ELECTRON IMPACT EXCITATION OF ATOMS (IONS) AND PLASMA MODELING

Abstract

The plasma described as fourth state of matter by I. Langmuir evoked great cu riosity and gave rise to an interesting research area in physics having numerous applications in the different field of science. The researchers throughout the globe have been endeavoring to understand the physics of natural and laboratory plas mas for many years. In this connection, various experimental and theoretical efforts are being made to investigate the different laboratory plasmas such as inductively coupled plasma, laser produced plasma, electron-beam generated plasma, direct cur rent glow discharges, capacitively coupled radio-frequency discharges, pulsed glow discharges, magnetron discharges, electron cyclotron resonance sources plasma and high temperature controlled fusion plasma, etc. These laboratory plasmas are the successful sources of different novel applications viz. surface modifications, etching, deposition of thin films, cleaning, surface hardening, fluorescence lamps, plasma dis plays, environmental applications, lasers, biomedical application, food processing, etc. Now a days, numerous techniques are being developed and employed in order to characterize the different laboratory plasmas depending upon their nature and experimental conditions. It is observed that among the different type of diagnostic techniques, the optical emission spectroscopy (OES) is one of the oldest and still extensively used method to obtain the plasma parameters. In this technique, the spectra of plasma emitted radiation is recorded and spectral analysis of the mea surements is done. However, the identified lines from the spectra of the plasma are not sufficient enough to characterize it or to obtain the plasma parameters. In fact, i ii it has to be further coupled with a suitably developed or imagined kinetic model which mimic the real plasma in the sense that all possible atomic and molecular processes occurring in the plasma are considered. Further, through such plasma model, theoretically the line emission intensities are calculated and then compared or matched with the OES measured spectral lines. The comparison thus provides the appropriate plasma parameters viz. electron temperature, electron density, state population of the different fine structure energy levels of the atomic species present in the plasma which emit the radiation. However, the development of a suitable plasma model is not straightforward. It requires the proper inclusion of all the significant kinetic processes occurring in the plasma i.e. various collisional and radiative processes viz. electron impact excitation, de-excitation, ionization, three body recombination, two body recombination and radiative decay, etc. depending on the nature of plasma. In order to consider these processes in the model their rate coefficients data is required and can be taken from the literature or need to be obtained by theoretical or experimental methods. Due to non-availability or scarcity of such data in the literature, specially, of the most dominant process i.e. electron impact excitation (EIE), different simplified plasma models viz. thermodynamic equilibrium (TE), local thermodynamic equilibrium (LTE), and coronal models have been proposed and utilized to characterize the different plasmas. In fact, such basic or primitive plasma models are not adequate for the plasma diagnostics as these do not represent the real picture of the plasma. In the recent days, therefore, efforts are being made to carry out the diagnostics of the different laboratory plasmas by using extensive collisional-radiative (CR) mod els. The development of a CR model for any such plasma requires essentially the atomic data of fine structure energy levels, transition probabilities and EIE cross sections of involved various transitions for the incorporation of different radiative and collisional processes occurring in plasma. Recently with the help of various iii OES measurements, many detailed CR models have been developed where the the oretically calculated EIE cross-sections and other atomic data have been used for the characterization of different inert gas viz. Ne, Ar, Kr and Xe plasma. Differ ent spectroscopic studies of various low temperature experimentally generated inert gas mixture plasma of Ar/N2, Ar/O2 and Ar/CO2 have been reported which have several industrial applications viz. thin film deposition, surface chemistry, material modification, etching and surface sterilization, etc. Only in the recent years, for the characterization of such mixture plasmas few detailed CR models have become available. In a similar way, many laser produced plasmas are being created using the different metal targets viz. Zn, Cu, Mg, Al, etc., but due to the lack of complete set of EIE cross-section data of these metal target atoms, the suitable plasma models have not been developed. In addition, there have been current interest in carry ing out the diagnostics of the low temperature inert gas plasma through their ionic emission lines observed in the spectral measurements. Their emission lines, in fact, play important role in understanding and in the diagnostics of the non-equilibrium plasmas, but so far there are not extensive studies with the measured ionic emission lines from different inert gas ions. This can also be attributed due the lack of de tailed EIE cross-sections for inert gas ions and consequently there is urgent need of the calculations of the same. It is observed that with the time, the accuracy and reliability of the atomic structure calculations have increased with the availability of better atomic structure computer codes and computation facilities. However, the reports of detailed or complete set of atomic and electron-collision data are still lacking for many neutral atoms and ions. Unfortunately, the measurements of the electron impact excitation cross-sections have been done only at few selected electron impact energies and also mostly for the transitions from the ground state to few selected excited upper states, which are thus not sufficient at all to develop an adequate or suitable CR model. Therefore, the iv requirement of detailed cross-sections for large number of fine structure transitions can only be fulfilled through the different theoretical approximation methods and these need to be taken up in a systematic and complete manner. In view of this, in the present thesis an attempt has been made to fulfill such requirements by carrying out the relativistic calculations for the neutral atoms and ions. Specifically for their f ine structure level excitation energies, radiative transition probabilities, EIE cross sections, and the rate coefficients and thereafter implementing these in developing suitable CR models to characterize the laboratory based plasmas. In the light of above discussions, in the present thesis, the developments of two different collisional-radiative models are considered for the characterize of low tem perature laser produced Zn plasma and Ar/N2 inert gas mixture plasma through their reported spectral line measurements. In addition, the thesis also reports the cross-section calculations of electron collisions with the inert gas singly ionized tar gets viz. Kr+ and Xe+ ions. For these ions, the calculations of linear polarization of radiation emitted by decay of the electron excited states are also reported as these can provide additional information for the diagnostics of their plasma. The entire work reported in the thesis has been described through six Chapters as given below. Chapters 1 & 2 provide an overview of plasma, role of electron collisions with the atoms or ions in the plasma, the plasma characterization parameters as well as the different plasma models available and used. Further, it explains, the development of a comprehensive collisional-radiative model along with the details of different atomic collisional-radiative processes and the solution of particle balance equation to obtain the population of atomic energy levels. In addition, the full description of the relativistic distorted wave method is discussed which is adopted for the calculation of fine-structure EIE cross-sections. It also outlines whole work of the thesis. Chapter 3 describes the development of a collisional-radiative model for the laser induced zinc plasma by considering all the important plasma kinetic processes. The v model considers several fine structure levels of Zn and their electron impact excita tion and de-excitation which play a dominant role in the laser induced plasma. In view of this, extensive calculations are performed using fully relativistic distorted wave theory to obtain the EIE cross-sections of the large number of fine structure transitions of zinc atom. Further, the calculated cross-sections are incorporated in the CR model for the plasma characterization. By solving the particle balance equation, the state population densities of the fine structure energy levels of the neu tral Zn atom are obtained, from which the CR model emission line intensities have been calculated. Further, the calculated intensities are matched with the available recently reported OES measurements and the plasma parameters are evaluated at different ambient pressures. Chapter 4 presents the development of a collisional-radiative model for the Ar/N2 mixture plasma. Model considers the well-known populating and depopulating chan nels along with the processes which account for the coupling of argon with nitrogen molecule. For the proper inclusion of excitation and de-excitation processes in the model, the fully relativistic EIE cross-sections of several fine structure transitions of argon have been incorporated. Further, this model is coupled with an available re cent spectroscopic measurements for a low pressure electron beam generated Ar/N2 plasma and the plasma parameters are evaluated by optimizing the model simulated intensities with the measured intensities. The electron density and temperature are extracted and investigated from the CR model of Ar/N2 mixture plasma at different concentration of N2. Chapters 5 & 6 report respectively the detailed EIE cross-section results of the electron impact excitation of Xe+ and Kr+ ions. The calculations are performed for the large number of transitions from their ground state to the different fine struc ture levels of the excited state configurations using the relativistic distorted wave method. For both the ions, the detailed EIE cross-sections results along with the vi corresponding rate coefficients are reported for all the considered fine-structure ex citation transitions in the wide range of incident electron energies. In addition, for the plasma modeling purposes the analytic fittings of the calculated cross-sections are also provided for each transition of both the ions. Further, the linear polariza tion of photon emissions is calculated and reported for all the dipole allowed decay transitions from the electron excited states. Chapter 7 finally summarizes the present thesis work, puts few important com ments and adds concluding remarks along with pointing out some future work which can be undertaken.