Optical activity in the vacuum ultraviolet.

Abstract

This review describes the recent development of vacuum ultraviolet circular dichroism (VUCD) spectrometers and the results that have been obtained with them. Optical activity is a property of material that can be displayed phenomenologi­ cally in either of two forms. Optical rotatory dispersion (ORD) refers to the rotation of the polarization plane of plane polarized light when the light is passed through material, that rotation being measured as a function of the radiation wave­ length. Circular dichroism (CD) is the difference between the absorption of left and right circularly polarized radiation. Equivalently, CD is the ellipticity induced in originally plane polarized radiation. An ellipticity of 0.001 degree corresponds to an absorbancc diffcrcncc bctwccn left and right circularly polarized light of 3 x 105 units of absorbance. Optical activity can be expressed mathematically in terms of a complex, frequency dependent function-the molecular susceptibility, in which the real (dispersive) part determines the optical rotatory dispersion and the imaginary (absorptive) part determines the circular dichroism. The nature of the susceptibility is such that these real and imaginary parts are related to one another through a Kronig­ Kramers transform. Each excited state of an optically active molecule contributes to the optical activity and, in principle, both the ORD and the CD at any wavelength should be resolvable into components arising from the various contributing optical transitions. In practice it is difficult to resolve the ORD since, being a dispersive function, it can be significantly large far from the region of absorption. Since CD is nonzero only in the region of absorption and is more easily resolved into components assignable to individual optical transitions, it has come to be the preferred technique by many people. Although one's introduction to molecular optical activity is.often in connection with the asymmetric carbon atom of elementary organic chemistry, Pasteur himself noted the possibility of helical arrangements also resulting in optical activity. In recent times helicity as a source of optical activity has received a great deal of