Ultrahigh-frequency NMR of Tm$^{3+}$ ions in single crystals of thulium ethylsulfate in high magnetic fields

Resonant transitions predicted earlier between low-lying electron-nuclear sublevels of the Tm$^{3+}$ ground state were observed at frequencies up to 700 MHz in a dielectric Van Vleck paramagnet — thulium ethylsulfate single crystal. It is shown that, due to the distortion of the $4f$-electron shell of a rare-earth ion in an applied magnetic field, the parameters of electron-nuclear interaction become field-dependent.