SCATTERING OF eT AND x-ray FROM ATOMS/MOLECULES : SPIN POLARIZATION AND CORRELATION EFFECTS

Abstract

The work reported in the thesis deals with the study of spin polarization and correlation effects on the scattering of electrons, positrons and x-ray from atoms and molecules. The whole work can be divided into three categories. Under the first category, we have calculated the spin polarization parameters and various scattering cross-sections such as differential, integrated-elastic, momentum transfer and the total cross-section for electrons scattered from heavy atoms such as closed-shell (Yb, Ra and Rn) and open-shell configuration atoms (Eu and Bi) and for positrons scattered from Xe, Yb and Hg atoms. Under the second category, we have studied x-ray and high-energy electron scattering from eighteen electron hydride series HCI, H2S, PH3, SH-14 and an organic molecule methanol (CH3OH) using a density functional theory. In addition we have also calculated the isotropic and directional Compton profiles, momentum density and various momentum expectation values of the silane (SiH4) molecule for several wavefunctions at different level of correlation and in the last category, we report our results of energy levels, oscillator strength, transition probabilities for all dipole allowed transitions among the 24 terms belonging (1s22s22p6) 3s2, 3s3p, 3p2, 3s3d, 3s4s, 3p3d, 3s4p, 3s4d, 3s4f and 3p4s configurations together with fine structure level of these terms for magnesium like chlorine (CI VI) using extensive configuration interaction wave function with relativistic effect through the Breit-Pauli Hamiltonian. Scattering of electrons from atoms and molecules has been the object of research for a long time. The recent technical advances in the study of scattering of spin-polarized electrons have made it possible to understand the the correlation polarization and absorption potential as devised for positron atom scattering. The effect of correlation on cross section for the series of molecules HCI, H2S, PH3, SiH4 and an organic molecule methanol (CH3OH) is considered. In recent years, the technical advances in experimental x-ray scattering and high-energy electron scattering have made it possible to obtain elastic and inelastic intensities with ever increasing accuracy. Elastic high-energy electron scattering from molecules provides information about electron distribution in molecules, while the total (elastic+inelastic) electron scattering intensities reflect electron correlation effects in the system via electron-electron distribution factor. X-ray scattering cross sections are closely related to these for the scattering of fast electrons and provide similar information about the electronic structure of atom and molecules. We report the elastic and total intensities for x-ray and high energy electron scattering from the eighteen electron hydride series HCI, H2S, PH3, SiH4 and an organic molecule methanol (CH3OH) as calculated from Kohn-Sham orbitals using B3LYP, BLYP and LSDA functionals and compared to Hartree-Fock and high quality multi-reference configuration interaction wavefunctions. In addition we also report our studies on electron momentum distribution (EMD) in molecular systems through the analysis of Compton line shapes in the gaseous phase. We have calculated the isotropic, directional Compton profiles, momentum distribution and various momentum expectation values and internally folded density (B(r)-function) of silane (SiH4) molecule using several wavefunctions at different level of correlation along with the Hohenburg:Kohrn density functional approach. This study provides an alternative way of understanding the electronic structure iv of molecules. Elements in Mg-isoelectonic sequence are astrophyscially abundant and accurate atomic data such as absorption oscillator strengths and transition probabilities are needed for the interpretation of accurate observational data. These atomic data are also useful in many astrophysical studies. In order to make a direct comparison with these observational data, accurate theoretical approach, which can include the electron correlation effects in the description of the target states. Configuration-Interaction (CI) calculations within a semi-empirical model potential framework are presented for oscillator strength in the Mg-isoelectronic series. We have done calculations for oscillator strengths and transition probabilities for 24 LS levels and for all 44 fine structure transitions of C1+5 Mg—isoelectronic. In our calculations, we include a fairly large number of configuration within the n=5 complex in the CI expansions. The relativistic effects in intermediate coupling are incorporated by means of the Breit-Pauli Hamiltonian. Our calculations agree reasonably well with the experimental and other theoretical results