SOME STUDIES ON THE SCATTERING OF PARTICLES WITH ATOMIC SYSTEMS

Abstract

The work reported in this thesis is author's attempt to study the interaction of photons and charged particles with atomic systems. The study of such collisi-on processes provides the most powerful means available to physicists for obtaining the detailed information about the forces in nature, not only of submicroscopic world but also about the origin of the stars in the universe. The study of scattering processes involving excited atomic systems in the initial states is of considerable interest in astrophysics, plasma physics, fusion research and also in the development of lasers and in the understanding of gaseous discharges. The present work aims at the study of (i) excited state single photon-ionization and multiphoton ionization processes and (ii) to investigate and understand the finer aspects of the dynamics of electron atom collisions. The main work is broadly divided in three categories. In the first category, we have studied the single photon and multiphoton ionization processes. The effect of multipole interference has been investigated in the photoionization process. In the second category we have investigated the spin averaged as well as spin dependent alignment and orientation parameters in electron impact excitation of atoms. In the third category we have investigated the joint electron-photon collisions with atoms. The thesis has been divided into seven chapters. The first chapter deals with the introduction to the various theoretical methods used to study the collision process ii and gives the basic idea of the work reported in all other chapters. A brief review of experimental work related to the present study is also given. In the second chapter we report a theoretical study of the photoionization from the excited states of sodium within the framework of the central - field approximation. Results in the electric dipole approximation are obtained for the photoionization cross section (a) from the excited 3p, 3d, 4s and 4p states and for the angular distribution asymmetry parameter ( 8) of the photoelectrons produced by linearly polarized photons. In this study for obtaining, and f3 we have preferred the use of the velocity formulation, since the length from which emphasizes regions of large 'r' will not be suitable for the excited state photoionization. On comparison with the available experimental data and other theoretical studies, our results for a and 8 are found to be in good agreement. In this chapter we also study the angular distri-bution in the electron - Na+ elastic scattering and compare our results with other available calculations. The study of photoionization from excited states of atoms offers an opportunity to investigate the ionization processes from a given state of total energy and angular momentum. Excited - state photoionization is of great importance in controlled thermonuclear research plasmas, stellar atmosphere and radiative recombination. During the past decades laser technology has permitted theoretical predictions of photoionization cross sections and photoelectron iii angular distributions of excited atomic states to direct experimental test. Due to their simple electronic configura-tions, the alkali metals have been popular atomic targets for b-oth theoretical and experimental investigations of these excited states. In the third chapter we report a study of the photoionization cross sections from 'ns' and 'np' atomic states of Na in the framework of the electric dipole plus quadrupole approximation. Results are obtained for photo-ionization with polarized photons both circularly and the linearly polarized. The effect of the electric quadrupole contribution to the total photoionization cross section is studied near the threshold region as well as in the high energy regions of polarized photons. From the above study we find that the quadrupole effect, though less significant at low energies, becomes very important towards high energies. It gives rise to dipole - quadrupole interference effect. Such interference effects are clearly seen in the photoioniza-tion cross section ratio with circularly and linearly polarized photons from optically prepared magnetic substates of angular momentum unity of sodium. In the fourth chapter we have studied the two photon ionization of Na from the initial excited state in the dipole plus quadrupole approximation. We have shown that for a certain range of photon wavelengths, the quadrupole effects in the two-photon ionization of Na are more important than dipole iv effects. The dipole-quadrupole interference effects are clearly demonstrated. Ratio's of cross sections for circularly and linearly polarized photons have been obtained over a specific range of frequencies. In the fifth chapter we have investigated the spin resolved alignment and orientation in the electron impact excitation of lithium using the two - potential localised exchange (TPLE) approach. Results are obtained for the resolution of angular momentum transfer perpendicular to the scattering plane (LL) in terms of the singlet (Si) and triplet (LT) components for the polarized electron impact (2s - 2p) excitation of lithium atom. The spin asymmetry parameter (A) and spin - resolved orientation parameters (x, L , LS, LT,y and polarization 'PI) in the nonresonant electron (2s - 3p) excita-tion of lithium are also investigated. When the scattering takes place between polarized electrons and unpolarized target of low Z atoms with unresolved fine structure levels, the exchange effects causes the partial depolarization of the polarized incident electrons. On the other hand when different atomic fine structure levels are well selected, a significant left-right scattering asymmetry is observed. This spin effect results from an interplay of atomic orbital angular momentum orientation,, atomic fine structure and exchange interactions. We also investigate theoretically the partial depolarization Pe/Pe and the left-right asymmetry function SA(i) for the scattering of the spin polarized electrons from the unpolarized 2p, and 3p, states of lithium. V In the sixth chapter, we investigate the joint electron photon collision process. The laser assisted elastic scattering of electron by metastable hydrogen atom is studied in the framework of the rotating wave approximation. The frequency of laser field is chosen to match the 2s - 3p •transition frequency in hydrogen atom. Variation of the cross section with laser intensity and with incident electron energy is investigated. The seventh chapter summarizes the work reported in the thesis and contains conclusions.