XAS and XMCD Beamlines

Soft X-ray monochromators and beamlines are the interface between modern high end 3rd generation synchrotron radiation sources and sophisticated state-of-the-art experimental stations. The combination of extreme surface quality of refractive optical elements and high precision mechanics and temperature stability makes VUV beamlines in themselves highly developed devices. Optical elements with nearly atomically flat surfaces along with their motional precision of down to 0.02” (= 5/1,000,000 of a degree) allowed BESSY to set new standards in photon energy resolution of VUV monochromators. Energy resolutions of modern VUV monochromators reach down to their physical limits, the so called diffraction limit.

The extreme quality of the VUV beamlines led to a number of outstanding experimental results in the related spectroscopic experiments. With photon energy resolutions far below 1 meV it is possible to study e.g. band gaps of superconductors, to disentangle different chemical environments of particular elements in solids, and to resolve vibrational and rotational degrees of freedom in molecules. Small VUV light spots on the samples allow for microscopic applications reaching spatial resolutions down to about 10 nm more than 100 times better than light optical microscopes. The application of the tuneability and polarization of the VUV radiation allows microscopies with element specifity and magnetic sensitivity.

Along with the enormous scientific success of X-ray circular magnetic dichroism (XMCD) techniques an increasing request for circularly or elliptically polarized sources in the soft X-ray and vacuum ultraviolet (VUV) radiation regime arose during the last 20 years. Today the most efficient and successful method to produce circularly polarized soft X-ray beams is to use polarized soft X-ray sources complemented by X-ray monochromators and beamlines which affect the polarization least possible.

The high end VUV sources in use at BESSY II are elliptical (= helical) undulators of the so called APPLE type. Here the undulator magnet structure is arranged in a way that electrons in the storage ring are forced onto helical trajectories. This leads to the emission of circularly polarized VUV radiation. Photon fluxes range up to 1015 photons/s/0.1% bandwidth @ 100 mA ring current. The brilliance, which additionally takes into account the area of the source and the divergence of the emission reaches up to 1019 photons/s/0.1% bandwidth/mm2/mrad2.

An alternative source for elliptically polarized VUV radiation is the off-plane emission from bending magnets. The degree of circular polarization from bending magnets can easily exceed 90% at the relevant excitation edges of the transition metals and the rare earth elements. Typical photon fluxes are 50 – 100 times smaller than at undulator sources.

The polarization of VUV radiation is conserved within the beamline as long as optical elements like mirrors or gratings are run far off their Brewster angles, i.e., close to normal or in grazing incidence. Thus, for circular VUV radiation every state-of- the-art grazing incidence plane-grating (PGM) or spherical-grating (SGM) monochromator is applicable.

An upcoming field in modern physics is the study of ultrafast processes in nature. These range from magnetization dynamics, ultrafast processes such as dissociation and synthesis in molecules and in surface physics, optimization of catalytic processes, the study of phase transitions like e.g. metal-insulator transitions of solids. State-of-the-art time resolution is of the order of 1 – 100 femtoseconds (fs = 10-15s). Via the Heisenberg uncertainty relation the photon energy resolution or bandwidth of a monochromator is connected to the temporal resolution achievable with the monochromatized radiation. Thus, special care must be taken in the beamline layout when dealing with ultrashort VUV pulses like in the BESSY-II femtosecond slicing setup.

The typical photon energy range of soft X-ray monochromator lies between about 50 to 1500 eV. Most of our beamlines use plane or spherical grating geometries. Plane grating monochromators (PGM) show a high flexibility, i.e. wide photon energy range without exchange of optical elements, along with extreme photon energy resolution. In some case spherical grating monochromators (SGM) can deliver higher photon flux at particular photon energies. In the case of the femtosecond slicing experiment we apply a reflecting zone plate monochromator (ZPM) because of its incomparably high transmission.

The Institute of Methods and Instrumentation supervises the following BESSY-II beamlines: List of Beamlines

The 60to6 project: an extremely wide photon energy interval beamline

The 60to6 port is one of the 3 branches of the newly developing double undulator of the Energy Material In-situ Laboratory project (EMIL) at BESSYII. The port offers unique tuneability and unparalleled brilliance between 60 eV and 6 keV and good source parameters up to 10 keV in comparison to other synchrotron sources as well as a focal point of soft and hard x-rays in the same interaction volume.

60to6 will be made available for external user experimental end-stations. Based on the long experience with the HIKE system, here the Institute of Methods and Instrumentation in Synchrotron Radiation research (G-I2) is developing the necessary investigation tools. Priority here has the development of the SpinDIVES (Spin, energy and angle resolved Depth profile, Interface and Volume Electron Spectroscopy) end-station in collaboration with Prof. G. Schönhense from University of Mainz where it is planned an advanced X-ray spectroscopy setup with a highly efficient spin filter scheme and high energy resolution, depth and lateral dimension in an energy range from 60 eV to 6 keV.