ABSTRACT
In discussing the problem of the transition intensities of Ln3+ activator ions in crystals, Van Vleck’s classic work completed over 55 years ago should be mentioned.31 Using the available experimental results concerning Ln3+ ion transition intensities, the author considered three possible mechanisms for interpreting their nature, namely those involving electric quadrupole, magnetic dipole (md), and forced electric dipole (ed) emission. He showed, in particular, that ed transitions between levels of the same electronic configuration may be attributed to noncentrosymmetric interactions of the activator ion with the crystalline surroundings. A more detailed analysis of the problem on Ln3+ ion transition intensities in crystals, performed in Reference 2, revealed that the probabilities of forced ed and md transitions may greatly exceed that of electric-quadrupole transitions. In the years that followed a variety of experimental and theoretical studies of the spectral properties of crystals (including insulating laser crystals) doped with Ln3+ ions conclusively confirmed the results of References 1,2. Investigations showed that in most cases the spectral-emission properties of laser crystals (which were available) doped with Ln3+ ions are due to forced ed transitions. In some cases significant contribution can also arise from md transitions. The present chapter takes this fact into account and is devoted to md and forced ed transitions. The initial basis for selection of experimental data were known intermanifold laser channels of the Ln3+ activator ions; information about which was introduced in Chapter 1.
