ELECTRON INDUCED PROCESSES IN ATOMIC SYSTEMS

Abstract

Electron atom scattering has been the most extensively studied subject in the area of atomic and molecular collision physics. The experimental and theoretical activities in this area have been accelerated over the past few years. This may be attributed to the growing, demand of the accurate cross section data in numerous fields e.g. for modeling and diagnostic purposes of variety of plasmas including fusion, as an input data to various astrophysical models to interpret spectral observations, laser physics etc. Recent advancement in the quality of experimental technology has made various new measurements possible. For example, with the introduction of electron — photon coincidence experiment technique it has been possible to get additional information about the collision dynamics of the electron-atom excitation process than those obtained through differential and total cross sections. These measurements provide Stokes parameters which give the information about the population of the excited magnetic sublevels and also about the shape and orientation of the charge cloud of the excited state of the atom. The ease with which now the production and detection of spin polarization of electrons have become possible, a new dimension is added to the exploration of atomic forces viz. the spin dependent electron-exchange and spin-orbit interactions. Besides this. large populations of excited states of target can be produced experimentally by laser irradiation to allow for the measurements of electron impact inelastic and superdastic scattering processes from these states. Such experiments for the electron induced transitions between the excited states of atoms are the latest additions to the present status of the collision physics. To interpret the experimental data, different reliable theoretical models have been developed. However, because of the complexities involved in carrying out sophisticated experiments on electron-atom collision the requirement of cross section data is mainly supplemented by different theoretical .calculations using suitable approximation methods. Theoretically, various perturbative and non-perturbative quantum mechanical approaches for studying -electron-atom excitations have been proposed and applied. The non-preturbative methods such as the close-coupling, R-matrix and variational methods promise quite reliable results especially, at low impact energies, but involve heavy ground and excited state wavefuncions of the oxygen atom. Results for the differential cross sections are compared with both experimental measurements and close-coupling and R-matrix calculations available for fine-structure unresolved transitions. For this purpose the RDW results for fine-structure transitions have been averaged over initial fine-structure levels and summed over final fine-structure levels. Integrated cross sections for the excitation of the 2p33s 3S state are also presented as extensive theoretical and experimental data have been reported for this transition. The calculations are also a further test of the applicability of the RDW method to the excitation of open-shell atoms. Chapter — 5 discusses the electron — photon (e, ey) coincidence studies for the transition 6.v6p I P I 6s2 .0 in mercury atoms with polarized electrons. This work has been done in the light of recent measurements for Stokes parameters P, (i = I, 4) by Munster group, Germany. For any process, which involves spin-polarized electrons and where the spin-orbit interaction is important, the Dirac equations are the natural way to describe the system. The Dirac equations explicitly involve the spin of the electrons of the system and include all one-particle spin-orbit interactions. Thus RDW calculations have been carried out to obtain Stokes parameters P, (i = 1, 4) and total degree of polarization Ptot at electron impact energies 15, 50 and 100 eV. The results are compared with the experimental data and theoretical calculations based on a five-state Breit—Pauli R-matrix approach and convergent close-coupling model. Chapter — 6 describes the study of electron impact excitation of the 6p7s 3P0,r states of lead atom from its ground 6p2 3P0 state. Recently, measurements have been made at incident electron energies E0 = 10, 20, 40, 60, 80, and 100 eV and scattering angles up to 150' by Belgrade group, Serbia. In the light of the measurements RDW results for the differential, integral, momentum transfer and viscosity cross sections are obtained at these energies and compared with the corresponding measured values. Chapter — 7 presents investigations for elastic scattering of electrons from lead atoms. This is in continuation to the work presented in Chapter — 6 for inelastic scattering of electrons from lead atoms and in the light of further measurements from the Belgrade group, Serbia for elastic scattering. However, for the study of elastic scattering of electrons from lead, a relativistic approach based on solving the Dirac equation with suitable complex optical model potential has been used. The cross section results are obtained using the method of phase shift analysis. Results are obtained for differential, integral, momentum transfer and viscosity cross sections in the energy range from 10 to 100 eV and compared with the recent measurements from the Belgrade group. The last Chapter — 8 gives the summary of the entire work presented in the thesis and the concluding remarks, comments and suggestions. vi