RELATIVISTIC AND NON-RELATIVISTIC STUDIES OF ELECTRON-ATOM COLLISIONS

Abstract

Electron-atom scattering is one of the most studied subject in the field of atomic collision physics. Especially in the recent years there has been tremendous progress both theoretically and experimentally in the study of various electron-impact excitations of the atoms. This is mainly due to the rapidly growing advances in the quality of experimental technology with which sophisticated experiments are becoming possible and these also lead to the development of powerful theoretical models. For example, with the introduction of electron-photon coincidence technique it has been possible to get additional information about the dynamics of the electron-atom excitation process than the one obtained through traditional differential (DCS) and total (TCS) cross section measurements. We know, with such experiments, alignment and orientation of the atom for an anisotropic excited state can be experimentally determined. Further, the ease with which production and detection of spin polarization of electrons have become possible, a new dimension has now been added to the exploration of atomic forces viz. the spin dependent electron exchange and spin-orbit interactions. The study of the excitation of an atom using spin-polarized electrons coupled with electron-photon coincidence technique is one of the latest addition to the present status of the collision experiments. In the past most studies were for the lighter atoms which could be described in LS-coupling scheme. For heavier atoms departures from LScoupling scheme become important during the scattering process and it is expected that relativistic effects through spin-orbit and(or) exchange interactions play key role. Thus the influence of spin dependent interactions on the parameters which describe the excitation process should be studied theoretically and experimentally with spin analysis. The work in the thesis addresses this aspect and presents the author's attempt to theoretically investigate the importance of relativistic effects in the study of electron impact excitations of atomic systems. Various perturbative and non-perturbative quantum mechanical approaches for theoretically studying the electron-atom excitations have been proposed and applied. Among these the distorted wave approximation methods have been known to be quite successful in explaining the experimental measurements and predicting the correct behaviour of various scattering parameters, especially at intermediate and higher energies (to which the thesis addresses). In order to incorporate the spin-orbit and exchange effects a relativistic version of the distorted wave theory is therefore needed. The thesis describes such a relativistic distorted-wave (RDW) method and presents it's application to various excitations in different closed and open shell atoms. In the RDW method, the target atom is represented by multi-configuration Dirac-Fock wave functions while the wave function for the scattered electron is calculated via the Dirac equations thus the relativistic effects are included to all orders. The whole work in the thesis is presented through six chapters. The first chapter gives an introduction to the subject of the thesis by consideiing the excitation of an atom by electrons and defines the possible collisional parameters experimentalists measure and their relation with the theoretically calculated scattering amplitudes. A detailed derivation of the distorted wave approximation (DWA) method in its non-relativistic and relativistic forms is given and the corresponding expressions for the direct and exchange transition matrices are obtained. A brief review of the available recent theoretical approximate methods used in the literature for electron-impact excitation of the atoms is also given. The second chapter deals with the excitation of alkaline-earth atoms viz magnesium and zinc. Excitations of the 31P1, 331'0 12/ 311)2 and 33D1,2n3 states of magnesium from the 31S0 ground state have been considered in the RDW approximation. A complete set of results for the excitation by unpolarized and spin-polarized beams of electrons is reported. The results are obtained for the DCS for the excitation process, state multipoles of the excited states after excitation and the Stokes parameters of their photon decay as well as the different generalized STU spin parameters. Where possible these results are compared with the available earlier non-relativistic close-coupling and DWA results as well as the experimental measurements. For similar excitations in zinc there are no earlier experimental or theoretical results reported so far. Therefore, the excitations of the lowest excited 4'P1 and 43P considered and the results for the DCS, coherence and correlation parameters and the spin polarization parameter are reported for the first time. The effect of configuration mixing of the ground state is also explored. Also, for comparison non-relativistic distorted wave calculations are performed where no earlier theoretical results are available i.e. for the excitations of the 31D and 33D states of Mg and the 4113 and 43P states of Zn...