Electron paramagnetic resonance (EPR) is a powerful technique to investigate the electronic and magnetic properties of a wide range of materials. We present the first combined terahertz (THz) field and frequency domain electron paramagnetic resonance (HFEPR/FDMR) spectrometer designed to investigate the electronic structure and magnetic properties of molecular systems, thin films and solid state materials in a very broad frequency range of 85-1100 GHz. In this paper, we show high resolution frequency-field (Zeeman) maps (170-380 GHz by 0-15 T) recorded on two single-molecule magnets, [Mn2(saltmen)2(ReO4)2] and [Mn2(salpn)2(H2O)2](ClO4)2, which give direct access to the field-dependence of the energy level diagram. Furthermore, supression of standing waves in the described system and the sensitivity in field and frequency domain operations is evaluated and discussed.

F + center, an electron trapped at oxygen vacancy (VO), was investigated in the oxygen deficient Y3Al5O12 (YAG) garnet crystals by electron paramagnetic resonance (EPR). The measurements were performed in the wide temperature interval 5450 K and the frequency region 9.4350 GHz with using both the conventional continue wave and pulse EPR technique. The pulse electron-nuclear double resonance was applied to resolve the hyperfine interaction of the trapped electron with surrounding nuclei. The measurements show that at low temperatures, T < 50 K, EPR spectrum of the F + center is anisotropic with g factors in the range 1.9991.988 and originates from three magnetically inequivalent positions of the center in garnet lattice according to different directions of the Al(IV) -VO -Al(VI) chains, where Al(IV) and Al(VI) are the tetrahedral and octahedral Al sites, respectively. As the temperature increases, the EPR spectrum becomes isotropic suggesting a motional averaging of the anisotropy due to motion of F + -center electron between neighboring oxygen vacancies. With further increase of the temperature to T > 200 K, we observed delocalization of the F + -center electron into the conduction band with the activation energy about 0.40.5 eV that resulted in substantial narrowing of the EPR spectral line with simultaneous change of its shape from the Gaussian to Lorentzian due to diminish up to zero of the Fermi contact hyperfine field at 27 Al and 89 Y nuclei. Such temperature behavior of the F + -center electron in YAG is completely similar to behavior of a donor electron in a semiconductor. Our findings is further supported by measurements of the conductivity and dielectric properties. In particular, these data show that the conduction electrons are not homogeneously distributed in the crystal: there are high-conductive regions separated by poorly-conductive dielectric layers. This leads to the so-called Maxwell-Wagner dielectric relaxation with huge apparent dielectric constant at low frequencies. To the best of our knowledge, this is probably the first observation of a donor-like behavior of F + center in wide band-gap insulating crystals.

We report on the investigation of Bismuth-doped pure silica glass without other co-dopant by the tech- nique of magnetic circular dichroism (MCD), which allows the direct probing of the ground state of optical centres. Taking into account the results of conventional optical spectroscopy, we show that the observed MCD bands belong to the centre responsible for the red photoluminescence in this material. Measurements of the temperature and field dependences indicate that the MCD effect is caused by the even-electron system. This, however, opposes the widespread opinion that Bi2+ ions are the origin of red photoluminescence in Bismuth-doped silica glasses. On the other hand, the lasing centre responsi- ble for the near infrared photoluminescence does not exhibit any magnetic optical activity connected to its ground state. As a consequence, we conclude that the ground state of lasing centre is a magnetic singlet with the effective spin S = 0.