exciting light is needed, including the phase shift between these components. The phase shift is produced by the complex index of refraction. Using the Fresnel-equations for structureless media, one finds that in CDAD measurements not only the third component of the Stokes vector is responsible for the differences in the excitation process, but also the second component and vice versa for LDAD. Both Stokes components change the sign if the initial helicity or linear polarisation is reversed resulting in an additional dichroism just of the other kind. (C) 1998 Elsevier Science B.V.