Resonance between electronic and vibronic levels of Tm3+ in yttrium aluminium garnet

An attempt to explain some of the Tm3+ spectra features in YAG in terms of the splitting of the zero-photon lines due to the even part of the linear vibronic interaction Hamiltonian with a single active phonon mode in the cases of resonance or quasi-resonance between a pure electronic and vibronic level is presented. The observed features in absorption and emission spectra are discussed and the active mode is identified. The experimental splittings and intensities allow estimation of the vibronic coupling matrix elements of about 4-5 cm(-1).