f ↔ f electric dipole transitions; old problems in a new light

Very well known difficulties of the theoretical description of the f ↔ f electric dipole transitions of lanthanides are addressed in the old language of the standard Judd–Ofelt (J–O) theory, which has been widely applied for more than 45 years. New light is shed on a competitive physical mechanism of these transitions, which is presented to complement the existing J–O model. Possible new solutions are given for the old problems of the negative intensity parameters obtained from the adjusting procedure. An alternative scheme for the reproduction of the hypersensitive transitions is discussed and a theoretical description of highly forbidden transitions 0–0 and 0–1 observed for Eu 3+ and Sm 2+ ions is presented.