COLLISION OF CHARGED PARTICLES FROM ATOMS AND MOLECULES

Abstract

The work reported in the thesis contains the results of the authoris,attempt to study the scattering of inter,- mediate and high energy electrons or positrons by simple atomic and molecular target system, using the various quantum mechanical approaches. The whole work has been divided into three main categories. Under the first category we haNe studied the inelastic scattering of electron by simple atoms like helium and lithium using various perturbative methods. Under the second category, we have studied the positron impact excitation of helium using same peiturbative approaches and in the last category we have calculated the elastic, inelastic, total high energy electron and x-ray scattering from the ten electron systems Ne, HP, H20, NH3 and CH4 using SCF-MO wave-function obtained in double zeta quality basis of Gaussian contracted wavefunctions within the framework of the first Born approximation. The thesis has been written in seven Chapters. The first Chapter gives a brief review of previous work and various quantum mechanical approaches which have been used in different manner. Among the various quantum mechanical approaches, the perturbative models such as distorted wave and its variants, eikonal Born series, modified Glauber approximation, first Born approximation are worth mentioning. In all these approaches the central idea has been to pay attention to the second order term and attempt tocalculate it as accurately as possible. This chapter forms the ground- work to the work reported in subsequent chapters. In recent years it has been noticed that the theoret-ical calculations suffer from two sources of uncertainties (i) adoption of an approximate model within the frame work of which the calculation is carried out (ii) choice of the bound state wavefunction to represent the initial and final states of the atomic targets as input to evaluate the scattering amplitude in that particular model, In the subse-quent chapters, i.e., 2,3, and 4 we have tried to minimise these uncertainties. For example, the Chapter 2 present our study of electron impact excitation of helium (116- 1s,211,) in the energy range (50500 eV) using the Coulomb-Born model. Closed form expressions for scattering amplitudes have been obtained with Fourier-decomposition of interaction potential and then the use of accurate correlated wavefunctions. The results of the present calculations have been compared with other available theoretical calculations and the experimental measurements. Chapter 3 contains the results of our calculation for the differential and total inelastic scatteriog cross-section, using electron impact excitation (113 21S, 21P) of helium in distorted wave approximation (beyond Coulomb Born model as reported in Chapter 2) at intermediate and high energies. The effect of the distortion of incident electron, contribution due to polarization of the target —vi-- and the exchange effect are appropriately taken in both the initial and final channels. Fourier decomposition of interaction potential between the projectile and target is taken and an accurate form factor has been used to calculate the transition matrix. The resulting radial Schr&linger equation was solved by a standard noniterative procedure given by Marriott and Percival (Proc.Phys.Soc.Z2 121(1958):. The present results show very good agreement with experi-ment. In Chapter 4, we extend our study (as outlined in Chapter 2 and 3) for 21S and 21P transition of helium atom by positron impact. As is well known that the positron impact studies differ from its similar counterpart study by electron in two ways (i) No exchange effects are present (iii) the static distortion potential now bears a negative sign. In this way we have carried out this study on the same footing as has been done for electron impact. Present results are compared with the earlier theoretical estimates. The Chapter 5, presents our results of electron impact excitation of 3s state of lithium in the energy range 20,-200 eV. Differential cross—sections for this transition are obtained in eikonai Born series, Modified Glauber approximation, second. Born, Glauber and first Born approximation. In addition, we have also calculated the generalised oscillator strength employing a variety of target wavefunctions. In this chapter. we shall also report the results of resonance transition (2s-2p) of .lithium atom in Modified Glauber approximation. In Chapter 6, we present our results concerning molecular high energy electron and x-ray scattering inten-sities for ten electron systems (Ne,HF,E[20, NH3 and CH4). The difference between the usual elastic intensities for electron and x-ray from nonvibrating but freely rotating molecules and the fully elastic intensities for scattering from the J,0 state are studied. The effect of molecular binding and various other trends and systematics in the intensities have been examined with the help of difference function computed between the present scattering intensities (total, elastic, inelastic) and that for the independent atom model (lAM). Chapter 7 summarizes the work reported in the earlier chapters and contains some comments, pointing out the drawback and the suggestions for their improvement.