Interaction of electronic excitations of Tm³⁺ ions with acoustic vibrations in KTm(MoO₄)₂

Electron paramagnetic resonance spectra of KTm(MoO₄)₂ were measured as a function of magnetic field between 3 and 11.5 cm⁻¹ at T = 2 K. We found that in addition to the absorption line caused by the electronic excitation of Tm³⁺ ions, the spectra contain sidebands. Far-infrared transmission measured with polarized light from 10 to 75 cm⁻¹ revealed vibration modes at 16.7 and 25.7 cm⁻¹ for polarizations E^ω ∥ a and E^ω ∥ c, respectively. We show that sidebands in the spectra of paramagnetic resonance result from a parametric resonance between the electronic excitations of the Tm³⁺ ions and the acoustic vibrations of the crystal lattice.