Failure to observe electron circular dichroism in camphor

Abstract

We have searched for evidence of electron circular dichroism, Lhe Preferential transmission of longirudinally polarized electrons lhrough a chiral medium, using stereoisomers of camphor. Such an effect was seen by Campbell and Farago in 1987 for 5 eV elecvons. Within OUT detection limit (- 0.02%) we have not observed asymmehic transmission at either an incident electron energy of 1 eV or 5 eV. ln preption for this inquiry, we also measured the total sanering cross section for camphor in the energy range 0.5-5 eV. Optical activity is the rotation of the plane of polarization of linearly polarized light as it passes through a chiral medium. A complementary phenomenon is circular dichroism: the preferential absorption of left- or right-handed circularly polarized light in a chiral medium. Farago (1980, 1981) pointed out that symmetry allows similar phenomena to be observed with electrons. Thus 'electron optical activity' is the rotation of a transverse electron spin vector in a plane perpendicular to the momentum in a chiral medium. Similarly, 'electron circular dichroism' is the preferential absorption of left- or right-handed longitudinally- polarized electrons in a vapour of chiral molecules. These parity violating effects arise because the target molecule lacks inversion symmetry. Electron circular dichroism also causes scattered electrons that were initially unpolarized to develop a polarization component in the plane of scattering. Beerlage ef al (1981) first searched for this effect but failed to see it within their experimental limits. A different experiment, performed by Campbell and Farago (1987). looked for preferential transmission of longitudinally polarized electrons through stereoisomers of camphor. They measured the mymetry