Knowledge Bank

Magnetic circular dichroism (MCD) is expressed $theoretically1$ by $\Delta k=(k--k+)=\{Af1+(B+C/kT)f0$} H. A-terms originate in ground or excited state degeneracy, B-terms represent first order mixing states by the field, and C-terms arise from ground state population differences; $f0$ and $f1$ are the zeroth and first derivatives of the line shape, and H is the applied field. The Laporte-forbidden transitions in the lanthanides are known to result from breakdown in parity induced by odd order contributions to the crystal field Hamiltonian (``forced electric-dipole transitions”), and have been successfully treated in absorption by $Judd2$ and $Ofclt.3$ He extend their theory to MCD. Due to the narrow line widths and highly paramagnetic nature of the lanthanides, the MCD is generally dominated by Large A terms, Whereas positive A terms are predicted for the allowed transitions, both positive and negative A terms are observed for the “forced electric-dipole transitions”. The sign of the MCD correlates in a simple fashion with the relative magnitudes of the various odd order terms in the crystal field potential.