STUDY OF ELASTIC AND INELASTIC PROCESSES IN ATOMIC SYSTEMS

Abstract

Maximum attention has focused on the studies of electron impact elastic and inelastic processes with atoms, ions as well as molecules in the field of atomic collisions physics. The specific interest in such electron induced fundamental processes have been due to their wide applications in various areas viz. plasma physics, astrophysics, laser physics, fusion research etc. Although to study such processes the experimental measurements as well as the theoretical calculations have started from very early time but the main progress has been achieved in the recent past with the availability of advanced experimental techniques. We can prepare now atomic targets in different selected states and the electron impact collision experiment can be performed with various available efficient detection systems. In fact, projectile electrons in the various spin polarize states can be produced whose collision with lighter and heavier atomic systems with well resolved states have been considered. The sophisticated experimental data have led to advance theoretical calculations to progress and thus accurate theoretical model have come up on the basis of which one can explain the newly reported experimental data. In spite of the advancement in experimental techniques, report on experimental data has been scarce and thus the theoretical studies have dominated the field to meet the demand of the required data of various electron induced processes. A complete quantum mechanical description of an electron atom collision process requires the determination of all the scattering amplitudes for the process. The differential cross section (DCS) and total cross section (TCS) for the process are obtained by taking the squared modulus of complex scattering amplitudes and averaging over the magnetic sub-states. Thus the complete information about the scattering amplitudes is not available. For example, in an electron impact excitation process one can get more information by analyzing the polarization of photons, emitted from the atom after the excitation, in coincidence with the scattered electrons. This is achieved in an experiment called scattered electron-emitted photon coincidence experiment where one measures the polarization of the emitted photon in terms of differential Stokes parameters. If the scattered electrons are not observed and only the polarization of the emitted photon is analyzed, such measurements lead to integrated Stokes parameters. By measuring of the Stokes parameters, we have the information about the shape and orientation of the charge cloud of the excited state of the atom (ion). In the present thesis, one of our aims has been to theoretically study the electron impact excitation processes in different heavier atoms and ions where spin-orbit interaction is very important and gives rise to the various fine structure levels. The studies of such processes require fully relativistic treatment where the projectile and target electrons are described through Dirac equations. We have considered a number of excitations which are important and having applications to plasma modeling in the frame work of relativistic distorted wave theory. We have focused on those transitions for which either new experimental result have been recently reported or earlier theoretical calculations did not exist. In addition, we have also considered the elastic scattering from atoms and molecules using suitable optical model potential method and explored the suitability of such method. Since the scattering amplitude cannot be evaluated exactly, different approximation methods have been adopted to study electron-atom collisions. The available methods for electron-atom collisions can be broadly divided into two categories i.e. non-perturbative and perturbative approaches. Among the non-perturbative approaches, the convergent close coupling (CCC), Rmatrix and variational methods have been successful in their non-relativistic form in the recent past. Such methods have been found suitable and extensively used in the low incident electron energy region. In the high energy region more channels open up requiring heavy computational efforts which makes the calculation impractical. Though, there have been recent efforts to develop techniques to extend the low energy methods to intermediate energy region as well as to incorporate the relativistic effects. Among the perturbative approaches, distorted wave theory has been found very successful at intermediate as well as higher incident electron energies in predicting the experimentally measured various scattering parameters. As compared to non-perturbative approaches the distorted wave method is relatively less expensive and has been used in its non-relativistic and relativistic forms. However, in the present work the fully relativistic distorted wave approximation theory is used to describe accurately the electron scattering from heavy atoms and ions where electron spin-orbit interaction is relatively large for both the projectile and target electrons. The relativistic effects can be included ideally through the use of the Dirac equations for describing both the atomic and continuum electrons involved in the collision process. In this way electron spin is automatically incorporated in a natural way. For studying elastic electron-atom and electron-molecule scattering, optical model potential approach has been found very successful and still being widely used recently. However, elastic scattering of electron from heavier atoms should be studied using relativistic approach where the optical model potential is used and the scattering is described through Dirac equations. Further, the electron elastic scattering from molecules using optical model potential is complicated by the fact that molecules are a multi-centered target with the nuclei of the constituent atoms being a center of charge. Thus one of the most important parts of a scattering calculation is to obtain the static potential which represents the interaction of the incident electron with the unperturbed charge distribution of the molecule. Calculating this potential is a difficult challenge if multicenter wave functions are used for the target. A method is developed to address this problem.