EPR spectroscopy endstation at the Novosibirsk free electron laser facility: Status 2025
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The electron paramagnetic resonance (EPR) endstation at the Novosibirsk free electron laser facility allows performing continuous wave and time-resolved EPR experiments under pulsed irradiation of the system under study by terahertz (THz, 25-111 cm−1, 90-400 μm; 0.75-3.3 THz) radiation. Recently, several improvements of experimental equipment and software have been carried out at the endstation to realize the pulsed EPR spectroscopy, including the use of THz radiation pulses. Herein, we present the current status of the EPR spectroscopy endstation. First, an X-band pulsed EPR spectrometer is described, which was developed for spin relaxation measurements and for applications in pulsed dipolar and hyperfine spectroscopy. The Atomize modular open source software is used to control the spectrometer. The software allows the operator to observe the processed signal (after Fourier transform, phase cycling, etc.) in real time, both for setting up the experiment (selection of magnetic field, setting of pulse intervals, etc.) and for monitoring the progress of the experiment. Second, two specialized probeheads are presented, which were created to perform different types of experiments under THz radiation in Faraday geometry. The first is a microwave probehead for pulsed EPR experiments, while the second allows an inductive detection of temperature-induced magnetization dynamics. To evaluate the performance, several coordination compounds were investigated using low-energy pulses of a terahertz free electron laser of the Novosibirsk free electron laser facility. These experimental capabilities were applied to investigate THz-assisted processes in a family of field-induced Co(II)-based single-ion magnets (SIMs) with S = 3/2, whose structural and magnetic properties had been thoroughly characterized in prior studies. By varying the wavelength of THz radiation, three cases were studied in detail: (i) off-resonance irradiation, (ii) THz-pumping of mS = −3/2 ↔ mS = −1/2 transitions, and (iii) THz-pumping of mS = +3/2 ↔ mS = +1/2 transitions. Experiments (i) revealed temperature-induced magnetization dynamics caused by lattice heating under THz irradiation. Experiments (ii) and (iii) clearly demonstrated direct excitation of spin transitions by THz radiation, resulting in a non-equilibrium population of the ground state. By tuning the THz wavelength, the SIM magnetization could be polarized in both directions: toward “heating” or “cooling”. In absolute values, at a base temperature of 3.4 K, THz excitation induced a 0.4 K temperature shift in the spin system. This work was supported by the Russian Science Foundation 23-73-00042.