The THz beamline exploits intense coherent synchrotron radiation (CSR, [1-2]) as emitted from special storage ring modes, for the study of magneto optical phenomena in the energy range from 3 to 150 cm-1. A dedicated THz electron paramagnetic resonance (THz-EPR) facility combines a broad range of excitation and detection schemes with extreme sample environments (in particular high magnetic fields and low temperatures). Research topics are related to manipulation and detection of high spin states in e.g., proteins, single molecule magnets, energy materials and materials relevant for future information technologies.
The THz beamline extracts CSR from the 2° dipole source (D112) after the slicing section at an acceptance of 60 mrad (h) x 15 mrad (v). A true optical transmission line transports CSR as emitted by ultra-short bunches in the low a mode (pulse length: < 10 ps, spectral range: 3-50 cm-1) and laser-induced by Femtoslicing  ( pulse length: ~ 200 fs, spectral range: 20-150 cm-1), respectively. Complementary, FIR-UV-VIS cw radiation and 1 mJ of synchronized fs laser pulses (800 or 400 nm) are available at the experiment. Sample environments include a superconducting magnet (Oxford Spectromag, -11 T to +11 T) equipped with a variable temperature insert (1.5 K-200 K), and an optical cryostat (Oxford Optistat, T = 1.5 K- 300 K). THz detection is achieved either with a high resolution FTIR-spectrometer (Bruker IFS 125-HR, min. bandwidth: 0.0063 cm-1) in combination with ultra-sensitive liquid helium cooled Si- or InSb – bolometers or fast Schottky diode THz detectors (ACST, time resolution 250 ps). Alternatively, transient THz signals may be directly detected via a time domain (TD) THz set-up. This dedicated TD THz scheme allows for a cross-correlation of THz pulses from the storage ring with the synchronized external fs-laser source (optical pump – THz probe).
Science drivers for the THz beamline
Optical layout and schematic of the instrumental set-up at the THz beamline. THz radiation extracted from the storage ring may be readily directed towards three different detection schemes.
Main science drivers are investigations in spin coupling energies of high spin transition metal and rare earth ions. Spin coupling energies are sensitive probes of the electronic structure and determine magnetic properties of compounds with unpaired electron spins. The latter are highly desired pieces of information, as high spin paramagnetic ions determine the function of many vital catalytic processes in proteins and synthetic complexes, as well as the properties of systems with large exchange couplings, e.g. single molecule magnets, energy materials or strongly correlated solids.
BESSY II THz-EPR Setup
Frequency Domain Fourier Transform THz-EPR (FD-FT THz-EPR)
EPR is capable of providing unique information on magnetic structure-function relationships of materials containing unpaired electron spins. However, conventional single frequency EPR frequently fails in cases where spin transition energies exceed the quantum energy of the spectrometer (typically < 4 cm-1). Recently, we have demonstrated that CSR [1-2] based FD-FT THz-EPR  provides a unique tool to overcome this restriction. Our novel approach allows for EPR excitations over a broad energy (3 cm-1 – 150 cm-1) and magnetic field range (-11 T - +11 T) in a single spectrometer. FD-FT THz-EPR has been successfully applied to high spin ions in single molecule magnets [4, 5, 6] catalytic mononuclear integer HS TMI complexes  and very recently even in proteins  and in strongly correlated solid state systems .