Hypersensitive pseudoquadrupole transitions in lanthanides

It is pointed out that the absorption lines of the lanthanides that are peculiarly sensitive to changes in the ligands are consistent with the selection rules for quadrupole radiation within the 4f shell. A number of sources for this sensitivity are examined, and it is concluded that it owes its origin to the inhomogeneity of the dielectric. According to this mechanism, there is an asymmetrical distribution of the dipoles induced by the electromagnetic field in the medium surrounding a lanthanide ion. The variation across the lanthanide ion of the electric vector is very much greater than for the case of a homogeneous dielectric, and the intensities of quadrupole transitions are enormously enhanced.