Physics
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Item Novel applications of high intensity femtosecond lasers to particle acceleration and terahertz generation(2008-11-20) York, Andrew Gregory; Milchberg, Howard M; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We have investigated new applications for high intensity femtosecond lasers theoretically and experimentally, including a novel method to accelerate electrons to relativistic energy and a new type of coherent lasing medium for amplification of few-cycle, high energy pulses of terahertz radiation. We report the development of corrugated `slow wave' plasma guiding structures with application to quasi-phase-matched direct laser acceleration of charged particles. These structures support guided propagation at intensities up to 2x10^17 W/cm^2, limited by our current laser energy and side leakage. Hydrogen, nitrogen, and argon plasma waveguides up to 1.5 cm in length with corrugation period as short as 35 microns are generated, with corrugation depth approaching 100%. These structures remove the limitations of diffraction, phase matching, and material damage thresholds and promise to allow high-field acceleration of electrons over many centimeters using relatively small femtosecond lasers. We present simulations that show a laser pulse power of 1.9 TW should allow an acceleration gradient larger than 80 MV/cm. A modest power of only 30 GW would still allow acceleration gradients in excess of 10 MV/ cm. Broadband chirped-pulse amplification (CPA) in Ti:sapphire revolutionized nonlinear optics in the 90's, bringing intense optical pulses out of large government facilities and into the hands of graduate students in small university labs. Intense terahertz pulses (>> 10 microjoules, <5 cycles), however, are still only produced at large accelerator facilities like Brookhaven National Labs. CPA is theoretically possible for terahertz frequencies, but no broadband lasing medium like Ti:sapphire has been demonstrated for terahertz. Dipolar molecular gases such as hydrogen cyanide (HCN), `aligned' by intense optical pulses, are a novel and promising medium for amplification of broadband few-cycle terahertz pulses. We present calculations that show rotationally excited molecules can amplify a few-cycle seed pulse of terahertz radiation: a sequence of short, intense optical pulses aligns a dipolar gas, driving the molecules into a broad superposition of excited rotational states. A broadband seed terahertz pulse following the optical pulses can then be amplified on many pure rotational transitions simultaneously. We also discuss plans and progress towards experimental realization of a few-cycle terahertz amplifier.Item Hierarchical Intermolecular Interaction Models of N-Heteroaromatic STM Adlayer Structures(2007-11-28) Evans, Diane; Reutt-Robey, Janice; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The molecular scale electronic device concept was initiated in 1974 with the semi-quantitative analysis of a hemiquinone molecule. Because of the molecule's electron donor and acceptor properties, and ability to transfer electrons along the -network, it was proposed that the molecule could perform as a circuit rectifier. Many investigations of molecular scale systems have occurred since then, in particular, of organic molecules with large, fused ring systems that spontaneously self-organize after deposition onto a substrate. The directionality and molecular specificity of hydrogen bonding differentiates it from the other weak interactions, driving molecules into specific arrangements and enabling spontaneous rearrangement after addition of only a small amount of enthalpic energy. A direct application of molecular recognition through self-assembly has been the design of patterned self-assembled monolayers (SAMs) for the construction of microelectrodes and supramolecular templates. However, the intermolecular interactions that drive ordered structures to form, including molecular chains and large aggregates, has not been well understood. To elucidate a quantitative description of the intermolecular forces of network systems of aromatics that control such features as packing density and porosity, two individual model heteroaromatic systems of 9-acridinecarboxylic acid and isonicotinic acid are investigated using both experimental and computational resources. Supported by scanning tunneling microscopy (STM) topographies, x-ray diffraction (XRD) data and x-ray photoelectron (XPS) spectra, this class of N-heteroaromatics adsorbed on Ag (111) serves as a model system to systematically investigate 2-dimensional intermolecular (2-D) interactions and their impact on forming different structural phases of molecular chain domains. To approach an understanding of the dynamics of N-heteroaromatic film growth, an intermolecular interaction model of 1-D single phase chains and clusters is performed. The model considers the anisotropy of the electrostatic force interactions to determine what charge arrangements (dipole, quadrupole, etc.) better characterize the molecular interactions. Furthermore, the competition between phase chain types is shown to be length dependent and in qualitative agreement with the coverage dependent STM structural phase composition.Item Ferrocene-based molecular electronics and nanomanufacturing of Pd nanowires.(2007-11-27) Wang, Lixin; Sita, Lawrence; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Two test structures were tried out for molecular junction formation and subsequent I-V characteristics measurements. One is formed by insertion of certain dithiol molecules into an alkanethiol self-assembled monolayer (SAM), followed by tethering the free thiol end with gold nanoparticles. The test structure can then be measured with CP-AFM. The matrix SAM, mixed monolayer with inserted dithiol molecules, and final test structure with gold nanoparticles were prepared and characterized by ellipsometry, AFM and STM. However, the CP-AFM measurements were very irreproducible, even on an alkanethiol SAM. This problem was analyzed and believed to come from two possible causes, namely thermal drift and deformation of the metalized tips. The other test structure was from insertion of molecules into nanogaps made by electromigration technique. Two molecules were tested and drastically different properties were observed from junctions with each molecule. For Fc-OPE molecules, near perfect conductance peaks (>0.6G0) were observed in some junctions and analysis indicates that such molecular junction contains only one or two molecules inside the nanogap. The formation of conductance peaks was analyzed with Landauer formula and a simple metal-molecule-metal model. Computational calculation also predicted high conductance through such junctions and the existence of resonant peaks. The junctions with OPE molecules, however, showed poor conductance. Possible causes such as molecular structure and easiness of molecular junction formation were discussed. In the second part of this dissertation, a new method was developed to fabricate Pd nanowires on HOPG surface using a sacrificial Cu film. The morphology and composition of the nanowires were characterized by AFM, SEM and XPS. The formation of such Pd nanowires was explained with a galvanic displacement mechanism and some test experiments were carried out to prove such growth mechanism. It was also found that the size of the Pd nanowires can be directly controlled by the thickness of the Cu film that was initially deposited. However, attempts to make Au, Pt and Ag nanowires with the same method failed, and possible causes were discussed.Item Quantum mechanical investigation on the vibrational relaxation of HF in collisions with H atoms(2007-05-10) Tao, Liang; Alexander, Millard; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We investigate the vibrational relaxation of HF(v=2-5) in collisions with H atoms by means of fully-quantum reactive scattering calculations. Our calculations are based on the global ab initio potential energy surface of Stark and Werner which includes, specifically, an accurate description on the reaction barrier and the van der Waals wells in the reactant and product arrangements. We attribute discrepancies between early fluorescence experiments and quasi-classical trajectory calculations to accuracies in the approximate potential energy surface used, in particular inaccuracies in the predicted barrier heights. By suitable linear combinations of the definite parity basis functions, we are able to separate the nominally indistinguishable inelastic relaxation pathways: (1) Inelastic vibrational relaxation unaccompanied by H atom exchange (2) Inelastic vibrational relaxation accompanied by H atom exchange In addition, reactive quenching also contributes to the overall vibrational removal of HF We report state-to-state and overall integral cross sections for each of these channels. The dominant removal process corresponds to vibrational relaxation without H-atom exchange. The magnitude of the vibrational relaxation cross sections are in reasonable overall agreement with the limited experimental data. We also observe sharp structure in the energy dependence of the HF(v=3) removal cross sections. We use an adiabatic-bender analysis to assign this structure to scattering resonances arising from quasi-bound van der Waals states in the HF-H entrance valley.Item A Biophysical Study of Clathrin Utilizing Light Scattering, Neutron Scattering and Structure Based Computer Modeling(2007-04-27) Ferguson, Matthew Lee; Nossal, Ralph J; Losert, Woflgang; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A principal component in the protein coats of certain post-golgi and endocytic vesicles is clathrin, which appears as a three-legged heteropolymer (known as a triske- lion) that assembles into polyhedral baskets principally made up of pentagonal and hexagonal faces. In vitro, this assembly depends on the pH, with baskets forming more readily at low pH and less readily at high pH. We have developed procedures, based on static and dynamic light scattering, to determine the radius of gyration, Rg, and hydrodynamic radius, RH, of isolated triskelia under conditions where basket assembly occurs. Calculations based on rigid molecular bead models of a triskelion show that the measured values can be accounted for by bending of the legs and a puckering at the vertex. We also show that the values of Rg and RH measured for clathrin triskelia in solution are qualitatively consistent with the conformation of an individual triskelion that is part of a "D6 barrel" basket assembly measured by cryo-EM tomography. We extended this study by performing small angle neutron scattering (SANS) experiments on isolated triskelia in solution under conditions where baskets do not assemble. SANS experiments were consistent with previous static light scattering ex- periments but showed a shoulder in the scattering function at intermediate q-values just beyond the central diffraction peak (the Guinier regime). Theoretical calcula- tions based on rigid bead models of a triskelion showed well-defined features in this region different from the experiment. A flexible bead-spring model of a triskelion and Brownian dynamics simulations were used to generate a time averaged scattering function. This model adequately described the experimental data for flexibilities close to previous estimates from the analysis of electron micrographs.Item Ultra-fast Dynamics of Small Molecules in Strong Fields(2006-04-24) Zhao, Kun; Hill, Wendell T; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Correlation detection techniques (image labeling, coincidence imaging, and joint variance) are developed with an image spectrometer capable of collecting charges ejected over 4\pi sr and a digital camera synchronized with the laser repetition rate at up to 735 Hz. With these techniques, molecular decay channels ejecting atomic fragments with different momenta (energies) can be isolated; thus the initial molecular configurations (bond lengths and/or bond angles) and orientations as well as their distributions can be extracted. These techniques are applied to study strong-field induced dynamics of diatomic and triatomic molecules. Specific studies included the measurements of the Coulomb explosion energy as a function of bond angle in linear (CO_2) and bent (NO_2) triatomics and the ejection anisotropy relative to the laser polarization axis during Coulomb explosions in both triatomic (CO_2 and NO_2) and diatomic (H_2, N_2 and O_2) systems. The experiments were performed with 100 fs, 800 nm laser pulses focused to 0.1 ~ 5 \times 10^15 W/cm^2. The explosion energy of NO_2 decreases monotonically by more than 25% from the smallest to the largest bond angle. By contrast, the CO_2 explosion energies are nearly independent of bond angle. The enhanced-ionization and static-screening models in two-dimension with three charge centers were developed to simulate the explosion energies as a function of bond angle. The predictions are consistent with the measurements of CO_2 and NO_2. The observed explosion signals as a function of bond angle for both triatomics show large-amplitude vibrations. The ejection angular distributions in triatomic (CO_2 and NO_2) and diatomic (H_2, N_2, and O_2) Coulomb explosions were measured; the contribution made to the ejection anisotropy by dynamic alignment was studied by comparing the images obtained with linearly and circularly polarized fields. Different angular distributions of the molecules are consistent with different ionization stages, induced dipole moments and rotational constants. The dynamic alignment of H_2 is found to be nearly complete. A larger dynamic alignment of CO_2 than that of N_2 or O_2 is consistent with that more electrons have been removed from CO_2 and the precursor molecular ion spends more time in the field prior to the explosion.Item ORBITAL-FREE DENSITY FUNCTIONAL THEORY OF ATOMS, MOLECULES, AND SOLIDS(2005-11-23) Chai, Jeng-Da; Weeks, John D.; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Density functional (DF) theory has proved to be a powerful way to determine the ground state energy of atoms, molecules, and extended systems. An important part of the theory requires one to determine the kinetic energy of the ground state of a system of N noninteracting electrons in a general external field. Kohn and Sham showed how this can be numerically calculated very accurately using a set of N orbitals. However this prevents the simple linear scaling in N that would arise if the kinetic energy could be directly expressed as a functional of the electron density, as is done with other components of the total energy like the exchange-correlation energy. Orbital free methods attempt to calculate the noninteracting kinetic energy directly by approximating the universal but unknown kinetic energy density functional. However simple local approximations are inaccurate and it has proved very difficult to devise generally accurate nonlocal approximations. We focus instead on the kinetic potential, the functional derivative of the kinetic energy DF, which appears in the Euler equation for the electron density. We argue the kinetic potential is more amenable to simple physically motivated approximations in many relevant cases. We propose a family of nonlocal orbital free kinetic potentials that reduce to the known exact forms for both slowly varying and rapidly varying perturbations and also reproduce exact results for the linear response of the density of the homogeneous system to small perturbations. A simple and systematic approach for generating accurate and weak ab initio local pseudopotentials describing a smooth slowly varying valence component of the electron density is proposed for use in orbital free DF calculations of molecules and solids. The use of these local pseudopotentials further minimizes the possible errors arising from use of the approximate kinetic potentials. A linear scaling method for treating large extended systems is proposed for fast computations. Our theory yields results for the total energies and ionization energies of atoms, and for the shell structure in the atomic radial density profiles that are in very good agreement with calculations using the full Kohn-Sham theory. We describe the first use of nonlocal orbital free methods to determine the ground-state bond lengths and binding energies of diatomic molecules. These results and the ground-state lattice parameters, and total energy of bulk aluminum and bulk silicon are in generally good agreement with detailed calculations using the full Kohn-Sham theory.Item General Theory of Nonuniform Fluids: From Hard Spheres to Ionic Fluids(2004-12-09) Chen, Yng-gwei; Weeks, John D.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The exclusion effects of repulsive intermolecular potential cores are often modeled by hard sphere fluids, for which an accurate Hydrostatic Linear Response (HLR) equation was previous developed by Katsov in 2001 for computing the density response to general external fields. In this dissertation the HLR equation is combined with various thermodynamic integration pathways to investigate the solvation free energy of cavity insertion which characterizes the entropic cost of solvating molecules in a fluid. A Shifted Linear Response (SLR) equation is developed to build in the exact limits of external fields varying in very small ranges and fluids confined in narrow spaces, where the HLR fails qualitatively. The SLR is derived from an expansion truncated at linear order about a reference density, and an Insensitivity Criterion (IC) is proposed for determining an optimal reference density. The slow 1/r decay of the Coulomb potential is characteristically long-ranged, but it also becomes strong at short distances. The structure of ionic systems exhibits an intricate interplay between the short and long length scales of their molecular potentials. A strategy is proposed for separating the Coulomb interaction between general charge distributions into a short-ranged piece u0(r) and a slowly varying piece u1(r). In the strong coupling states of the ionic systems that we have studied, mimic systems with only the short-ranged part u0(r) are found to show very similar correlation functions. The slow decays of ion-ion and ion-dipole interactions give rise to unique long-wavelength constraints on ionic fluid structure. Local Molecular Field Theory (LMF), which maps an external field in the full system to a mimic system in the presence of a renormalized field, can correct the mimic correlations by embodying contributions from u1(r). The LMF has been applied to both uniform and nonuniform model ionic systems, and accurate results for bulk correlation functions, internal energy and the density distribution in a confined system are obtained. For a system of counterions confined by charged walls, the LMF and the mimic system have especially helped shed light on many phenomena that had previously lacked coherent physical interpretations and consistent approximations.Item The role of the F spin-orbit excited state in the F+H2 and F+HD reactions(2004-07-22) tzeng, yi-ren; Alexander, Millard H; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this dissertation we study the role of the F spin-orbit excited state (F*) in the F+H2 and F+HD reactions using quantum mechanical calculations. The calculations involve multiple potential energy surfaces (the Alexander-Stark-Werner, or ASW, PESs), and include an accurate treatment of the couplings (non-adiabatic, spin-orbit, and Coriolis) among all three electronic states. For the F+H2 reaction, we calculate the center-of-mass differential cross sections and laboratory-frame angular distributions at the four different combinations of collision energies and hydrogen isotopomer investigated in the experiments of Neumark et al. [J. Chem. Phys., 82, 3045 (1985)]. Comparisons with the calculations on the Stark-Werner (SW) and Hartke-Stark-Werner (HSW) PESs, which are limited to the lowest electronically adiabatic state, show that non-adiabatic couplings greatly reduce backward scattering. Surprisingly, we find the shapes of both the CM DCSs and LAB ADs are insensitive to the fraction of F* presented in the F beam. For the F+HD reaction, we calculate the excitation functions and product translational energy distribution functions to study the reactivity of F*. Comparisons with the experiment by Liu and co-workers [J. Chem. Phys., 113, 3633 (2000)] confirm the relatively low reactivity of spin-orbit excited state (F*) atoms. Excellent agreement with the experiment is obtained under the assumption that the F*:F concentration ratio equals 0.16:0.84 in the molecular beam, which corresponds to a thermal equilibrium of the two spin-orbit states at the experimental temperature (600K). From the accurate calculation of the F* reactivity and its relatively small contribution to the overall reactivity of the reaction, we attribute discrepancies between calculation and experiment to an inadequacy in the simulation of the reactivity of the F ground state, likely a result of the residual errors in the ground electronic potential energy surface.Item Electronic Properties of Carbon Nanotubes studied in Field-Effect Transistor Geometries(2004-05-12) Dürkop, Tobias; Fuhrer, Michael S; PhysicsDue to their outstanding properties carbon nanotubes have attracted considerable research effort during the last decade. While they serve as an example of a 1-dimensional electron system allowing one to study fundamental quantum effects nanotubes-especially semiconducting nanotubes-are an interesting candidate for next-generation transistor application with the potential to replace silicon-based devices. I have fabricated nanotubes using chemical vapor deposition techniques with various catalysts and gas mixtures. The nanotubes produced with these techniques vary in length from 100 nm to several hundreds of micrometers. While data taken on shorter metallic and semiconducting devices show Coulomb blockade effects, the main part of this work is concerned with measurements that shed light on the intrinsic properties of semiconducting nanotubes. On devices with lengths of more than 300 um I have carried out measurements of the intrinsic hole mobility as well as the device-specific field-effect mobility. The mobility measured on these nanotube devices at room temperature exceeds that of any semiconductor known previously. Another important consideration in nanotube transistor applications are hysteresis effects. I present measurements on the time scales involved in some of these hysteresis effects and a possible application of the hysteresis for memory devices.