STUDY OF ELECTRON INTERACTION WITH ATOMS AND IONS

Abstract

The present thesis concerns with the three broad aspects. The first one is to provide detailed theoretical investigations on excitation of highly charged xenon ions due to electron impact. Such a study is important for modeling fusion plasma, for example, in case of International Thermonuclear Experimental Reactor (ITER), xenon is expected to be used as a coolant. Therefore, several charged species of xenon are likely to be present in ITER plasma and their atomic data is highly essential for plasma modeling. The second aspect is to study electron impact excitation of atoms and ions which are heavier than zinc and found to exist in the interstellar medium as predicted by the observations from Goddard High Resolution Spectrograph (GHRS) aboard the Hubble Space Telescope (HST). There are no excitation cross section results reported for these systems in spite of a great demand of appropriate and reliable theoretical calculations to improve our understanding of astrophysics related to the interstellar medium. Finally, the third aspect is to study the elastic scattering of electrons from free as well as confined Ca atom inside a spherically symmetric fullerene cage using a relativistic optical model potential approach. Trapped atoms show significant changes in their physical and chemical properties with respect to their corresponding isolated species. These difference in properties can be exploited for many practical applications, nano sciences biological sciences etc. This allows us to understand the influence of confinement, not only on spectroscopic properties, but on scattering parameter also. The entire work of the thesis is presented through 1-7 Chapters and has been briefly described below Chapter – 1 contains an introduction to the gradual progress and shifting of emphasis pertaining to the studies on electron-atom collisions in view of the current interests. The relativistic distorted wave (RDW) method, used in the present thesis to describe electron impact excitation of atomic systems, is described in detail. This method has been implemented in major part of the thesis for studying the electron impact excitations of different atomic systems. Chapter – 2 presents our investigation on the electron impact excitation of the fine-structure levels of the ground state configuration 5p2 to the excited states of the configuration 5p6s in tin atom. We used RDW method to perform these calculations within the 𝑗𝑗 coupling scheme. The bound state wave functions required in these calculations are obtained using GRASP2k program. Accuracy of these wavefunctions is decided by comparing our calculated excitation energies and oscillator strengths for the different transitions with the available measurements and other theoretical calculations. We presented differential cross sections at incident electron energies 20, v 50, 80 and 100 eV and integrated cross sections are presented in the incident electron energy range of 5 to 100 eV. In chapter – 3 we consider electron impact excitation of the fine-structure transitions from the 𝑛𝑠2𝑛𝑝 𝑃1/2,3/2° 2 states to the excited 𝑛𝑠2𝑛𝑑 𝐷3/2,5/2 2,𝑛𝑠𝑛𝑝2 𝐷3/2,5/2 2 and 𝑛𝑠2(𝑛+1)𝑠 𝑆1/2 2 states of Sn+ (n =5) and Pb+ (n = 6) ions. The relativistic distorted-wave approximation has been used to calculate the integral cross sections for these transitions. Our calculated oscillator strengths using multiconfiguration Dirac-Fock (MCDF) method are compared with the results from NIST database, measurements and other theoretical calculations. The cross sections are presented for all the considered fine-structure transitions in the scattered electron energy range of 20 to 100 eV. In case of the resonance transition 6𝑠26𝑝 𝑃1/2° 2→6𝑠2𝑛𝑑 𝐷3/2 2 in Pb+, cross sections are compared with the only available recent measurements [Gomonai et al. Eur. Phys. J. D 71 31 (2017)] and good agreement is found at the high incident electron energies. Further, the analytical fit to the cross sections are reported. Chapter – 4 deals with the detailed investigations of electron impact excitation of 37 transitions among the fine-structure levels of 5s2, 5s5p, 5s5d, 5s6s and 5s6p configurations of In+. We performed RDW calculations to report cross section for these transitions for scattered electron energies upto 200 eV. The multi configuration bound state wavefunctions for initial and final states are obtained and used in cross section calculations. Our calculated oscillator strengths are compared with the results from NIST database and other theoretical calculations. Comparison of the cross sections for the transition 5𝑠2𝑆0 1−5𝑠5𝑝𝑃1° 1 is presented with the measurements reported by [Gomonai et al. Nucl. Instr. Meth. Phys. Res. B 233 250 (2005)]. We have also given detailed analytical fitting of all the cross section results for plasma modeling purposes. Chapter – 5 contains a comprehensive study of electron impact excitation of Ge-like (Xe22+) to Cu-like (Xe25+) highly charged xenon ions. We consider dipole allowed fine-structure transitions in the wavelength range 9-25 nm and use relativistic distorted wave method to calculate the electron excitation cross sections for the different transitions of the four ions in the scattered electron energy range from excitation threshold to 1500 eV. The accuracy of the MCDF wavefunctions for the ionic bound states is examined by providing a detailed comparison of the wavelengths and oscillator strengths of the considered transitions in these ions with the previously reported measurements and other theoretical calculations, where available. Further, assuming the electron energy distribution to be governed by Maxwell distribution in plasma, the excitation rate coefficients are determined from the cross section results in the range of electron vi temperature up to 50 eV. We also provide analytical fitting of our cross sections for their direct and convenient implementation in any plasma model. In chapter – 6, we study elastic scattering of electrons from free and confined Ca atoms using relativistic approach based on solving the Dirac equation with suitable complex optical model potential. The confining potential is taken to be an attractive spherically symmetric potential well of an endohedral fullerene C60 cluster. We analyse the influence of entrapment on spectroscopic properties of Ca by determining the electron correlation energy, ionization potential and dipole polarizability as a function of increasing depth of the potential well. In these calculations, we have employed Ca atom wavefunctions obtained from relativistic coupled cluster (RCC) as well as Dirac-Fock method. We compare our results for correlation energies with the previous non-relativistic calculations employing multi-configuration Hartree–Fock (MCHF) method. Using the obtained charge density and phase shift analysis technique, the differential and integrated cross sections are presented for elastic scattering of electrons from free and confined Ca atoms and effect of potential depth is described on these properties. Finally, the last Chapter – 7 summarizes the entire work presented in the thesis as well as gives the concluding remarks, comments and suggestions.