Date of Award
May 2020
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Physics and Astronomy
Committee Member
Joan Marler
Committee Member
Chad Sosolik
Committee Member
Brad Meyer
Committee Member
Stephen Kaeppler
Abstract
Modeling the spectra of astrophysical environments requires knowledge of the absolute cross sections for the relevant interactions occuring between ions and neutrals. Two of the common processes are a) charge transfer, which has been shown to be the source of cometary x-rays, and b) spontaneous emission, which is used as an abundance diagnostic in studies of stellar atmospheres and other exotic environments. Understanding the source of observed emission lines, e.g. charge exchange or de-excitation, will require an understanding of the electronic structure of each element in the environment being studied. This dissertation examines the charge exchange process with singly charged ions and investigates spontaneous emission from heavy atoms and ions.
Charge exchange measurements are typically performed either with a gas cell or with a gas jet. For absolute cross sections, a gas cell provides a well defined pressure and path length. The design and experimental procedure for a gas cell based experiment optimized for interactions expected to have large cross sections is discussed. Charge exchange cross sections are reported for the four symmetric ion-neutral reactions He+ - He, Ne+ - Ne, Ar+ - Ar, and Kr+ - Kr between 0.2 - 5.0 keV and compared to the available experimental and theoretical results. Extrapolation of a fit to the current data using the theoretically suggested functional form provides good agreement even with the available high energy experimental results. A modified gas cell with improved gas conductance and the ability to scan the collision energy without altering the ion source parameters is also described.
For interactions where it is desired to extract both the product ions and/or photons from the interaction region, a crossed beam set-up is required. A gas jet provides near-complete optical access to the interaction region but a less rigidly defined pressure and path length. Two gas jets were designed for future cossed-beam experiments involving solar wind ions and simple neutrals. The first design is intended for delivering neutral beams with central densities of order 10^6 - 10^7 cm-3 to the CUEBIT drift tubes for producing metal ions. The operation of the jet is consistent with a simple theoretical model. A second gas jet, intended for operation at higher backing pressures, is presented and expected to produce dense targets (n > 10^10 cm-3) to a crossed-beam apparatus downstream from CUEBIT.
In environments containing heavy elements, emission lines may be used to estimate the abundances of these heavy species. Therefore accurate knowledge of the electronic structure of both the ground and higher charge states of the elements is required for interpretation of the astrophysical spectra where these elements may be found, e.g. kilonovae or solar wind interactions. The electronic structure of Au I - II was studied by observing emission from gold targets ablated inside the Compact Toroidal Hybrid plasma apparatus at Auburn University. Gold lines were identified by their time-dependent behavior when compared to similar spectra of nickel targets. New emission lines are found by comparing the observed spectra to Ritz wavelengths calculated from level energies in the literature. Level lists and line lists of dipole-allowed transitions spanning 187 - 800nm are reported for Au I \& II with resolution d(Lambda) / Lambda ~ 10^-4. It is expected that these results will contribute to the understanding and interpretation of spectra from neutron star mergers and chemically peculiar stars.
Recommended Citation
Bromley, Steven James, "Atomic Data Needs in Laboratory Astrophysics: Experimental Methods for Spectroscopy and Charge Exchange with Ions" (2020). All Dissertations. 2601.
https://open.clemson.edu/all_dissertations/2601