J. Phys. B: At. Mol. Opt. Phys. 33 (2000) 4939–4956. Printed in the UK PII: S0953-4075(00)16280-7 Circular dichroism in differential and integrated cross sections of free–free transitions in high-energy electron–atom scattering Aurelia Cionga†, Fritz Ehlotzky‡ and Gabriela Zloh† † Institute for Space Science, PO Box MG-23, R-76900 Bucharest, Romania ‡ Institute for Theoretical Physics, University of Innsbruck, Technikerstrasse 25, A-6020 Innsbruck, Austria Received 11 August 2000 Abstract. In this paper high-energy electron scattering by hydrogen atoms in the presence of a laser field of moderate power and higher frequencies will be considered. For a superposition of a linearly and a circularly polarized laser beam we shall show that circular dichroism in the angular distribution can be predicted for the nonlinear two-photon transitions, if the dressing of the atomic target by the laser field is treated in second order of perturbation theory in which the coupling between the atomic bound and continuum states plays an essential role. Of special interest is that for particular configurations circular dichroism can be encountered not only in the differential but also in the integrated cross sections. To the best of our knowledge this is the first example of this kind and therefore this effect should become more easily accessible to observation. 1. Introduction In the course of his experimental investigations on dispersion in liquids and gases, Cotton [1] detected circular dichroism. Since then dichroism has become a well known concept in classical optics. It denotes the property of certain materials to have absorption coefficients that depend on the state of polarization of the incident radiation field [2, 3]. This general notion was taken over into the investigation of problems of atomic or molecular interactions with a laser field. Of particular interest became the concept of circular dichroism in an angular distribution (CDAD). It refers to the differences between the fluxes of scattered or ionized electrons measured at definite spatial directions, caused by left and right circularly polarized (CP) laser light [4]. CDAD was found, for example, in the photoionization of polarized atoms and of oriented molecules [5]. In our present work we shall use the definition of CDAD that was introduced by Manakov et al [6], namely as the angular dependence of the difference between the differential cross sections (DCS) evaluated for the two helicities of the laser photons. In particular, we shall denote by CD the special case in which the angular integration of the differences between the left and right CP signals yields a non-vanishing result. In recent years, in the investigation of several other processes stimulated by laser fields, increasing care has been devoted to the effects caused by the different states of polarization of such fields. For example, the double photoionization of helium [7], x-ray scattering by unoriented systems [8, 9], electron impact excitation of autoionizing states of atoms excited by CP lasers [10] and several multiphoton processes [11–19] were investigated along these lines. 0953-4075/00/214939+18$30.00 © 2000 IOP Publishing Ltd 4939