Magnetic Circular Dichroism

Magnetic optical activity (MOA) is induced in all matter by a longitudinal magnetic field. Magnetic optical rotation (MOR, the Faraday effect) and mag­ netic circular dichroism (MCD) arise respectively from the difference in refrac­ tive indices and absorption coefficients of left and right circularly polarized (CP) light due to the applied magnetic field. MOR occurs in both transparent and absorbing spectral regions ; MCD is an absorptive phenomenon only. MCD is the difference between left and right CP Zeeman spectra and provides no new information when the Zeeman components of a transition are resolved. However, MCD and MOR can be measured in broad bands where conventional Zeeman effects are undetectable, and their value lies essentially in extending the CP Zeeman experiment to broad spectra. At the beginning of the 1960s little was known about MOA in regions of broad absorption. Now, a decade later, the study of the dispersion of MOA through electronic absorption bands is a very active field of research in chemistry, physics, and biophysics. Instrumentation for the measurement of MOR and MCD and theoretical techniques for the analysis of MOR and MCD data have developed to a high level of sophistication. The nature of the phenomenon and the types of information to be obtained from its study are broadly understood. A general review of this field is therefore timely and is the purpose of this article. We discuss only MOA in absorbing regions and exclude MOR work in trans­ parent regions. The latter is (in principle) contributed to by all transitions of the system and is therefore generally less informative. Conventional CP Zeeman spectroscopy is also excluded. Emphasis is on MCD, since this has become the predominant technique; relevant MOR work is not excluded however. The scope of the article is determined by relevance to chemistry. Thus, color centers are discussed, since MOA work in this area has had a profound impact on work in chemistry; semiconductors, on the other hand, are excluded.