THE INFLUENCE OF A LINEAR PHOTON POLARIZATION LIGHT ON MEASUREMENTS OF THE CIRCULAR-DICHROISM IN PHOTOEMISSION

We measured the CDAD from NO on Pt(lll) at the SX/700-III monochromato r at the synchrotron BESSY in Berlin. We found high asymmetries up to 60% using a photon energy of 40 eV. Surprisingly, this is higher than the degree of circular polarisation that should be between 30% and 50% for the experimental parameters used. This obvious contradiction can be explained if an additional dichroism arising from a strongly linear polarised component of the radiation at the SX/700-III is taken into account. The polarisation of this component has the orientation of +/- 45 degrees with respect to the synchrotron plane. This linear polarisa tion changes its sign if the circular polarisation is changed, too. Fi rst experiments on this linear dichroism show large asymmetries for CO /Pt(111), comparable to the CDAD asymmetries.