DICHROIC EFFECTS IN PHOTOEMISSION FROM PURE AND OXIDIZED RUBIDIUM ANDPOTASSIUM MONOLAYERS ON PLATINUM(111)

Well-ordered monolayers of rubidium and potassium on platinum Pt(111) have been studied by LEED, angular resolved UPS and XPS excited by cir cularly polarised synchrotron radiation from the electron storage ring BESSY (Berlin). The order of pure and oxidised monolayers at T = 300 K was investigated by LEED. CDAD has been measured for the Rb 3d, Rb 4 p and K 3p states in the case of the (root 3 x root 3)R30 degrees stru ctures of pure Rb and K. Exposure of oxygen to the alkali monolayers l eads to disordered alkali oxide layers. After additional heat treatmen t ordered structures are observed again. In all cases Of ordered monol ayers CDAD asymmetries up to 30% are found. This is surprising, becaus e the observed initial states have spherical symmetry in particular, a t least the p(1/2) state. We explain these results by photoelectron di ffraction, including neighbouring adsorbate (Rb, K) and underlying sub strate (Pt) atoms. Variations of the CDAD asymmetry between 0% and 30% are found to be dependent on the kinetic energy of the photoelectrons up to 190 eV and the angle of observation, respectively.