EUROPHYSICS LETTERS Europhys. Lett., 17 (8), pp. 727-732 (1992) 7 February bis 1992 Circular Dichroism in Photoemission from Nonmagnetic, Low4 Solids: a Conspicuous Effect of) the Photon Spin. G. SCHONHENSE (*), C. WESTPHAL, J. BANSMANN (*) and M. GETZLAFF Fakultat &r Physik, Universitat Bielefeld - D-4800 Bielefeld, Germany (received 26 June 1991; accepted in final form 9 December 1991) PACS. 73.20D - Electron states in low-dimensional structures. PACS. 78,203 - Optical rotatory power. PACS. 79.60 - Photoemission and photoelectron spectra. Abstract. - A new phenomenon has been observed in solid-state photoemission with circularly polarized light. Its outward appearance is similar to the circular dichroism in photoabsorption. For special observation geometries, large photoelectron intensity asymmetries arise upon rever- sal of the photon helicity. The effect is not based on spin-orbit interaction, as is demonstrated by the example of photoemission from graphite (2 = 6). Similar asymmetries are expected for all single-crystal solids thus opening up a new class of experiments providing new tests for photoe- mission theories. Detailed information on the dynamics of photoexcitation of a solid can be gained, as discussed in terms of a simple orbital model. Electron- and/or photon-spin-dependent phenomena have been investigated previously by photoemission from solids which significantly show either exchange or spin-orbit inter- action [l, 21, i.e. for ferromagnets or for high-Z materials. In ferromagnets, electron-spin orientation is already present in the initial state, whereas it can be optically induced in non- magnetic solids by circularly polarized light via spin-orbit coupling. At the surface, spin- orbit forces give rise to spin-dependent electron transmission through the solid-vacuum in- terface which may affect photoelectron intensities [3]. The purpose of this letter is to present experimental evidence of a new photon-spin-de- pendent phenomenon which does not fit into these previous categories. Its different nature is obvious since it occurs without significant spin-orbit or magnetic-exchange interactions. Experimental data for single-crystal graphite (Z = 6) at low photon energies (hv = 20 to 40 eV) reveal that the effect results from an electronic dipole transition. We will elucidate its physical origin in terms of a simple orbital picture, by extending theoretical work on iso- lated molecules and atoms [4-61. In previous experiments [7], similar asymmetries have been observed in photoemission from oriented adsorbed molecules, verifying the quantita- tive predictions of Dubs et al. [6]. Owing to the outward appearance of the phenomenon it will be termed circular dichroism in photoemission, or, more precisely, circular dichroism in the angular distribution of photoelectrons (CDAD) [4-61. In its broadest sense the term dichroism refers to selective absorption of one of the two orthogonal polarization states of an incident photon beam. For reasons of symmetry, the appearance of circular dichroism requires either a chiral target (such as a molecule without (*) Present address: Institut fur Physik, Johannes Gutenberg-Universitat, Staudingerweg 7, D-6500 Mainz, Germany.