Browsing by Subject "Ionization"
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Item Dynamics of noble gas cluster expansion driven by intense pulses of extreme ultraviolet light(2009-08) Murphy, Brendan Francis, 1976-; Ditmire, Todd R.; Keto, John W.The interaction of intense laser pulses with nanometer scale atomic clusters has been an active area of study since the advent of amplified femtosecond lasers. In the case of infrared irradiation of noble gas clusters, direct field-driven ionization results in the ejection of energetic electrons, high ion charge states, and Coulomb explosion of the ion core of the clusters. These processes result from electron motion driven by the cluster potential and the large ponderomotive potential of the laser field. When extreme ultraviolet (XUV) pulses interact with clusters, the mechanisms responsible for the infrared response are 'turned off' because the ponderomotive potential is very small. We have conducted cluster experiments at 38nm using focused XUV pulses produced by high harmonic generation with a 15TW Ti:Sapphire laser. We measured the charge states and kinetic energy spectra of ions produced in the interaction, and observe substantial ion population up to Xe⁵⁺, with a small number of Xe⁶⁺-Xe⁸⁺ ions produced by collisional ionization by hot plasma electrons. The ion kinetic energy spectrum indicates a hydrodynamic expansion at an ion temperature of 8eV. This is in stark contrast to intense infrared/cluster interactions, where clusters are stripped of electrons to a large degree and expand by Coulomb forces, resulting in far higher ion kinetic energy for similar degrees of ionization.Item Electrospray ionization mass spectrometric techniques for the study of molecular recognition(2005) Sherman, Courtney Lawrence; Brodbelt, Jennifer S.The ability of electrospray ionization mass spectrometry (ESI-MS) to quantitatively analyze the distribution of complexes resulting from molecular recognition in solution was modeled, and ESI-MS techniques were developed to analyze complexes involving several different types of novel compounds in different areas of molecular recognition and supramolecular chemistry. To better understand the relationship between ion abundances observed by ESI-MS and concentrations of host-guest complexes in solution, mathematical models based on equilibrium partitioning theory were developed to relate ESI-MS ion abundances to relative solution concentrations of complexes resulting from host-guest binding. The predictions of these new models were evaluated and experimentally confirmed through the analysis of complexes of crown ethers with alkali metal cations in an ESI quadrupole ion trap mass spectrometer, yielding a greater understanding of the behavior of host-guest complexes in ESI-MS, allowing for more accurate measurements of solution binding interactions. The self-assembly of ligand-metal-ligand sandwich complexes involving a novel quinoxaline-containing crown ether was studied to evaluate the contribution of pi-stacking interactions between the ligands towards the overall stability of the complexes. Donor-acceptor pi-stacking interactions between the electron-poor quinoxaline group and electron-rich benzene groups from benzo- or dibenzo-18-crown-6 were found to significantly enhance the formation of mixed-ligand sandwich complexes. A synthetic pyrrole-inosine nucleoside, capable of forming an extended three-point Hoogsteen-type hydrogen-bonding interaction with guanine, was shown to bind guanosine selectively over other individual nucleosides, and ESI-MS results indicated the formation of specific complexes between the pyrrole-inosine nucleoside and two different quadruplex DNA structures. The specificity of the pyrrole-inosine nucleoside for quadruplex DNA suggests that it or similar structures based on this binding modality may ultimately demonstrate utility as anti-tumor agents. The interactions between a novel enediyne drug and various cytidine-containing oligonucleotides were studied, and the structures of the DNA-enediyne adducts known to lead to cytidine-specific DNA cleavage were examined. Collisionally activated dissociation of the adducts confirmed their strength and suggest a direct linkage between the enediyne and the cytidine nucleobase, likely the result of a nucleophilic attack by the cytidine amine.Item Low noise avalanche photodiodes with an impact-ionization-engineered multiplication region(2002-08) Wang, Shuling, 1972-; Campbell, JoeAn avalanche photodiode (APD) is frequently the photodetector of choice in high-bit-rate, long-haul fiber optic communication systems due to its higher sensitivity, relative to a PIN photodiode, afforded by its internal gain. However, this can only be accomplished given that the multiplication noise is low. Impactionization-engineering (I2 E) is a novel approach that incorporates materials with different impact ionization threshold energies (Eth) into the multiplication region of APDs for low noise, high gain, and low dark current. A series of multiplication region structures with record-low multiplication noise were developed on both GaAs and InP substrates; an excess noise level comparable to silicon APDs was achieved on I2 E structures grown on GaAs. Unlike “superlattice” or “staircase” structures, the band gap continuities are not involved in the working mechanism of I2 E APDs. Monte Carlo simulation has revealed the spatial modulation effect of the impact ionization events in these heterostructure devices, which makes the ionization process more deterministic than in homojunctions, thus yielding lower noise. These low-noise I2 E multiplication region structures are promising in improving APD performance once they are implemented into SACM structures, with working wavelengths including 800-900nm, 1.3µm, and 1.55µm.Item Nanowire sharpening : application to field ionization(2021-05-03) Leviyev, Alex; Raizen, Mark G.Nanowires show potential for a wide range of fields, from developing next generation solar cells, to detecting viruses, to field ionizing gasses. Their uses in such disparate fields are due to the extreme flexibility in which nanowires can be manufactured and customized to order. The heart of a nanowire, however, is it’s tip. Here surface charges accumulates in extreme densities when the wire is biased, consequently producing large electric fields that are then used in creative ways for exciting applications. My aim for this thesis is three-fold. First, I intend to establish a context for nanowires. Why were these structures studied in the first place? What are some exciting application areas? What makes nanowires unique? Etc... This will set the stage for subsequent sections, and provide the salt and pepper that will make the main course more flavorful. The second is to present an overview of relevant results from the literature that I will later build upon. There are analytical models of varying complexity examining the type of protrusions we are interested in here, as well as numerous numerical studies. We will look at them to get a deeper intuition for whats happening, and use them as a basis for comparison later on. Finally, I will present my results and discuss their consequences. As we will see, sharpening a cylindrical-post nanowire of height H = 1um, starting radius of curvature r₀ = 100nm, and base size b = 2 · r₀ = 200nm can enhance the field further by an order of 100. In addition, the dielectrophoretic (DEP) force present due to strong electric field gradients can significantly alter cooled gas beam trajectories towards the nanowire tips. Non-cooled gas beams also display room temperature trajectory deviations for species with large polarizability to mass ratios. This leads to the conclusion that at lower temperatures (and even at room temperature) the field ionization cross section of a nanowire array is significantly increased under certain conditions when taking into account DEP coupling.Item Transport of protostellar cosmic rays in turbulent dense cores(2022-07-08) Fitz Axen, Margot; Offner, StellaRecent studies have suggested that low-energy cosmic rays (CRs) may be accelerated inside molecular clouds by the shocks associated with star formation. We use a Monte Carlo transport code to model the propagation of CRs accelerated by protostellar accretion shocks through protostellar cores. We calculate the CR attenuation and energy losses and compute the resulting flux and ionization rate as a function of both radial distance from the protostar and angular position. We show that protostellar cores have non-uniform CR fluxes that produce a broad range of CR ionization rates, with the maximum value being up to two orders of magnitude higher then the radial average at a given distance. In particular, the CR flux is focused in the direction of the outflow cavity, creating a ’flashlight’ effect and allowing CRs to leak out of the core. The radially averaged ionization rates are less than the measured value for the Milky Way of [zeta] ≈ 10⁻¹⁶ s⁻¹; however, within r ≈ 0.03 pc from the protostar, the maximum ionization rates exceed this value. We show that variation in the protostellar parameters, particularly in the accretion rate, may produce ionization rates that are a couple of orders of magnitude higher or lower than our fiducial values. Finally, we use a statistical method to model unresolved sub-grid magnetic turbulence in the core. We show that turbulence modifies the CR spectrum and increases the uniformity of the CR distribution but does not significantly affect the resulting ionization rates.