Carbon K-edge polarimetry with Cr/Sc multilayers

We investigate the polarization changes simultaneously occurring with intensity changes due to carbon contamination of optical components at two plane grating beam lines PGM-1 and PGM-2 at BESSY II. The two beam lines are very similar to each other and connect to the same APPLE II type undulator UE56/2. The spectra measured with initial horizontal and vertical polarization differ for the two beam lines. For polarimetry measurements the BESSY 8-axis polarimeter is equipped with a Cr/Sc multilayer in transmission as phase retarder and a Cr/Sc reflecting multilayer operating near the Brewster angle as analyzer. The polarimetry results also differ for the two beam lines. Only at PGM-2 we observe no decline of the degree of polarization within experimental errors i.e. the degree of polarization at PGM-2 is always found P > 0.96. We find big changes of the polarization which rapidly vary across the carbon edge. A tentative interpretation predicts the orientation of the dipoles in the contaminating carbon layers. Circular polarization is largely recovered (80% and higher) at PGM-2 thru out the carbon edge by setting the undulator shift to approximately compensate the polarizing properties of the beam line.