UMD Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/3
New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a given thesis/dissertation in DRUM.
More information is available at Theses and Dissertations at University of Maryland Libraries.
Browse
2 results
Search Results
Item Optical and quantum interferences in strong field ionization and optimal control(2017) Foote, David B.; Hill, Wendell T.; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)For decades, ultrafast laser pulses have been used to probe and control strong-field molecular dynamics, including in optimal control experiments. While these experiments successfully recover the optimal control pulses (OCPs), they have a limitation -- it is generally unknown how the OCP guides the target system to its final state. This thesis is concerned with "unpacking" OCPs to explain how they achieve their control goals. The OCPs that inspired this work consisted of pulse trains; a twin-peaked pulse (TPP) is the simplest example. Consequently, TPPs with variable interpeak delay and relative phase were employed in this work to study ionization, the first step in many control experiments. Two types of interference influence ionization from a TPP: optical interference (OI) between the electric fields of the two peaks, and quantum interference (QuI) between the electron wavepackets produced by the two peaks. Two sets of experiments were performed to determine what roles OI and QuI play in controlling ionization from a TPP and how they in turn influence subsequent molecular dynamics. The first set of experiments measured the total ionization yield induced by the TPPs. It was found that OI was principally responsible for changing the ion yield; QuI-induced oscillations were not observed. Small imperfections in the shape of the TPP (i.e., pedestals and subordinate peaks) were found to have a surprisingly large influence in the OI, highlighting the need for researchers in molecular control experiments to characterize the temporal profile of their pulses accurately. A time-dependent perturbation theory simulation showed that the signatures of QuI in the ionic continuum vanish when measuring {\it total} electron yield, but appear in {\it energy-resolved} electron yields. The second set of experiments measured photoelectron energy distributions from a TPP with a velocity map imager to search for QuI. The experiments were performed at high intensities (~10^14 W/cm^2) where the ponderomotive energy tends to wash out the fine energy structures of QuI. The thesis ends by proposing a modified, low-intensity experiment that will allow for the first unambiguous observation of QuI in non-resonant, multiphoton ionization.Item Learning to Forget: An Interference Account of Cue-Independent Forgetting(2009) Tomlinson, Tracy Darlene; Dougherty, Michael R; Psychology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Memory suppression is investigated in inhibition paradigms that produce cue-independent forgetting. Because the forgotten items are not retrieved even when tested with an independent, semantically related cue, it has been assumed that this forgetting is due to an inhibition process. However, this conclusion is based on comparing inhibition to classic interference theory with a single stage of recall. Yet, memory models, which produce forgetting through a process of interference, include both a sampling and a recovery stage of recall. A neo-classic interference theory is proposed, which assumes that interference exists during recovery as well as sampling and can explain cue-independent forgetting. Three behavioral studies tested predictions of the neo-classic interference theory. Experiment 1 found support for recovery interference in testing key predictions of the theory within the think/no-think paradigm. Most importantly, learning to quickly press enter produced as much cue-independent forgetting as no-think instructions. Experiment 2 tested the role of word frequency in terms of sampling and recovery, but failed to obtain cue-independent forgetting. Experiment 3 reversed the order of blocks and produced original cue forgetting following retrieval practice with independent cues, which provided a clear manipulation of recovery strength. Lastly, a mathematical model (SAM-RI) of neo-classic interference theory was specified that captures data from Experiment 1, Experiment 3, and is extended to the greater retrieval induced forgetting.