ELECTRON (POSITRON) EXCITATION OF HYDROGEN, HELIUM AND THE IONS OF THEIR ISOELECTRONIC SEQUENCES

Abstract

Electron and/positron impact excitations of the atoms and/atomic ions are the most fundamental processes in atomic physics. A wide variety of classical, semiclassical and quantal theories have been proposed or used to describe these processes theoretically. Among various quantum mechanical theoretical studies the distorted wave methods have become known in recent years as being rather simple but of wide applicability. There is however, no single distorted wave method, but many versions of that. The present work in the thesis is the author's attempt to investigate the scattering of electrons and/positrons by simple atomic targets of hydrogen-and helium-like using a suitable distorted wave theory. The whole work presented in the thesis can be broadly divided into three categories. Out of these, the first one deals with the single electron excitation of the atomic targets from their ground states. The second involves the single electron excita-tion of the atomic systems initially frOm their metastable excited states and finally, the third category is related with the double electron excitation of the autoionizing levels. For the sake of convenience and proper presentation of the work the thesis is divided into ten chapters as briefly described below. The first chapter gives the general introduction to the subject and the basic idea of the work reported in all other chapters. The brief review of the previous works, recent theoretical and experimental developments in connection with ii) collisional processes considered in the present study are also discussed. Various approximate methods which have been referred or used in the present work are: briefly described and discussed. In the second' chapter, the study of the electron and positron impact excitation of n = 3 states in the hydrogen is presented. Results for differential and total. cross sections as well as the polarization fractions in the range of incident 'projectile energy 30-200 eV are reported. In addition to -the cross sections the electron (positrons) - coincidente parame-ters for the excited 3p and 3d states in hydrogen are also obtained for the first time. All these various results are, evaluated using a consistent version of distorted wave Born approximation (DWBA) approach. The DWBA method considers the distortion of the projectile in both the initial and final channels by appropriate static field of 'the target, the effec.t; of polarization of the target by the projectile and the exchange effect (in case of electron impact) between the electrons of the projectile and target. Comparison of calculated results with other theoretical and experinntal measurements are also presen-ted and discussed. A good agreement is obtained between the present calculated and recently measured electronphoton coinci-dence rates for 32P. and 32D. states in hydrogen. The third chapter of the thesis deals the study of the 11S -> 21S, 21P excitations in helium by impact of electrons and positrons. Various results viz, differential and total cross sections for both the excitations as well as coincidence and circular polarization parameters for 21P excitation are obtained. For the calculation the similar distorted wave Born approximation as adopted for the hydrogen (in second chapter) is used. To avoid the uncertainties in the calculated results due to input bound state wavefunctions of the helium atom the accurate wavefunctions of the many parameters (53 terms) correlated type have been used. Further, all various results are found to give reasonable agreement when compared with the reported experimental and other reliable theoretical data.