CHIRALTEM - Circular Dichroism in the TEM

The similarities between X-ray Absorption Near Edge Structure (XANES) and energy loss near edge structures (ELNES) in the Transmission Electron Microscope (TEM) have long been recognised, and attempts to "export" X-ray Magnetic Linear Dichroism (XMLD) to the electron microscope were successful [1, 2]. Linear dichroism is the dependence of the absorption cross section on the orientation of the polarisation vector of the absorbed photon. It can be measured in the TEM because the counterpart to the polarization vector of a photon is the momentum transfer in an inelastic electron scattering event. Selecting a particular momentum transfer in diffraction mode in an ELNES experiment allows to observe linear natural dichroism [3]. This was done in a few cases, e. g. in graphite, in h-BN and in AlB2. Since dichroism is caused by the natural or magnetically induced anisotropic electronic structure of the material the notation "anisotropy" is more common than "dichroism" in TEM experiments. The absorption cross-section in circular dichroism depends on the helicity (right or left) of the incident photon. Whereas XMLD has been measured in the TEM it was believed that X-ray Magnetic Circular Dichroism (XMCD) cannot be done with electrons. It was thought that in analogy to the circularly polarized X-ray beam a spin polarized beam of electrons is necessary. A recent discovery [4] showed that this is not true. By a sophisticated experimental setup circular dichroism is observable in a TEM equipped with an energy spectrometer or energy filter.