Theory of optical spectra of impurity centres in crystals: general consideration of quadratic vibronic coupling

Electron–vibrational interaction in 4f–5d optical transitions in Ba, Ca, Sr thiogallates doped with Eu2+ ions

“…[20]. In the 1970s the radiative and non-radiative processes have been extensively studied [21][22][23] and the basic model (linear vibronic coupling) was extended with due account for anharmonicity [24] and shift of the vibrational frequencies [25].…”

Line Shapes in Infrared Absorption by Solids and by Atomic or Molecular Species Embedded in Solids

“…The theory assumes linear electron−phonon coupling, which is accurate enough for dealing with optical transitions in the infrared . Quadratic coupling is far beyond our needs because it implies substantial modification of the electronic bonds . Anharmonicity is also not considered because we are not occupied here with higher order effects or fine details of spectra.…”