X-band EPR setup with THz light excitation of Novosibirsk Free Electron Laser: Goals, means, useful extras

Temperature Dependence of the Sensitivity of PVDF Pyroelectric Sensors to THz Radiation: Towards Cryogenic Applications

The application of terahertz (THz) science in industrial technology and scientific research requires efficient THz detectors. Such detectors should be able to operate under various external conditions and conform to existing geometric constraints in the required application. Pyroelectric THz detectors are among the best candidates. This is due to their versatility, outstanding performance, ease of fabrication, and robustness. In this paper, we propose a compact pyroelectric detector based on a bioriented poled polyvinylidene difluoride film coated with sputtered metal electrodes for in situ absorption measurement at cryogenic temperature. The detector design was optimized for the registration system of the electron paramagnetic resonance (EPR) endstation of the Novosibirsk Free Electron Laser facility. Measurements of the detector response to pulsed THz radiation at different temperatures and electrode materials showed that the response varies with both the temperature and the type of electrode material used. The maximum signal level corresponds to the temperature range of 10-40 K, in which the pyroelectric coefficient of the PVDF film also has a maximum value. Among the three coatings studied, namely indium tin oxide (ITO), Au, and Cu/Ni, the latter has the highest increase in sensitivity at low temperature. The possibility of using the detectors for in situ absorption measurement was exemplified using two typical molecular spin systems, which exhibited a transparency of 20-30% at 76.9 cm-1 and 5 K. Such measurements, carried out directly in the cryostat with the main recording system and sample fully configured, allow precise control of the THz radiation parameters at the EPR endstation.

Inductive detection of temperature-induced magnetization dynamics of molecular spin systems

The development and technological applications of molecular spin systems require versatile experimental techniques to characterize and control their static and dynamic magnetic properties. In the latter case, bulk spectroscopic and magnetometric techniques, such as AC magnetometry and pulsed electron paramagnetic resonance, are usually employed, showing high sensitivity, wide dynamic range, and flexibility. They are based on creating a nonequilibrium state either by changing the magnetic field or by applying resonant microwave radiation. Another possible source of perturbation is a laser pulse that rapidly heats the sample. This approach has proven to be one of the most useful techniques for studying the kinetics and mechanism of chemical and biochemical reactions. Inspired by these works, we propose an inductive detection of temperature-induced magnetization dynamics as applied to the study of molecular spin systems and describe the general design and construction of a particular induction probehead, taking into account the constraints imposed by the cryostat and electromagnet. To evaluate the performance, several coordination compounds of VO2+, Co2+, and Dy3+ were investigated using low-energy pulses of a terahertz free electron laser of the Novosibirsk free electron laser facility as a heat source. All measured magnetization dynamics were qualitatively or quantitatively described using a proposed basic theoretical model and compared with the data obtained by alternating current magnetometry. Based on the results of the research, the possible scope of applications of inductive detection and its advantages and disadvantages in comparison with standard methods are discussed.

Synthesis and photoinduced behavior of DPP-anchored nitronyl nitroxides: a multifaceted approach

Understanding and controlling spin dynamics in organic dyes is of significant scientific and technological interest. The investigation of 2,5-dihydropyrrolo[4,3-c]pyrrolo-1,4-dione derivatives (DPPs), one of the most widely used dyes in many fields, has so far been limited to closed-shell molecules. We present a comprehensive joint experimental and computational study of DPP derivatives covalently linked to two nitronyl nitroxide radicals (DPPTh-NN2). Synthesis, single crystal X-ray diffraction study, photophysical properties, magnetic properties established using steady-state and pulse EPR, fast spin dynamics, and computational modelling using density functional theory and ab initio methods of electronic structure and spectroscopic properties of DPPTh-NN2 are presented. The single-crystal X-ray diffraction analysis of DPPTh-NN2 and computational modeling of its electronic structure suggest that effective conjugation along the backbone leads to noticeable spin-polarization transfer. Calculations using ab initio methods predict a weak exchange interaction of radical centers through a singlet ground state of DPPTh with a small singlet-triplet splitting (ΔEST) of about 25 cm-1 (∼0.07 kcal mol-1). In turn, a strong ferromagnetic exchange interaction between the triplet state of DPPTh chromophore and nitronyl nitroxides (with J ∼ 250 cm-1) was predicted.

Comparative Study of Single Crystal and Polymeric Pyroelectric Detectors in the 0.9-2.0 THz Range Using Monochromatic Laser Radiation of the NovoFEL

The development of efficient and reliable sensors operating at room temperature is essential to advance the application of terahertz (THz) science and technology. Pyroelectric THz detectors are among the best candidates, taking into account their variety, outstanding performance, ease of fabrication, and robustness. In this work, we compare the performance of six different detectors, based on either LaTiO3 crystal or different polymeric films, using monochromatic radiation of the Novosibirsk Free Electron Laser facility (NovoFEL) in the frequency range of 0.9-2.0 THz. The main characteristics, including noise equivalent power and frequency response, were determined for all of them. Possible reasons for the differences in the obtained characteristics are discussed on the basis of the main physicochemical characteristics and optical properties of the sensitive area. At least three detectors showed sufficient sensitivity to monitor the shape and duration of the THz macropulses utilizing only a small fraction of the THz radiation from the primary beam. This capability is crucial for accurate characterization of THz radiation during the main experiment at various specialized endstations at synchrotrons and free electron lasers. As an example of such characterization, the typical stability of the average NovoFEL radiation power at the beamline of the electron paramagnetic resonance endstation was investigated.

Modern physical methods for the molecular design of single-molecule magnets

Many paramagnetic metal complexes have emerged as unique magnetic materials (single-molecule magnets), which behave as conventional magnets at the single-molecule level, thereby making it possible to use them in modern devices for data storage and processing. The rational design of these complexes, however, requires a deep understanding of the physical laws behind a single-molecule magnet behaviour, the mechanisms of magnetic relaxation that determines the magnetic properties and the relationship of these properties with the structure of single-molecule magnets. This review focuses on the physical methods providing such understanding, including different versions and various combinations of magnetometry, electron paramagnetic and nuclear magnetic resonance spectroscopy, optical spectroscopy and X-ray diffraction. Many of these methods are traditionally used to determine the composition and structure of new chemical compounds. However, they are rarely applied to study molecular magnetism.

The bibliography includes 224 references.

A novel dual mode X-band EPR resonator for rapid in situ microwave heating

A unique dual mode X-band Continuous Wave (CW) EPR resonator designed for simultaneous EPR measurement and rapid microwave (MW) induced sample heating is described. Chemical reactions subjected to a flow of energy and matter can be perturbed away from the thermodynamic equilibrium by imposing a rapid shock or physical change to the system. Depending on the magnitude of the perturbation, these changes can dictate the subsequent evolution of the entire system, allowing for instance to populate non-equilibrium reactive intermediate states. Temperature jump (T-jump) experiments are a common method to achieve such perturbations. Most T-jump experiments are based on Joule Heating methods or IR lasers. Here we demonstrate the principle of rapid sample heating based on microwaves. The benefits of MW heating include (i) rapid and efficient heating (i.e. using a tuned resonant cavity, >99% efficient power transfer to the sample can be achieved), and (ii) volumetric heating (i.e. the entire sample volume rises in temperature at once, since heat is generated in the sample instead of being transferred to it). Accordingly, the key concept of the design is the use of a cavity resonator allowing EPR detection (at 9.5 GHz) and simultaneous sample heating (at 6.1 GHz). Temperature increments of 50 °C within a few seconds are possible. This is evidenced and illustrated here by probing the temperature-induced variation of the rotational dynamics of 16-doxyl stearic acid methyl ester (16-DSE) spin probe grafted on the surface of sodium dodecyl sulphate (SDS) micelles in water, as well as copper (II) acetylacetonate in chloroform. Rapid changes in the rotational dynamics of the paramagnetic centres provide direct evidence for the in situ and simultaneous EPR measurement-heating capabilities of the resonator. Improvements afforded by the use of pulsed MW sources will enable faster heating time scales to be achieved. In the longer term, this current study demonstrates the simple and direct possibilities for using MW heating as a means of performing T-jump experiments.

Theoretical Basis for Switching a Kramers Single Molecular Magnet by Circularly-Polarized Radiation

The d-group Kramers ions, having strong zero field splitting (ZFS) with axial symmetry and a negative D value for the ZFS Hamiltonian, are widely considered as candidates for use as single molecular magnets (SMMs). An important need is the means to switch the SMM between its states in a reasonably short and predictable period of time, which is generally not available. We propose an approach, Zeeman-far infrared (ZeFIR) double resonance, in which circularly polarized alternating magnetic fields in the far infrared (FIR) range induce selective magnetic dipole transitions between different Kramers doublets of the SMM and polarized microwave (mw) pulses transfer excitation inside the upper Kramers doublet. A combination of FIR and mw pulses allows unidirectional switching between +S and -S states of the ion. The proposed approach is considered for a model quartet system with total spin S = 3/2, which seems to be the most promising object for selective resonance manipulations of its states by circularly polarized radiation.

Electronic Modulation of THz Radiation at NovoFEL: Technical Aspects and Possible Applications

The Novosibirsk Free Electron Laser (NovoFEL) facility is able to produce high-power tunable terahertz (THz) laser radiation in quasi-continuous mode. The ability to control/shape this THz radiation is required in a number of user experiments. In this work we propose a modulation approach suitable for free electron lasers based on recuperation design. It allows for generating THz macropulses of a desirable length, down to several microseconds (limited by a quality factor of FEL optical resonator). Using this approach, macropulses in the time window from several microseconds to several hundred microseconds have been shown for three possible frequency ranges: mid-infrared (~1100 cm⁻¹), far-infrared (~200 cm⁻¹) and THz (~40 cm⁻¹). In each case, the observed rise and decay of the macropulse have been measured and interpreted. The advantage of using short macropulses at the maximum peak power available has been demonstrated with the time-resolved Electron Paramagnetic Resonance (EPR) spectroscopy.