Development and experimental verification of a theory for high-field, ultralow-temperature magnetic linear dichroism of glasses containing molecular chromophores with spin doublet ground states

Abstract

The first observation of magnetic linear dichroism in a metalloprotein Kramers system is reported, namely, that exhibited by the porphyrin moiety in ferricytochrome c (spin S=12). The measurements were conducted at low temperatures and in strong magnetic fields to maximize the signal intensity, which is intrinsicly weak in this case. The theory used in the interpretation of the wavelength dependence of this magneto-optical effect is based on the rigid-shift approximation in which the dichroic spectra are expressed as a sum of zeroth, first, and second derivatives of the underlying electronic absorption band. Similar to the case for magnetic circular dichroism, magnetic linear dichroism is caused by the Zeeman interactions of the molecular-chromophore electrons with an applied magnetic field. Two kinds of Zeeman interaction are considered, termed “inner state” and “outer state” depending on, respectively, whether or not they act between the components of a single Kramers doublet of the chromophore. It is formally demonstrated that the zeroth-derivative term for an electric-dipole transition between Kramers doublets arising from inner-state Zeeman interactions (nominally the strongest effect) completely cancels in the powder average over a randomly oriented ensemble of chromophores for all values of temperature and field. This cancellation has a profound effect on the magnetic linear dichroism of molecular chromophores suspended in glasses, as the effect now entirely relies on a set of weaker residual terms, each one having its own spectroscopic characteristics. The residual contributions have been estimated on the basis of electronic-term-energy differences and bandwidths in ferricytochrome c, resulting in the identification of the inner- and outer-state terms C1 and F0 as the dominant signatures. This prediction is in agreement with the experimental data for the shape and dependence on applied field and temperature of the magneto-dichroic spectrum for this chromophore. The magnetic linear dichroisms arising from inner- and outer-state Zeeman interactions in a system with S=12, like the one considered here, are of comparable magnitude, which makes the information content of magnetic linear dichroism distinctly different from that obtainable from magnetic circular dichroism, where the inner-state contributions dominate. From this analysis, magnetic linear dichroism emerges as the preferred magneto-optical spectroscopy for assessing outer-state Zeeman interactions under cryogenic conditions and is, in this respect, complementary to magnetic circular dichroism.