SOME ASPECTS OF ELECTRON IMPACT EXCITATION AND IONIZATION OF ATOMS

Abstract

The scattering of electrons from atoms/molecules has been one of the most studied subjects of atomic and molecular collision physics. The topic has been of increasing interest, both theoretically and experimentally, even in the recent years. The particular interest in such work is due to its numerous applications in various fields of science viz. plasma physics, astrophysics, laser physics etc. Several processes may result due to electron impact on atomic systems, which can be put into two categories, i.e. elastic and inelastic processes. The work reported in the thesis addresses to two specific inelastic processes viz. electron impact excitation and electron impact double ionization of atomic systems. The whole work is therefore presented in two separate parts, Part I and II, dealing with excitation and double ionization processes, respectively. In the studies for electron impact excitation of atoms, the detection and preparation of spin polarized electron beams and electron-photon coincidence techniques have made it possible to get additional information about the dynamics of an electron-atom excitation process over those obtained from the traditional cross section measurements. Recently, there has been special emphasis in terms of Bederson's idea of 'complete scattering experiment' which would provide a total quantum mechanical description of the excitation process. The preparation of initially polarized incident electron beams as well as polarization analysis of the emitted photon or spin polarization of the scattered electrons are the tools with which the complete scattering experiment, in principle, can be achieved. The work presented in Part I of the thesis focuses primarily on the calculation of the collisional parameters, which are needed to define the electron impact excitation process completely. Various theoretical methods for studying the electron-excitations have been proposed and applied. Among these, the distorted wave approximation theory has been quite successful in explaining experimental measurements especially at the intermediate incident electron energies. Therefore, the calculations are performed using relativistic version of distorted wave theory [i.e., relativistic distorted wave (RDW) method]. The method incorporates relativistic effects (i.e. spin orbit and exchange effects) to all orders automatically, which becomes important for heavy atoms. In the RDW method, the atomic target states are represented by multi-configuration Dirac-Fock wavefunctions while the wavefunctions for the incident and scattered electron distorted waves are calculated via Dirac equations directly. The whole work of Part I of the thesis is presented through four chapters. Chapter 1 gives an introduction to the study of electron impact excitation of atoms and presents a brief review of the available recent theoretical methods used in the literature. Distorted wave approximation and RDW method are explained in detail. Also the different scattering parameters, which are measured experimentally, are defined and their relation with the theoretically calculated scattering amplitudes through the reduced density matrix theory are given. Chapter 2 of this part deals with electron impact excitation of D-states of two alkaline earth atoms viz. calcium and strontium. The excitations of these atoms from their ground 'S0 states to n l'3D states (n=3,4,5 for Ca and 4,5,6 for Sr) are considered in the framework of RDW theory. The results are obtained for differential cross sections and Stokes parameters for these transitions. The calculations are performed by taking different multi-configuration wavefunctions for the ground and excited states and the effect of multi-configuration is thus explored. Chapter 3 of Part I deals with the excitation of mercury atoms from their ground state 6'S0 to the excited 6'P, and 63P, states by impact of polarized electrons. The RDW method is used for the calculation of scattering amplitudes for these transitions. Here also the effect of configuration interaction is explored by performing two types of calculations, with incorporation of configuration interaction in the representations of ground and excited sates. The results for generalized Stokes parameters along with the complete parameter set required to describe the 6'S0 &Pi and 6'S0 — 63P, excitation processes are presented. Where possible the results are compared with the available experimental measurements. iii Finally, Chapter 4 of this part provides a summary of the work presented in Part I along with our conclusions and comments. In electron impact double ionization (DI) studies, since the first (e, 3e) measurement, there has been enormous progress in these studies. Most of the theoretical studies have been focused on DI of helium, however (e, 3e) experiments on helium proved to be difficult due to low DI cross sections. Recent (e, 3e) experiments on helium have created a new dimension for the electron impact DI studies. In the light of these experimental measurements on helium, in Part II we studied some aspects of electron impact DI of atoms. Even for the simplest system like helium the electron impact DI leads to a four- body problem in final state. Most of the approaches reported so far consider the interaction of projectile and target in first Born approximation, which is suitable for the kinematics in which the incident and scattered electrons are fast. Further, the ejected electrons in the field of the residual ion constitute a three-body problem. Various approaches have been discussed in literature to deal with this problem. In the kinematical regime where the incident electron is fast and momentum transferred by the incident electron to the target is small, the three Coulomb wave method (3C method) is found to work well. The work presented in the thesis uses 3C method for the description of the ejected electrons with the Coulomb wave representing the interaction between the two ejected electrons approximated by Gamow factor. Part II of the thesis is written in four chapters. Chapter 1 of this part gives introduction to this part of the thesis, i.e. electron impact DI of atoms and presents a brief review of the experimental and theoretical work carried out during past few years. Chapter 2 of Part II deals with electron impact double ionization of ortho and para-helium. The angular distribution of five-fold differential cross section (FDCS) for the process is studied in a variety of kinematical arrangements with the aim to study the effect of asymmetry of target wavefunction on the FDCS. The shake-off and two-step mechanisms are considered. iv Chapter 3 of this part presents a comparative picture of (e, 3e) on the helium iso-electronic series members, taking into consideration three targets H", He and Li+ having iso-electronic structure but increasing nuclear charge. Chapter 4 of this part presents (e, 3e) on helium as a test of e-e correlation in helium. It is well known that the FDCS is quite sensitive to the target e-e correlations. Various analytical wavefunctions have been proposed for helium. The availability of (e, 3e) data on helium has provided a tool to test these wavefunctions. Two different wavefunctions for helium have been considered and FDCS is calculated. The results are compared with the experimental data. Finally, Chapter 5 of this part provides summary and concluding remarks on the work reported in Part II.