This Is AuburnElectronic Theses and Dissertations

Electron dynamics in inner-shell photo-ionization, dielectronic recombination and electron impact excitation




Wang, Qianxia

Type of Degree

PhD Dissertation




This dissertation includes three main topics: Dynamics of outgoing inner shell photoionized electrons and Auger decay electrons, an investigation into the recently proposed mechanism of below threshold dielectronic recombination and electron impact fine-structure excitation of Ne+ and Ne2+ at very low temperatures. For the first topic, we compare our time-dependent numerical method with a widely used analytical one at different photo-electron energies. We determine that the time-dependent numerical method is more accurate in the low and medium energy region. We apply this method to investigate inner shell photo-ionization followed by Auger decay in Kr at low energies (including as low as zero atomic units of energy). We investigate an interesting feature in the relative angular distribution of the outgoing electrons. To confirm the validity of this phenomenon, we use a Classical Trajectory Monte Carlo Method to re-simulate the process, with similar results being generated. A physical mechanism is identified for the unusual features in the relative angular distribution. The second topic concerns a newly proposed mechanism for dielectronic recombination. The traditional definition for dielectronic recombination only counts the contribution of above threshold (or positive energy) resonances which can be accessed via free electrons. This definition leads to several problems when the temperature is very low. For example, the dielectronic recombination coefficient becomes very sensitive to the energy position of resonances and whether they are above or below threshold. Moving a resonance by a few meV from above to below threshold can change the dielectronic recombination rate coefficient by large values, often factors of 10 or 100. A new recombination mechanism was proposed recently, pointing out that certain below threshold resonances should also be counted in the rate coefficient calculation due to an interaction with Rydberg electrons. This new mechanism has not been an experimentally confirmed. We proposed that below threshold satellite line observation can be used to prove the existence of this new mechanism. We also develop a method to identify ions with near threshold resonances and several key ions are selected to re-evaluate their dielectronic recombination rate coefficient with the inclusion of below threshold resonances contributions. For the last topic, we calculate collision strengths, effective collision strengths and excitation rate coefficients for fine-structure electron impact excitation of Ne$^+$ and Ne$^{2+}$ using different R-matrix techniques and different target expansions. We compare target energies and Einstein A coefficients with NIST values, and compare calculated effective collision strengths with existing values. A recommended dataset is provided for each ion, along with an estimate of the uncertainty in the collision data. This work has important applications in ultra-low temperature astrophysical plasmas.