ELECTRON-ATOM-PHOTON INTERACTIONS IN A LASER FIELD

We are currently studying the scattering of low and intermediate energy electrons by sodium atoms in a laser field. The experimental method is discussed, with particular attention to atomic recoil after resonant photon absorption. Total scattering cross sections by 3 2 ~ sodium atoms are presented. In this paper we will discuss the experiments being conducted in the New York University Atomic Beams Laboratory, which involve the scattering of low and intermediate energ-j electrons by alkali atoms in ground and laser-excited states. The principal goals of this research are, 1) to effect as close a comparison of ab-initio calculations for simple few-body systems with experiment as possible, 2) to reduce the collision experiment to a determination of separate observables, e.g., direct and exchange scattering amplitudes, selected angular momentum orientations, relative phases of scattering matrix elements, etc., using state selection and analysis, 3) to obtain reliable absolute cross sections for these, without requiring normalization to theory or other experiments, 4) to achieve sufficient redundancy in these state-sensitive cross section measurements to overdetermine scattering amplitudes, permitting detailed scrutiny of timeindependent collision theory, 5) to use the laser as a tool in state-preparation and analysis for both fine and hyperfine states, as well as short-lived electronically excited states, 6) to look for direct intense-field effects on electron-atom collisions, and finally, 7) to study the atom-laser interaction itself. From the point of view of this work, therefore, electron-atom scattering in the presence of laser interactions can be considered to be a natural extension of field-free scattering, which makes it possible to perform better collision experiments, but which do not add any intrinsically new physical processes to the two-body scattering problem. In 6) above, this of course changes; however generally speaking very strong fields are required. On the other hand, even when the laser field is not "strong", it is necessary to understand the laser-atom interaction in order to obtain absolute excited-state cross sections. This last point will be discussed more fully later. We will touch on all six items noted above in the present paper, concentrating of course on items 6, 7 and we will attempt to show how they relate to one another. We consider "one-electron" atoms, such as the alkalis or atomic hydrogen, and refer the reader to the literature for discussing of collision theory and details of previous work1. For electron scattering by alkali atoms, "close coupling" calculations have been extremely successful in predicting behavior of ground-state Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985123 C1-242 JOURNAL DE PHYSIQUE csllision cross sections. This is in part due, of course, to the relatively simple nature of the interaction, in which electron-correlation only plays a role through (cooperative) core polarization effects. In addition, because of the large (and spherical) core shielding, close-coupling convergence is extremely rapid, particularly with respect to optically-connected electric dipole-type matrix elements. Accordingly these systems can serve as stringent test cases for comparison of theory and experiment. The experimental work described here has been performed using variations of the atomic beam "recoil" technique2, wherein observation of the scattering is made on the "heavy" partner, i.e. the atom, rather than the electron. The method possesses the intrinsic advantages that (1) it is possible to directly observe change in the atomic magnetic quantum number, caused either by spin-exchange or by angular momentum projection changes; (2) absolute values of cross sections can be obtained, without need for knowledge of the atomic beam density; ( 3 ) laser-excited atoms can be spatially separated, using photon-recoil, from unexcited atoms enabling determination of absolute excited-state cross sections. The work discussed herein was performed on a new atomic beams apparatus, which is described briefly below. Mutually orthogonal rectangular electron, atom and laser beams intersect in an interaction volume; the (ground-state) atom beam is velocityand hyperfine-state-selected by a tunable hexapole electromagnet. It is recoil-scattered by both electron and laser beams. After travelling through a field-free "drift regionu 3 m long its spatial dispersion is analyzed by a surface ionization detector of square cross sectional area, capable of displacement in two dimensions. Fig. 1 shows a schematic view of the experimental setup. SINGLE-MODE RING DYE