THEORY FOR MAGNETIC LINEAR DICHROISM OF ELECTRONIC-TRANSITIONS BETWEEN TWOFOLD-DEGENERATE MOLECULAR SPIN LEVELS

Magnetic linear dichroism (MLD) spectroscopy is a relatively new techn ique which previously has been almost exclusively applied to atoms. Th ese investigations have revealed that the study of MLD, in conjunction with electronic absorption and magnetic circular dichroism (MCD) spec troscopies, provides significant additional information concerning the electronic structure of atoms. More recent measurements have indicate d that MLD is also observable from transition ions in inorganic compou nds and metalloproteins. While the theory for atomic MLD has been work ed out in considerable detail during the last two decades, an MLD theo ry of practical utility for the analysis of the spectra derived from t he majority of paramagnetic molecules is not available. In the present contribution, the MLD of an electric-dipole-allowed transition betwee n twofold-degenerate molecular spin levels is analyzed, assuming nonsa turating conditions. As for atomic systems, it is found that the MLD o f a single molecule is dominated by the term G(0). However, this term vanishes in the powder average evaluated for a randomly oriented ensem ble of molecules, leading to a drastic reduction of the MLD differenti al absorption for systems with spin S=1/2 compared to that observed fo r systems with higher ground-state spin. It is found that MLD and MCD spectroscopies on solution samples have complementary spin-state speci fic sensitivities which suggest that the two methods can be used to se lectively probe the individual metal sites in multicenter metalloprote in assemblies. (C) 1998 American Institute of Physics.