Nuclear Instruments and Methods in Physics Research B79 (1993) 85-87 North-Holland NIUMI B Beam Interactions with Materials 8 Atoms Ion accelerators and electron scattering * D.W. Mueller, B.R. Chalamala, J.E. Furst, C.T. Newell, S.M. Pak, R.S. Smith, L. Song, Z. Xu, Y. Zhang, J.X. Zhou and D.E. Golden Physics Department and Center for Materials Characterization, University of North Texas, Denton, TX 76203, USA Several problems of fundamental importance to physics involving highly correlated and few body systems are being addressed in our laboratories. In particular, the breakup of three low energy charged particles is being studied using crossed beams of electrons and ions. The systems involved can consist of the excitation and dissociation of molecular ions or the resonant excitation-auto-dou- ble ionization (READI) of simple atomic ions. The use of particle accelerators in conjunction with electron beams provide a unique combination of tools for studies of this nature. We have constructed an apparatus to measure multiparameter angular correlations between reaction products following electron impact ionization of ionic targets to obtain alignment and orientation information. With molecular targets, ground and excited state potential energy diagrams may also be obtained. We will measure the momentum of all reaction products and provide a stringent experimental test of near threshold three-body theory which could lead to a general theoretical treatment for three particles interacting via a Coulomb field. 1. Introduction Theoretical treatments of three-body systems have been limited by the need to account for long range Coulomb interactions between each pair of interacting particles. Ionizing collisions between electrons or ions and atoms are inherently systems of this type which commonly appear in the study of fundamental systems and in a vast number of applications such as plasma modeling for fusion and stellar atmospheres [l]. More complete theqretical descriptions and definitive experi- mental tests are still needed. By measuring the mo- menta of all reaction products in coincidence experi- ments, we should be able to provide insight into this fundamental problem. Recent experiments using trochoidal analyzers and merged beams techniques to measure electron impact excitation cross sections for ions has provided a signifi- cant advance in studies of this type [2,3]. High resolu- tion measurements of the electron impact ionization are also indicative of the advances which are being made in this field [4,.5]. We intend to expand on these advances by developing new technology where neces- sary and by implementing existing technology in new applications. We have constructed an apparatus to study electron impact ionization of ions which should provide addi- tional insight into the mechanisms involved. Using mul- tiparameter coincidence techniques, we will detect both outgoing electrons from ionization processes and will * Research initiated under grant #PHY 8803562 from the National Science Foundation. be able to analyze the momenta of both particles simultaneously. Coupled with a high resolution elec- tron gun, this information should have broad applica- tion to the difficult problem of three bodies interacting via the long range Coulomb force. If sufficient accuracy can be achieved, information regarding the relative phase of the outgoing electron wavefunctions can be obtained. 2. Experimental arrangement A schematic of our apparatus is shown in fig. 1. A, mass selected ion beam is guided into the interaction region where it is crossed with a beam of electrons. After passing through the interaction region, the ion beam enters an electrostatic analyzer where it is sepa- rated according to its charge-to-energy ratio and the ionized ions are counted. The electron beam which has passed through the interaction region is designed to enter a trochoidal analyzer where the momenta of the scattered electrons are determined. The interaction and analysis regions are maintained at a pressure of 1O-g-1O-‘o Torr. The detector section of the trochoidal analyzer is of the discrete anode MAMA [6] design. It has the advan- tage over the resistive anode and wedge and strip designs in that the pulse pair resolution can be on the order of 10 ns. The decoding electronics for this design is comprised of digital emitter-coupled logic (ECL) and can also accommodate these parameters. These anodes are of custom design and will be manufactured in collaboration with the microelectronics industry. Inter- 0168-583X/93/$06.00 0 1993 - Elsevier Science Publishers B.V. All rights reserved I. ATOMIC/MOLECULAR PHYSICS