Status of the Twin Helical Undulator Soft X-ray Beamline at SPring-8: Performance for Circular Dichroism Measurements

摘要

The status of the soft x-ray beamline BL25SU at SPring-8 is reported. The beamline performance for circular dichroism measurements using the helicity switching of helical undulator radiation is focused on. Light spots on a sample of the opposite helicities have been observed with a photoelectron emission microscope. The vertical profiles of the spots have almost the same line shape and a positional difference of ~ 0.5 μm. The horizontal spot positions coincide, but the horizontal spot widths are different by ~ 7 %. The energy difference of the monochromatized light between the opposite helicities has been estimated to be less than or comparable to 1 meV in the Ne 1s absorption region. The kick angle of the kicker magnets has been increased from 0.2 to 0.3 mrad to avoid penetration of the higher harmonics.