Charge and force on a conductive sphere between two parallel electrodes: A Stokesian dynamics approach
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catalysis (homogeneous), solar (photovoltaic), bio↗
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Structure and dynamics of cadmium telluride studied by x-ray and inelastic neutron scattering
We present a combined study of density functional theory, x-ray diffraction, and inelastic neutron scattering examining the temperature dependent structure and lattice dynamics of commercially available cadmium telluride. A subtle change in the structure is evinced near 80 K, which manifests also in the measured phonon density of states. There is no change to the long-range ordered structure. The implications of the change in relation to structural defects are discussed.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Electromagnetic effects on dynamics of high-beta filamentary structures
The impacts of the electromagnetic effects on blob dynamics are considered. Electromagnetic BOUT++ simulations on seeded high-beta blobs demonstrate that inhomogeneity of magnetic curvature or plasma pressure along the filament leads to bending of the blob filaments and the magnetic field lines due to increased propagation time of plasma current (Alfvén time). The bending motion can enhance heat exchange between the plasma facing materials and the inner scrape-off layer (SOL) region. The effects of sheath boundary conditions on the part of the blob away from the boundary are also diminished by the increased Alfvén time. Using linear analysis and BOUT++ simulations, it is found that electromagnetic effects in high temperature and high density plasmas reduce the growth rate of resistive drift wave instability when resistivity drops below a certain value. The blobs temperature decreases in the course of its motion through the SOL and so the blob can switch from the electromagnetic to the electrostatic regime where resistive drift waves become important again.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Dynamic control of metastable remanent states in mesoscale magnetic elements
The formation of the vortex-antivortex-vortex (v-av-v) metastable remanent states in elongated magnetic elements have been systematically investigated using micromagnetic modeling. It is demonstrated that the v-av-v magnetization pattern can be effectively stabilized by exciting the single vortex state with an external RF field. Furthermore, we show that a set of different polarity combinations of the vortex cores can be achieved by adjusting the frequency and amplitude of the excitation field. The corresponding dynamic response in time- and frequency-domain has also been presented. Owing to the diversity of the collective modes with different vortex-antivortex combinations, this system may open promising perspectives in the area of spin transfer torque oscillators.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Excited state non-adiabatic dynamics of pyrrole: A time-resolved photoelectron spectroscopy and quantum dynamics study
The dynamics of pyrrole excited at wavelengths in the range 242-217 nm are studied using a combination of time-resolved photoelectron spectroscopy and wavepacket propagations performed using the multi-configurational time-dependent Hartree method. Excitation close to the origin of pyrrole’s electronic spectrum, at 242 and 236 nm, is found to result in an ultrafast decay of the system from the ionization window on a single timescale of less than 20 fs. This behaviour is explained fully by assuming the system to be excited to the A{sub 2}(πσ{sup ∗}) state, in accord with previous experimental and theoretical studies. Excitation at shorter wavelengths has previously been assumed to result predominantly in population of the bright A{sub 1}(ππ{sup ∗}) and B{sub 2}(ππ{sup ∗}) states. We here present time-resolved photoelectron spectra at a pump wavelength of 217 nm alongside detailed quantum dynamics calculations that, together with a recent reinterpretation of pyrrole’s electronic spectrum [S. P. Neville and G. A. Worth, J. Chem. Phys. 140, 034317 (2014)], suggest that population of the B{sub 1}(πσ{sup ∗}) state (hitherto assumed to be optically dark) may occur directly when pyrrole is excited at energies in the near UV part of its electronic spectrum. The B{sub 1}(πσ{sup ∗}) state is found to decay on a timescale of less than 20 fs by both N-H dissociation and internal conversion to the A{sub 2}(πσ{sup ∗}) state.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Simultaneous broadband laser ranging and photonic Doppler velocimetry for dynamic compression experiments
A diagnostic was developed to simultaneously measure both the distance and velocity of rapidly moving surfaces in dynamic compression experiments, specifically non-planar experiments where integrating the velocity in one direction does not always give the material position accurately. The diagnostic is constructed mainly from fiber-optic telecommunications components. The distance measurement is based on a technique described by Xia and Zhang [Opt. Express 18, 4118 (2010)], which determines the target distance every 20 ns and is independent of the target speed. We have extended the full range of the diagnostic to several centimeters to allow its use in dynamic experiments, and we multiplexed it with a photonic Doppler velocimetry (PDV) system so that distance and velocity histories can be measured simultaneously using one fiber-optic probe. The diagnostic was demonstrated on a spinning square cylinder to show how integrating a PDV record can give an incorrect surface position and how the ranging diagnostic described here obtains it directly. The diagnostic was also tested on an explosive experiment where copper fragments and surface ejecta were identified in both the distance and velocity signals. We show how the distance measurements complement the velocity data. Potential applications are discussed.
46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
Multidiagnostic analysis of ion dynamics in ultrafast laser ablation of metals over a large fluence range
The dynamics of ions in ultrafast laser ablation of metals is studied over fluences ranging from the ablation threshold up to ≈75 J/cm{sup 2} by means of three well-established diagnostic techniques. Langmuir probe, Faraday cup, and spectrally resolved intensified charge coupled device imaging simultaneously monitored the ions produced during ultrafast laser ablation of a pure copper target with 800 nm, ≈50 fs, Ti: Sapphire laser pulses. The fluence dependence of ion yield is analyzed, resulting in the observance of three different regimes. The specific ion yield shows a maximum at about 4–5 J/cm{sup 2}, followed by a gradual reduction and a transition to a high-fluence regime above ≈50 J/cm{sup 2}. The fluence dependence of the copper ions angular distribution is also analyzed, observing a gradual increase in forward-peaking of Cu ions for fluences up to ≈10 J/cm{sup 2}. A broader ion component is observed at larger angles for fluences larger than ≈10 J/cm{sup 2}. Finally, an experimental characterization of the ionic angular distribution for several metallic targets (Mg, Al, Cr, Fe, Cu, and W) is carried out at a relatively high fluence of ≈66 J/cm{sup 2}. Interestingly, the ion emission from the volatile metals shows a narrow, forward-peaked distribution, and a high peak ion yield compared to the refractory metals. Moreover, the width of ionic angular distributions presents a striking correlation with the peak ion yield.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Oxygen transport in off-stoichiometric uranium dioxide mediated by defect clustering dynamics
Oxygen transport is central to many properties of oxides such as stoichiometric changes, phase transformation, and ionic conductivity. In this paper, we report a mechanism for oxygen transport in uranium dioxide (UO{sub 2}) in which the kinetics is mediated by defect clustering dynamics. In particular, the kinetic Monte Carlo method has been used to investigate the kinetics of oxygen transport in UO{sub 2} under the condition of creation and annihilation of oxygen vacancies and interstitials as well as oxygen interstitial clustering, with variable off-stoichiometry and temperature conditions. It is found that in hypo-stoichiometric UO{sub 2−x}, oxygen transport is well described by the vacancy diffusion mechanism while in hyper-stoichiometric UO{sub 2+x}, oxygen interstitial cluster diffusion contributes significantly to oxygen transport kinetics, particularly at high temperatures and high off-stoichiometry levels. It is also found that di-interstitial clusters and single interstitials play dominant roles in oxygen diffusion while other larger clusters have negligible contributions. However, the formation, coalescence, and dissociation of these larger clusters indirectly affects the overall oxygen diffusion due to their interactions with mono and di-interstitials, thus providing an explanation of the experimental observation of saturation or even drop of oxygen diffusivity at high off-stoichiometry.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
The impact of plasma dynamics on the self-magnetic-pinch diode impedance
In this study, the self-magnetic-pinch diode is being developed as an intense electron beam source for pulsed-power-driven x-ray radiography. The basic operation of this diode has long been understood in the context of pinched diodes, including the dynamic effect that the diode impedance decreases during the pulse due to electrode plasma formation and expansion. Experiments being conducted at Sandia National Laboratories' RITS-6 accelerator are helping to characterize these plasmas using time-resolved and time-integrated camera systems in the x-ray and visible. These diagnostics are analyzed in conjunction with particle-in-cell simulations of anode plasma formation and evolution. The results confirm the long-standing theory of critical-current operation with the addition of a time-dependent anode-cathode gap length. The results may suggest that anomalous impedance collapse is driven by increased plasma radial drift, leading to larger-than-average ion v r × B θ acceleration into the gap.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Phonon dynamics and Urbach energy studies of MgZnO alloys
The Mg{sub x}Zn{sub 1−x}O alloy system is emerging as an environmentally friendly choice in ultraviolet lighting and sensor technologies. Knowledge of defects which impact their optical and material properties is a key issue for utilization of these alloys in various technologies. The impact of phase segregation, structural imperfections, and alloy inhomogeneities on the phonon dynamics and electronic states of Mg{sub x}Zn{sub 1−x}O thin films were studied via selective resonant Raman scattering (SRRS) and Urbach analyses, respectively. A series of samples with Mg composition from 0–68% were grown using a sputtering technique, and the optical gaps were found to span a wide UV range of 3.2–5.8 eV. The extent of the inherent phase segregation was determined via SRRS using two UV-laser lines to achieve resonance with the differing optical gaps of the embedded cubic and wurtzite structural domains. The occurrence of Raman scattering from cubic structures is discussed in terms of relaxation of the selection rules due to symmetry breaking by atomic substitutions. The Raman linewidth and Urbach energy behavior indicate the phase segregation region occurs in the range of 47–66% Mg. Below the phase segregation, the longitudinal optical phonons are found to follow the model of one-mode behavior. The phonon decay model of Balkanski et al. indicates that the major contributor to Raman linewidth arises from the temperature-independent term attributed to structural defects and alloy inhomogeneity, while the contribution from anharmonic decay is relatively small. Moreover, a good correlation between Urbach energy and Raman linewidth was found, implying that the underlying crystal dynamics affecting the phonons also affect the electronic states. Furthermore, for alloys with low Mg composition structural defects are dominant in determining the alloy properties, while at higher compositions alloy inhomogeneity cannot be neglected.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Energy dynamics and current sheet structure in fluid and kinetic simulations of decaying magnetohydrodynamic turbulence
Simulations of decaying magnetohydrodynamic (MHD) turbulence are performed with a fluid and a kinetic code. The initial condition is an ensemble of long-wavelength, counter-propagating, shear-Alfvén waves, which interact and rapidly generate strong MHD turbulence. The total energy is conserved and the rate of turbulent energy decay is very similar in both codes, although the fluid code has numerical dissipation, whereas the kinetic code has kinetic dissipation. The inertial range power spectrum index is similar in both the codes. The fluid code shows a perpendicular wavenumber spectral slope of k{sub ⊥}{sup −1.3}. The kinetic code shows a spectral slope of k{sub ⊥}{sup −1.5} for smaller simulation domain, and k{sub ⊥}{sup −1.3} for larger domain. We estimate that collisionless damping mechanisms in the kinetic code can account for the dissipation of the observed nonlinear energy cascade. Current sheets are geometrically characterized. Their lengths and widths are in good agreement between the two codes. The length scales linearly with the driving scale of the turbulence. In the fluid code, their thickness is determined by the grid resolution as there is no explicit diffusivity. In the kinetic code, their thickness is very close to the skin-depth, irrespective of the grid resolution. This work shows that kinetic codes can reproduce the MHD inertial range dynamics at large scales, while at the same time capturing important kinetic physics at small scales.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Cluster dynamics models of irradiation damage accumulation in ferritic iron. II. Effects of reaction dimensionality
The black dot damage features which develop in iron at low temperatures exhibit significant mobility during in situ irradiation experiments via a series of discrete, intermittent, long range hops. By incorporating this mobility into cluster dynamics models, the temperature dependence of such damage structures can be explained with a surprising degree of accuracy. Such motion, however, is one dimensional in nature. This aspect of the physics has not been fully considered in prior models. This article describes one dimensional reaction kinetics in the context of cluster dynamics and applies them to the black dot problem. This allows both a more detailed description of the mechanisms by which defects execute irradiation-induced hops while allowing a full examination of the importance of kinetic assumptions in accurately assessing the development of this irradiation microstructure. Results are presented to demonstrate whether one dimensional diffusion alters the dependence of the defect population on factors such as temperature and defect hop length. Finally, the size of interstitial loops that develop is shown to depend on the extent of the reaction volumes between interstitial clusters, as well as the dimensionality of these interactions.
36 MATERIALS SCIENCE↗
Laser-to-hot-electron conversion limitations in relativistic laser matter interactions due to multi-picosecond dynamics
High-energy short-pulse lasers are pushing the limits of plasma-based particle acceleration, x-ray generation, and high-harmonic generation by creating strong electromagnetic fields at the laser focus where electrons are being accelerated to relativistic velocities. Understanding the relativistic electron dynamics is key for an accurate interpretation of measurements. We present a unified and self-consistent modeling approach in quantitative agreement with measurements and differing trends across multiple target types acquired from two separate laser systems, which differ only in their nanosecond to picosecond-scale rising edge. Insights from high-fidelity modeling of laser-plasma interaction demonstrate that the ps-scale, orders of magnitude weaker rising edge of the main pulse measurably alters target evolution and relativistic electron generation compared to idealized pulse shapes. This can lead for instance to the experimentally observed difference between 45 MeV and 75 MeV maximum energy protons for two nominally identical laser shots, due to ps-scale prepulse variations. Our results indicate that the realistic inclusion of temporal laser pulse profiles in modeling efforts is required if predictive capability and extrapolation are sought for future target and laser designs or for other relativistic laser ion acceleration schemes.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Dynamical screening of van der Waals interactions in nanostructured solids: Sublimation of fullerenes
Sublimation energy is one of the most important properties of molecular crystals, but it is difficult to study, because the attractive long-range van der Waals (vdW) interaction plays an important role. Here, we apply efficient semilocal density functional theory (DFT), corrected with the dynamically screened vdW interaction (DFT + vdW), the Rutgers-Chalmers nonlocal vdW-DF, and the pairwise-based dispersion-corrected DFT-D2 developed by Grimme and co-workers, to study the sublimation of fullerenes. We find that the short-range part, which accounts for the interaction due to the orbital overlap between fullerenes, is negligibly small. Our calculation shows that there exists a strong screening effect on the vdW interaction arising from the valence electrons of fullerenes. On the other hand, higher-order contributions can be as important as the leading-order term. The reasons are that (i) the surface of fullerene molecules is metallic and thus highly polarizable, (ii) the band gap of fullerene solids is small (less than 2 eV), and (iii) fullerene molecules in the solid phase are so densely packed, yielding the high valence electron density and small equilibrium intermolecular distances (the first nearest neighbor distance is only about 10 Å for C{sub 60}). However, these two effects make opposite contributions, leading to significant error cancellation between these two contributions. We demonstrate that, by considering higher-order contributions and the dynamical screening, the DFT + vdW method can yield sublimation energies of fullerenes in good agreement with reference values, followed by vdW-DF and DFT-D2. The insights from this study are important for a better understanding of the long-range nature of vdW interactions in nanostructured solids.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Communication: State-to-state dynamics of the Cl + H{sub 2}O → HCl + OH reaction: Energy flow into reaction coordinate and transition-state control of product energy disposal
Quantum state-to-state dynamics of a prototypical four-atom reaction, namely, Cl + H{sub 2}O → HCl + OH, is investigated for the first time in full dimensionality using a transition-state wave packet method. The state-to-state reactivity and its dependence on the reactant internal excitations are analyzed and found to share many similarities both energetically and dynamically with the H + H{sub 2}O → H{sub 2} + OH reaction. The strong enhancement of reactivity by the H{sub 2}O stretching vibrational excitations in both reactions is attributed to the favorable energy flow into the reaction coordinate near the transition state. On the other hand, the insensitivity of the product state distributions with regard to reactant internal excitation stems apparently from the transition-state control of product energy disposal.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Online remote control systems for static and dynamic compression and decompression using diamond anvil cells
The ability to remotely control pressure in diamond anvil cells (DACs) in accurate and consistent manner at room temperature, and at cryogenic and elevated temperatures, is crucial for effective and reliable operation of a high-pressure synchrotron facility such as High Pressure Collaborative Access Team (HPCAT). Over the last several years, a considerable effort has been made to develop instrumentation for remote and automated pressure control in DACs during synchrotron experiments. We have designed and implemented an array of modular pneumatic (double-diaphragm), mechanical (gearboxes), and piezoelectric devices and their combinations for controlling pressure and compression/decompression rate at various temperature conditions from 4 K in cryostats to several thousand Kelvin in laser-heated DACs. Because HPCAT is a user facility and diamond cells for user experiments are typically provided by users, our development effort has been focused on creating different loading mechanisms and frames for a variety of existing and commonly used diamond cells rather than designing specialized or dedicated diamond cells with various drives. In this paper, we review the available instrumentation for remote static and dynamic pressure control in DACs and show some examples of their applications to high pressure research.
47 OTHER INSTRUMENTATION↗
Self-consistent modelling of electrochemical strain microscopy in mixed ionic-electronic conductors: Nonlinear and dynamic regimes
The frequency dependent Electrochemical Strain Microscopy (ESM) response of mixed ionic-electronic conductors is analyzed within the framework of Fermi-Dirac statistics and the Vegard law, accounting for steric effects from mobile donors. The emergence of dynamic charge waves and nonlinear deformation of the surface in response to bias applied to the tip-surface junction is numerically explored. The 2D maps of the strain and concentration distributions across the mixed ionic-electronic conductor and bias-induced surface displacements are calculated. The obtained numerical results can be applied to quantify the ESM response of Li-based solid electrolytes, materials with resistive switching, and electroactive ferroelectric polymers, which are of potential interest for flexible and high-density non-volatile memory devices.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
The dynamics of Al/Pt reactive multilayer ignition via pulsed-laser irradiation
Reactive multilayers consisting of alternating layers of Al and Pt were irradiated by single laser pulses ranging from 100 μs to 100 ms in duration, resulting in the initiation of rapid, self-propagating reactions. The threshold intensities for ignition vary with the focused laser beam diameter, bilayer thickness, and pulse length and are affected by solid state reactions and conduction of heat away from the irradiated regions. High-speed photography was used to observe ignition dynamics during irradiation and elucidate the effects of heat transfer into a multilayer foil. For an increasing laser pulse length, the ignition process transitioned from a more uniform to a less uniform temperature profile within the laser-heated zone. A more uniform temperature profile is attributed to rapid heating rates and heat localization for shorter laser pulses, and a less uniform temperature profile is due to slower heating of reactants and conduction during irradiation by longer laser pulses. Finite element simulations of laser heating using measured threshold intensities indicate that micron-scale ignition of Al/Pt occurs at low temperatures, below the melting point of both reactants.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗