The design of a versatile high-precision eight-axis ultrahigh-vacuum-compatible polarimeter is presented. This multipurpose instrument can be used as a self-calibrating polarization detector for linearly and circularly polarized UV and soft-x-ray light. It can also be used for the characterization of reflection or transmission properties (reflectometer) or polarizing and phase-retarding properties (ellipsometer) of any optical element. The polarization properties of Mo/Si, Cr/C, Cr/Sc, and Ni/Ti multilayers used in this polarimeter as polarizers in transmission and as analyzers in reflection have been investigated theoretically and experimentally. In the soft-x-ray range, close to the p edges of Sc, Ti, and Cr, resonantly enhanced phase retardation of the transmission polarizers of as much as 18 degrees has been measured. With these newly developed optical elements the complete polarization analysis of soft-x-ray synchrotron radiation can be extended to the water-window range from 300 to 600 eV.

A beamline for advanced dichroism ͑BACH͒, to perform light polarization dependent experiments in the 35-1600 eV photon energy range is under construction at the ELETTRA Synchrotron Radiation Source in Trieste, Italy. The radiation source, based on two APPLE-II helical undulators, is designed for high photon flux and high resolving powers. The photons dispersion system is based on a Padmore variable angle spherical grating monochromator with a typical resolving power of 20 000-6000, 20 000-6000, and 15 000-5000 in the energy ranges 35-200 eV, 200-500 eV, and 500-1600 eV, respectively. Two separate branches after the monochromator allow setting two independent experimental chambers. The photon flux in the experimental chamber͑s͒, calculated at the best resolutions achievable and with the aperture of the slits set at 10 m, is expected to be above 10 11 photons's with linearly or circularly polarized light. In addition, a fourth grating operates in the 400-1600 eV range to provide a higher flux, 10 12 photons's with smaller resolving power ͑10 000-2000͒, allowing fluorescence and x-ray scattering experiments. The refocusing section͑s͒, based on plane elliptical mirrors in a Kirkpatrick-Baez scheme, will provide on the sample, a nearly free-aberration spot͑s͒, whose dimensions are expected to be 200ϫ10 m 2 (horizontalϫvertical). In the following, the general layout of the beamline is reported and the characteristics of the optical elements, as well as the optical performances ͑resolving powers and efficiencies of the monochromator, flux, and spot dimensions͒ are described in detail.

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