Such a full tunability of the polarization is necessary on DESIRS. Indeed, except for the trivial cases of vertical and horizontal linear polarizations that will be transmitted without distortion by the optics of the beamline (the s and p components of light correspond to the principal axis of the optics), any elliptical polarization will be strongly distorted by the reflections on the optics since their incidence are neither normal nor grazing. In particular a pure Circularly Polarized Light (CLP) produced at the undulator level would be transformed, at the sample location, into a general strongly elliptical polarization whose Stokes parameters will change with the photon energy (because of the photon-energy dependence of the Fresnel coefficients of the optics reflecting material).
Figure 1: Measured ellipse at the sample level in the linear vertical polarization of the undulator. The linear vertical polarization rate is 98 % showing that this polarization is perfectly transmitted by the beamline optics.
Figure 2: Measured ellipse at the sample level in the linear horizontal polarization of the undulator. The linear horizontal polarization rate is 99 % showing that this polarization is perfectly transmitted by the beamline optics.
Figure 3: Measured ellipse at the sample level in the general tailored elliptical polarization of the emitted radiation at the undulator level. The ellipse at the undulator level has been chosen so that it transforms into a perfect circular polarization (CPL) at the sample location. The circular polarization rate is 97 % (for both helicities left- and right-CPL).
Therefore, for a large set of photon energies, it is necessary to determine the elliptical polarization to start with at the undulator level that will be transformed into a pure CPL (left and right) at the sample location. Such a determination is achieved by measuring the polarization ellipse just upstream from the sample (after the last optics) with a home-made VUV polarimeter [1] which optics can be inserted at any time in vaccuo into the beam. Based upon reflections on rotating prisms, such a device is able to measure accurately (1 % relative precision) the 4 Stokes parameters of the incoming SR : S1 (normal axis linear component), S2 (45°-tilted axis component), S3 (circular component and S4 (unpolarized component). Once the ellipse measured at a given photon energy for a given trial configuration of the undulator, the necessary changes in the magnetic fields (strength and phase) can be determined leading to a quite perfect CPL.
A comprehensive polarimetry campaign has recently allowed the full calibration of the beamline in the CPL mode in the 6.5 to 35 eV range, with measured circular polarization rates in the 92 to 99 % range.
Such a variable polarization facility allows the study of symmetry properties of matter probed in the valance shell such as alignment processes in the VUV photoexcitation, electron/ion vectorial correlations in dissociative photoionization, and photochemical and photophysical processes in chiral compounds involving several types of circular dichroisms. Some of these topics have already been investigated since the opening of the beamline in early 2008.
[1] L. Nahon and C. Alcaraz, Applied Optics 43, 1024 (2004).
