ABSTRACT
In this chapter the hyperfine induced transitions are discussed, since this issue must be addressed in a monograph concerned with hyperfine interactions in lanthanides in crystalline environments. In 1962 Wybourne suggested2 that under certain circumstances highly forbidden transitions might become allowed in a crystalline environment by a nuclear magnetic moment mixing nearby crystal field levels, particularly in the case of holmium salts. Baker and Bleaney3 reported an apparent violation of selection rules in their paramagnetic resonance studies and suggested a mechanism based on the Jahn-Teller effect. It was only in the late 1980’s that high resolution optical studies of hyperfine structure by Popova and her associates4 revealed that indeed the interaction of nuclear magnetic dipole and electric quadrupole moments with 4 f −electrons leads to the observation of the “forbidden transitions”. Remarkably, the possibility of hyperfine induced transitions started with the spectroscopic observation of gaseous nebulae. In this chapter we first define what is meant by forbidden transitions and after a brief sketch of the early history of hyperfine induced transitions we outline the calculation of such processes in atoms and ions focussing particularly upon the recently observed highly forbidden transition5 (2s2p) 3 Po0 → (2s2) 1Se0 in N I V . This discussion is finalized by the analysis of possible importance of the hyperfine interactions in the description of the f ←→ f transitions in lanthanides in crystals.
