The KMC-3 XPP beamline at BESSY II Helmholtz-Zentrum Berlin für Materialien und Energie

The KMC-3 beamline is installed at teh bending magnet of the BESSY II synchrotron light source. It provides focused beam of monochromatic X-ray light at energies between 2.2 and 14 keV. It is dedicated to two experiments: X-ray Pump Probe (XPP) and CryoEXAFS.


Introduction
The XPP/KMC-3 is a middle range X-ray beamline providing monochromatic light between 2.2 and 14 keV for di raction and absorption spectroscopy.Optionally the monochromator can be easilly removed from the optical path providing focussed white beam at the sample.Two permanent experiments mounted in the experimental hutch are dedicated to time-resolved x-ray di raction and absorption spectroscopy experiments (EXAFS, XANES).In addition, the beamline equipment comprises an ultrafast laser as a pump source for time-resolved experiments.

Instrument Application
The KMC3 beamline is rather versatile and may be used for di erent experiments, including energy dispersive re ectometry and di ractometry.However, the main goal is to provide the monochromatic beam for time resolved di raction and absorption spectroscopy experiments.Typical experiments which can be performed at the beamline are mainly variations or combinations of the two permanent experiments mounted at the beamline: XPP-di raction and CryoEXAFS.

X-ray Pump-Probe Di raction
XPP experiment is run by the Joint Research Group between HZB and University Potsdam, Prof M. Bargheer.80 cm diameter vacuum vessel is mounted around the focal spot of the last mirror.It encompasses a sample goniometer with a cryostat and slit/pinhole system to precisely tune the footprint at the sample.Pulsed laser beam is introduced into the beamline before the sample chamber, and can be focussed to the same spot at the sample.Di erent X-ray detectors are mounted outside of the vacuum and can be rotated up to scattering angle of 90 • (Helmholtz-Zentrum Berlin für Materialien und Energie, 2016; Iurchuk et al., 2016;Navirian et al., 2014;Roshchupkin et al., 2016;Vadilonga et al., 2017).

CryoEXAFS
Cryo EXAFS is permanently mounted at mobile table, and can be connected to vacuum after the di raction experiment.Having only several two Be windows in the optical path allows to perform EXAFS and XANES experiments down to K-line of Sulphur (experimentally not yet veri ed).However, measurements were already performed (not yet published) on Potassium K-edge (3.6 keV) and Ruthenium L3-edge (2.8 keV) using user-supplied experimental chambers.Standard EXAFS experiment is mounted in vacuum (optionally He-atmosphere) and works in transmission and uorescence geometry.The experiment is provided by the Cooperative Research Group of Prof. H. Dau, Free University Berlin (Görlin et al., 2016;Zaharieva et al., 2016).

Source
Source characteristics of the BESSY II dipole magnet 13.2 are summarized in the table 1.

Optical Design
The optical layout of the beamline is shown in gure 1.The bending magnet source D 13.1 is sagittaly and meridionaly collimated by the rotational paraboloid mirror M1.It is located at a distance of 16.9 m from the source.The double crystal (DCM) monochromator is located at a distance of 21.9 m from the source.Then the beam is refocused by the rotational paraboloid mirror M2 located at a distance of 24.0 m from the source (Fig. 1).

Monochromatic beam
In normal operation the beamline employs the Si monochromator and both mirrors M1 and M2.This can be used for a wide range of experiments providing a monochromatic, tunable x-ray beam horizontally and vertically focused on the sample position.A spatial resolution of about 150 µm can be achieved in this way over the whole energy range.E nergy resolution is ∆λ /λ ≈ 4525˘5000 depending on the energy (Fig. 2).

White Beam
In this con guration only the mirror system without the monochromator are in the optical path.The rst monochromator crystal is vertically translated out of the beam and M2 is lowered 25 mm to receive the white beam.The exit window, as well as the experimental setup must be manually lowered 25 mm to accommodate the white beam.The energy spectra of the beamline in two modes are shown in the gure 3.

Technical Data
The beamline is built in Ultra-High-Vacuum windowless technique separated from the experiment by 150 µm thick beryllium window.The optical concept incorporates two focusing/refocusing options, which can be used alternatively to provide a large exibility in terms of desired focal size, energy resolution (Fig. 2) and photon ux (Fig. 3) for di erent experiments.Depending on the ux requirements, experiments can be mounted in focus or at the distance between 1 m and 2 m behind the focus.Permanent di raction XPP experiment is mounted in focus to match the focus of the pumping laser.CryoEXAFS experiment is mounted 2 m behind the focus to avoid the radiation damages.Figure 4 shows the beam cross section at di erent distances from the focal spot.

Figure 2 :
Figure 2: Energy bandwidth of the monochromatic beam.

Figure 3 :
Figure 3: Comparison of the ux of the white and monochromatic beam in the focal spot.

Figure 4 :
Figure 4: The shape of the focal spot at di erent positions in the experimental hutch.

Table 3 :
Technical data of the KMC-3 beamline.