The orientation and frequency-field dependencies of continuous wave electron paramagnetic resonance (EPR) spectra were measured for the first time for impurity Ho³⁺ ions in yttrium orthosilicate single crystal in the frequency range of 140 - 200GHz. The value of the zero field splitting of electron-nuclear levels and the magnetic characteristics of Ho³⁺ ions substituting yttrium in the Y2 crystallographic sites are determined.

Electron spin echo envelope modulation (ESEEM) spectroscopy provides detailed information on electron-nuclear hyperfine interactions and has also been successfully applied to photoexcited triplet states. However, the interpretation of two-dimensional (2D) field-swept ESEEM patterns in disordered triplet systems remains challenging. In this study, we provide analytical and numerical descriptions of ESEEM in photoexcited triplet states (S = 1) within the high-field regime. We derive analytical expressions for the nuclear modulation frequencies and depths associated with individual electron paramagnetic resonance (EPR) transitions at specific resonance positions where the external magnetic field aligns with the principal axes of the ZFS tensor. At these positions, the ESEEM response simplifies and exhibits characteristic differences between the two allowed EPR transition branches, providing direct access to the sign of the ZFS parameter D and to the orientation of the hyperfine coupling tensor. Numerical simulations of 2D ESEEM spectra confirm the analytical predictions and demonstrate pronounced intensity asymmetries that depend on these parameters. The developed approach is applied to the analysis of 2D ESEEM experimental data for the photoexcited triplet state of the endohedral fullerene Y₃N@C₈₀, providing a consistent interpretation of hyperfine interactions involving both ⁸⁹Y and ¹³C nuclei. The results establish 2D ESEEM as a powerful method for the characterization of photoexcited triplet states in polycrystalline samples.

This work is devoted to the investigation of paramagnetic centers in a single crystal of hexagonal boron nitride (h-BN) with beryllium introduced via high-temperature diffusion. Electron paramagnetic resonance (EPR) spectra and their dependence on the orientation of the magnetic field relative to the crystallographic axes and temperature were measured in hexagonal boron nitride crystals. The observed dependencies are explained by the presence of two types of paramagnetic centers. The first type consists of three exchange-interacting paramagnetic centers, each with spin S = 1/2, with g-values of g_∥ = 2.0 and g_⊥ = 2.7, which allow for the observation of lines with effective g-values of g_∥ = 6.0 ± 0.3 and g_⊥ =3.5 ± 0.2 at 16 K. The exchange coupling constant was estimated from the temperature dependence of the g-factor at 16-120 K range. The second type of centers is attributed to a defect with spin S = 1/2, exhibiting g-values of g_∥ = 2.18 ± 0.01 and g_⊥ = 2.06 ± 0.01, with a distinct orientation in the ab plane of the crystal.