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 polar ised light on the angle of photon incidence. We measured the differenc e in the angle-resolved photoemission intensities from Cs-Sp semi-core levels excited by photons of opposite helicity, known as CDAD and of o rthogonal linear polarisation-LDAD. We found that the dependence of di chroism on the angle of incidence can not be described by the equation s used in the photoemission model for free atomic states. A complex co nsideration 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 i ndex of refraction. Using the Fresnel-equations for structureless medi a, one finds that in CDAD measurements not only the third component of the Stokes vector is responsible for the differences in the excitatio n process, but also the second component and vice versa for LDAD. Both Stokes components change the sign if the initial helicity or linear p olarisation is reversed resulting in an additional dichroism just of t he other kind. (C) 1998 Elsevier Science B.V.