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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.

 

 

 



The end-stations offer to the users a wide range of combined spectroscopies. In brief, the following spectroscopies are available at each endstation:
Branch A; PES (Photoelectron Spectroscopy), ResPES (Resonant Photoemission Spectroscopy), ARPES (Angle Resolved Photoemission Spectroscopy), XAS (X-ray Absorption Spectroscopy) in total electron yield (both drain current and a channeltron are available), partial electron yield (using the electron analyzer), total fluorescence yield (photodiode), partial fluorescence yield (using the fluorescence grating spectrometer), XMCD (X-ray Magnetic Circular Dichroism) in remanence, X(M)LD [X-ray (Magnetic) Linear Dichroism] in remanence, SXES (Soft X-ray Emission Spectroscopy) and inelastic scattering spectroscopy RXES (Resonant X-ray Emission Spectroscopy), Low Energy Electron diffraction (LEED), Surface preparation and sample growth facilities. Variable sample temperature 40 K-2000 K. Optical ports for laser in-laser out. UHV environment. Currently used also for pump-probe time resolved XAS in total fluorescence yield.

 


Branch B; XAS (X-ray Absorption Spectroscopy) in total eletron yield (drain current) and total fluorescence yield (photodiode), XMCD (X-ray Magnetic Circular Dichroism) and X(M)LD [X-ray (Magnetic) Linear Dichroism] under high magnetic fields (superconducting magnet) and at variable temperature 1.8 K-340 K. Optical ports for laser in-laser out. A surface preparation and sample growth facility is in preparation. UHV environment.
 

Branch C; Available for user endstations (long-term projects)

THE ENDSTATIONS (click to open a new page)


UNDULATORS: ENERGY RANGES

Since BACH employs two different undulators for different energy ranges, the user must select properly the undulator to work with. Important Note: the two undulators cannot be used simultaneously!!! When one of them is in use (gap below 100 mm) the other must be at the maximum gap (100 mm) and zero phase and zero tapering. If both gaps are below 100 mm a beam dump will occur and all beamlines will loose the beam!



8.1 (High Energy) From 175 eV-1600 eV First harmonic:175 eV-730 eV 2.0 GeV
    First harmonic:247 eV-1030 eV 2.4 GeV
8.2 (Low Energy) From 35/42 eV to more than 600 eV First harmonic (highest flux):35 eV-440 eV 2.0 GeV
    First harmonic (highest flux):42 eV-620 eV 2.4 GeV

Since BACH employs two different undulators for different energy ranges, the user must select properly the undulator to work with. Important Note: the two undulators cannot be used simultaneously!!! When one of them is in use (gap below 100 mm) the other must be at the maximum gap (100 mm) and zero phase and zero tapering. If both gaps are below 100 mm a beam dump will occur and all beamlines will loose the beam!



 

The photons dispersion system is based on the Padmore VASGM (Variable Angle Spherical Grating Monochromator) scheme and it includes four different interchangeable spherical gratings. The first three gratings guarantee 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.

MONOCHROMATOR: ENERGY RANGES

Grating Optimized Energy Range Resolving Power Flux with slits at 10  mm (high resolution)
SG1 44 eV-349 eV (35 eV-42 eV available under special request) high resolving power: 20000-6000 flux>1012 ph/s
SG2 158 eV-597 eV high resolving power: 20000-6000 flux>1010 ph/s
SG3 491 eV-1600 eV high resolving power: 15000-5000 flux>1010 ph/s
SG4 301 eV-1600 eV medium resolving power: 10000-2000 flux>1011 ph/s
 

 

Last Updated on Friday, 15 February 2019 09:00