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Beamline Description

Overview

BACH delivers a photon beam with high intensity and brilliance in the soft x-rays energy range (35 - 1650 eV) with the full control of the polarization of the light for the realization of electronic and photon spectroscopy experiments with polarization dependence. The following techniques are implemented: XPS (X-ray Photoelectron Spectroscopy), UPS (Ultraviolet Photoelectron Spectroscopy) , XAS (X-ray Absorption Spectroscopy), XMCD (X-ray Magnetic Circular Dichroism), XMLD (X-ray Magnetic Linear Dichroism), SXES (Soft X-ray Emission Spectroscopy), RXES (Resonant X-ray Emission Spectroscopy).

The radiation source is based on two APPLE-II elliptical undulators that are used alternatively in order to optimize the flux.

The photon dispersion system is based on a VASGM dispersion system (Variable Angle Spherical Grating Monochromator) and it includes four different interchangeable spherical gratings. The first three gratings allow resolving powers of 20000-6000, 20000-6000 and 15000-5000 in the energy ranges 35-200 eV, 200-500 eV and 500-1600 eV respectively. In addition, a fourth grating operates in the 300-1600 eV range providing higher flux with reduced resolving power (10000-2000), suitable for x-ray emission experiments Three separate branches, following the monochromator, allow setting three independent experimental chambers.

The flux in the experimental chamber, measured with the high resolution gratings for linearly polarized light at the best achievable resolution, ranges between 1.2·1012 photons/s at 125 eV and 6·1010 photons/s between 900 eV and 1250 eV.  A gain of nearly one order of magnitude is obtained for the high brilliance grating, in accordance with theoretical predictions. Flux beyond 4·1011 photons/s was measured over nearly the complete energy range covered by this high brilliance grating. The results from polarization measurements confirm a polarization above 99.7% for both, linearly and circularly polarized mode at low energies. Circular dichroism experiments indicate a circular polarization beyond 90% at the Fe L2/L3 edge near 720 eV.

The photon beam is split in three branches, located downstream a common monochromator, allowing to operate with three independent end-stations (which cannot however be used simultaneously). The three refocusing sections (A-C in Fig. 1) are based on plane-elliptical mirrors in a Kirkpatrick - Baez configuration, providing nearly aberration free spots on the samples. At the branch A and B the light spot-size on the sample can be as small 250x20 μm2 (horizontal-vertical) and 250x30μm2 , respectively. However, for PES and XAS experiments, when beam damage effects are critical, the spot size can be increased. The typical beam size for PES is set to 300x300μm2.

 

 

 


Last Updated on Friday, 15 February 2019 09:00