High-frequency EPR: An occasion for revisiting ligand field theory

The use of high-field electron paramagnetic resonance (EPR) made possible for the first time to obtain detailed information on the anisotropy of integer-spin systems, which are usually silent at conventional EPR frequencies. The data obtained so far by different research groups on non-Kramers ions are reviewed within the framework of ligand field theory at different levels of approximations. Special attention is paid to the use of the angular overlap model, which resulted to be very powerful both in terms of qualitative and quantitative explanation of the experimental results.