Phonon-assisted optical bands of nanosized powdery SrAl₂O ₄:Eu²⁺ crystals: Evidence of a multimode Pekarian

A stoichiometric powder composed of nanosized grains of SrAl ₂O₄:Eu²⁺ was synthesized by combustion method at 500 C with the subsequent calcination at 1000 C. The zero-phonon line position, parameter of the Stokes shift, heat release factor and effective phonon energy were studied experimentally and analyzed in the framework of the multimode Pekar-Huang-Rhys model. Experimental data show that the optical 4f-5d transitions in Eu²⁺ ion exhibit a broad asymmetric electron-vibrational bands with a pronounced structure near the maxima. The form-function of the absorption and luminescence bands are theoretically analyzed in the framework of the model of the linear electron-vibrational interaction assuming strong coupling with the local vibration (estimated Pekar-Huang-Rhys parameter a=2S=10 and frequency ω=509 cm^-1) and relatively weak interaction with the crystal phonons. The last results in an effective temperature dependent broadening of the discrete lines corresponding to the local vibrations and to a specific shape of the whole phonon assisted band (multimode Pekarian). Providing specific interrelation between the key parameters the calculated absorption and luminescence bands exhibit peculiar temperature dependent structured peaks in a qualitative agreement with the experimental data.