INELASTIC SCATTERING OF CHARGED PARTICLES FROM ATOMS, IONS AND MOLECULES

Abstract

The work reported in this thesis is author's attempt to study the various excitation processes during the scattering of charged particles with atomic and molecular target systems. The whole work which is based on a quantum mechanical approach is categorised into three sections. In 'the first section a detailed theoretical study of the inelastic scattering of charged particles with hydrogen, helium and lithium target atoms, is presented. In the second section of the thesis the inelastic scattering of electrons by helium-like ions in the presence of a plasma is studied. In the third and last section we have studied the collisional quenching (deexcitation) of metastable helium ion with diatomic molecules. In the first chapter of this thesis, we have given a general introduction of the subject and discussed the various theoretical and experimental developments in atomic collision processes of relevance to the present study. Various approximate methods, which have been referred/used in the present work, are briefly reviewed in this chapter. This chapter also forms the basis to the work reported in subsequent chapters. -iv- In chapter II, we study several excitations of hydrogen atom (from ground and metastable state) by charged particles (electron, positron and proton) impact at inter-mediate and high energies in the framework of a distorted wave approximation. A comparative study is made by taking distorted wave (Coulomb wave) either in the initial channel, or in the final channel or in both the channels. The exchange effect in the theory is included through the Ochkur-Bonham approximation. Significant improvement over the first Born approximation has been achieved specially at large angles. A good agreement is also obtained with other theoretical and experimental results (wherever available). In chapter III, an extension of above approach is made to the inelastic scattering of helium atom by charged particles impact at intermediate and high energies. The differential and total cross sections for singlet-singlet and triplet-triplet excitations in helium atom are calculated. The results are compared with the other more sophisticated calculations and experimental data. Further we continue our study of inelastic scattering of electrons and positrons by lithium atom in chapter IV. We follow a two-potential approach and take the scattered particle wave function to be distorted, both in the initial and final channels. For electron scattering the exchange effect is also taken into account. Angular correlation. parameters (A, , ,A(c), and 41.) and the diffe- rential cross sections for the resonance transition (2s-2p) are computed. Electron impact excitation-of the forbidden transition (2s-3s) is also studied in this chapter. From this study it is found that the two—potential approach provides very good results for the resonance transition in lithium atom. In chapter V, we study the electron impact excita-tion (115-21P) of helium like ions in a dense plasma. The collision strength for this transition is calculated in the framework of a two—potential approach alongwith the Debye—Wickel model for plasma screening. In the last chapter VI, we study the collisional quenching of metastable helium ion by diatomic molecules (H2' N2' 02' CO and NO). We have used a time—dependent pertur-bation approach to study this quenching process. The effect of the long range and the short range forces is explicitly included in the interaction potential. The results for the quenching cross sections and deexcitation rate constants are presented and compared with the available experimental data.