Photoionization of Oriented Systems and Circular Dichroism

Abstract

Circular dichroism (CD) spectroscopy is a modification of normal absorption spectroscopy using circularly polarized light instead of unpolarized light for determining the difference in the absorption coefficients for right and left circularly polarized light, respectively, in optically active samples. Thus, there is nothing mysterious about CD spectroscopy. However, relative to most other spectroscopic techniques there seems to be a large psychological barrier in its application and at least physicists often regard CD, and optical activity in general, as a rather obscure technique without any useful application. There are some understandable reasons for this attitude, manifesting themselves strikingly in the fact that CD is hardly ever treated in elementary physics textbooks: Though optical activity is one of the oldest known physical phenomena, it was only 30 years ago that instruments for measuring CD spectra became available. Samples which are optically active consist of so-called “chiral” crystals or “chiral” molecules; these are crystals or molecules which are not superimposable onto their mirror image. In the majority of cases, molecules with this property consist of so many atoms that physicists believe them not to be suitable for basic physical investigations. Up till now there exists no straightforward theory of optical activity; data are in most cases interpreted on the basis of more or less empirical rules with restricted validity and only in very few cases is it possible to extract from the data the structural information about the investigated molecules being included in the spectra. Furthermore there is still a lack of systematic and reliable data for a quantitative comparison with theory.