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Application of Airborne Infrared Remote Sensing to the Study of Ocean Submesoscale Eddies
摘要： This paper explores the use of infrared remote sensing methods to examine submesoscale eddies that recur downstream of a deep-water island (Santa Catalina, CA). Data were collected using a mid-wave infrared camera deployed on an aircraft flown at an altitude of 3.7 km, and research boats made nearly simultaneous measurements of temperature and current profiles. Structure within the thermal field is generally adequate as a tracer of surface fluid motions, though the imagery needs to be processed in a novel way to preserve the smallest-scale tracer patterns. In the case we focus on, the eddy is found to have a thermal signature of about 1 km in diameter and a cyclonic swirling flow. Vorticity is concentrated over a smaller area of about 0.5 km in diameter. The Rossby number is 27, indicating the importance of the centrifugal force in the dynamical balance of the eddy. By approximating the eddy as a Rankine vortex, an estimate of upward doming of the thermocline (about 14 m at the center) is obtained that agrees qualitatively with the in-water measurements. Analysis also shows an outward radial flow that creates areas of convergence (sinking flow) along the perimeter of the eddy. The imagery also reveals areas of localized vertical mixing within the eddy thermal perimeter, and an area of external azimuthal banding that likely arises from flow instability.
关键词： infrared imagery,surface current,remote sensing of environment,submesoscale eddies,kinematics and dynamics
更新于2025-09-23 15:22:29
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Optimization and Validation of Efficient Models for Predicting Polythiophene Self-Assembly
摘要： We develop an optimized force-field for poly(3-hexylthiophene) (P3HT) and demonstrate its utility for predicting thermodynamic self-assembly. In particular, we consider short oligomer chains, model electrostatics and solvent implicitly, and coarsely model solvent evaporation. We quantify the performance of our model to determine what the optimal system sizes are for exploring self-assembly at combinations of state variables. We perform molecular dynamics simulations to predict the self-assembly of P3HT at ～350 combinations of temperature and solvent quality. Our structural calculations predict that the highest degrees of order are obtained with good solvents just below the melting temperature. We find our model produces the most accurate structural predictions to date, as measured by agreement with grazing incident X-ray scattering experiments.
关键词： organic photovoltaics,molecular dynamics,coarse-graining
更新于2025-09-23 15:22:29
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The effect of grain-size on fracture of polycrystalline silicon carbide: A multiscale analysis using a molecular dynamics-peridynamics framework
摘要： A robust atomistic to mesoscale computational multiscale/multiphysics modeling framework that explicitly takes into account atomic-scale descriptions of grain-boundaries, is implemented to examine the interplay between grain-size and fracture of polycrystalline cubic silicon carbide (3C-SiC). A salient feature of the developed framework is the establishment of scale-parity between the chosen atomistic and the mesoscale methods namely molecular dynamics (MD) and peridynamics (PD) respectively, which enables the ability to model the effect of the underlying microstructure as well as obtain relevant new insights into the role of grain-size on the ensuing mechanical response of 3C-SiC. Material properties such as elastic modulus, and fracture toughness of single crystals and bicrystals of various orientations are obtained from MD simulations, and using appropriate statistical analysis, MD derived properties are interfaced with PD simulations, resulting in mesoscale simulations that accurately predict the role of grain-size on failure strength, fracture energy, elastic modulus, fracture toughness, and tensile toughness of polycrystalline 3C-SiC. In particular, it is seen that the fracture strength follows a Hall-Petch law with respect to grain-size variations, while mode-I fracture toughness increases with increasing grain-size, consistent with available literature on brittle fracture of polycrystalline materials. Equally importantly, the developed MD-PD multiscale/multiphysics framework represents an important step towards developing materials modeling paradigms that can provide a comprehensive and predictive description of the microstructure-property-performance interplay in solid-state materials.
关键词： Peridynamics,Polycrystalline,Multiscale modeling,3C-SiC,Grain boundaries,Molecular dynamics
更新于2025-09-23 15:22:29
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Long-term lidar observations of wintertime gravity wave activity over northern Sweden
摘要： This paper presents an analysis of gravity wave activity over northern Sweden as deduced from 18 years of wintertime lidar measurements at Esrange (68° N, 21° E). Gravity wave potential energy density (GWPED) was used to characterize the strength of gravity waves in the altitude regions 30–40 km and 40–50 km. The obtained values exceed previous observations reported in the literature. This is suggested to be due to Esrange’s location downwind of the Scandinavian mountain range and due to differences in the various methods that are currently used to retrieve gravity wave parameters. The analysis method restricted the identification of the dominating vertical wavelengths to a range from 2 to 13 km. No preference was found for any wavelength in this window. Monthly mean values of GWPED show that most of the gravity waves’ energy dissipates well below the stratopause and that higher altitude regions show only small dissipation rates of GWPED. Our analysis does not reproduce the previously reported negative trend in gravity wave activity over Esrange. The observed interannual variability of GWPED is connected to the occurrence of stratospheric warmings with generally lower wintertime mean GWPED during years with major stratospheric warmings. A bimodal GWPED occurrence frequency indicates that gravity wave activity at Esrange is affected by both ubiquitous wave sources and orographic forcing.
关键词： Meteorology and atmospheric dynamics (waves and tides,instruments and techniques)
更新于2025-09-23 15:22:29
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Adsorption Dynamics of Redox Active Species onto Polarized Surfaces of Sensitized NiO
摘要： Mesoporous NiO films were deposited by means of a screen printing technique onto fluorine-doped tin oxide transparent electrodes and consequently sensitized with Erythrosin B (EryB) dye. The obtained colored NiO material was used as a working electrode in a three-electrode cell to study the evolution of the triple semiconductor/dye/electrolyte interface upon electrochemical polarization in dark conditions. The electrolyte was a solution of I3?/I? in acetonitrile, with the redox couple representing the typical redox shuttle of dye-sensitized solar cells (DSCs). The adopted electrochemical conditions were devised in order to simulate the actual electrical environment of the NiO/dye photocathode in a light-soaked DSC. The use of a benchmark sensitizer EryB and of the most widely used redox mediator I3?/I? is particularly meaningful for the study of the adsorption dynamics and the determination of possible degradative phenomena on the basis of the behavior of numerous analogue systems. Therefore, for the first time, the evolution of the NiO/EryB/I3?/I? multiple interface was investigated combining the electrochemical characterization with ex situ spectroscopic analysis by means of X-ray photoelectron spectroscopy. The resulting picture shows that EryB in the immobilized state promotes the redox processes based on the I3?/I? couple. Moreover, the EryB sensitizer inhibits the phenomena of recombination between the metal oxide semiconductor and the redox couple.
关键词： X-ray photoelectron spectroscopy,Sensitized NiO,Dye-sensitized solar cells,Redox active species,Adsorption dynamics,Polarized surfaces
更新于2025-09-23 15:22:29
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Impact of Organic Spacers on the Carrier Dynamics in 2D Hybrid Lead-Halide Perovskites
摘要： We have carried out non-adiabatic molecular dynamics simulations combined with time-dependent density functional theory calculations to compare the properties of the two-dimensional (2D) (BA)2(MA)Pb2I7 and three-dimensional (3D) MAPbI3 (where MA = methylammonium and BA = butylammonium) materials. We evaluate the different impacts that the 2D-confined spacer layer of butylammonium cations and the 3D-confined methylammonium cations have on the charge carrier dynamics in the two systems. Our results indicate that while both the MA+ and BA+ cations play important roles in determining the carrier dynamics, the BA+ cations exhibit stronger non-adiabatic couplings with the 2D perovskite framework. The consequence is a faster hot-carrier decay rate in 2D (BA)2(MA)Pb2I7 than in 3D MAPbI3. Thus, tuning of the functional groups of the organic spacer cations in order to reduce the vibronic couplings between the cations and the Pb-I framework can offer the opportunity to slow down the hot-carrier relaxations and increase the carrier lifetimes in 2D lead-halide perovskites.
关键词： carrier dynamics,2D hybrid lead-halide perovskites,time-dependent density functional theory,non-adiabatic molecular dynamics,organic spacers
更新于2025-09-23 15:21:21
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Fast and robust generation of singlet state via shortcuts to adiabatic passage
摘要： In this paper, we propose a protocol to fast and robustly generate two-atom singlet state by designing the evolution operator with the help of quantum Zeno dynamics. The population of the intermediate state can be controlled by system parameters. The pulses in the protocol can be ?tted as Gaussian functions, which are bene?cial to the experimental feasibility. Besides, the performance of various decoherence factors, such as spontaneous emission, cavity decay and ?ber photon leakage, is discussed by numerical simulations. The results show that the protocol is fast and robust against decoherence and operational imperfection. Finally, the protocol is generalized to realize three-atom singlet state by the same principle.
关键词： Quantum Zeno dynamics,Shortcuts to adiabatic passage,Singlet state
更新于2025-09-23 15:21:21
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Ultrafast X-ray Transient Absorption Spectroscopy of Gas-Phase Photochemical Reactions: A New Universal Probe of Photoinduced Molecular Dynamics
摘要： Time-resolved spectroscopic investigations of light-induced chemical reactions with universal detection capitalize recently on single-photon molecular probing using laser pulses in the extreme ultraviolet or X-ray regimes. Direct and simultaneous mappings of the time-evolving populations of ground-state reactants, Franck?Condon (FC) and transition state regions, excited-state intermediates and conical intersections (CI), and photoproducts in photochemical reactions utilize probe pulses that are broadband and energy-tunable. The limits on temporal resolution are set by the transit- or dwell-time of the photoexcited molecules at specific locations on the potential energy surface, typically ranging from a few femtoseconds to several hundred picoseconds. Femtosecond high-harmonic generation (HHG) meets the stringent demands for a universal spectroscopic probe of large regions of the intramolecular phase-space in unimolecular photochemical reactions. Extreme-ultraviolet and soft X-ray pulses generated in this manner with few-femtosecond or sub-femtosecond durations have enormous bandwidths, allowing the probing of many elements simultaneously through excitation or ionization of core?electrons, creating molecular movies that shed light on entire photochemical pathways. At free electron lasers (FELs), powerful investigations are also possible, recognizing their higher flux and tunability but more limited bandwidths. Femtosecond time-resolved X-ray transient absorption spectroscopy, in particular, is a valuable universal probe of reaction pathways that maps changes via the fingerprint core-to-valence resonances. The particular power of this method over valence-ionization probes lies in its unmatched element and chemical-site specificities. The elements carbon, nitrogen, and oxygen constitute the fundamental building blocks of life; photochemical reactions involving these elements are ubiquitous, diverse, and manifold. However, table-top HHG sources in the “water-window” region (280?550 eV), which encompasses the 1s-absorption edges of carbon (284 eV), nitrogen (410 eV), and oxygen (543 eV), are far from abundant or trivial. Recent breakthroughs in the laboratory have embraced this region by using long driving-wavelength optical parametric amplifiers coupled with differentially pumped high-pressure gas source cells. This has opened avenues to study a host of photochemical reactions in organic molecules using femtosecond time-resolved transient absorption at the carbon K-edge. In this Account, we summarize recent efforts to deploy a table-top carbon K-edge source to obtain crucial chemical insights into ultrafast, ultraviolet-induced chemical reactions involving ring-opening, nonadiabatic excited-state relaxation, bond dissociation and radical formation. The X-ray probe provides a direct spectroscopic viewport into the electronic characters and configurations of the valence electronic states through spectroscopic core-level transitions into the frontier molecular orbitals of the photoexcited molecules, laying fertile ground for the real-time mapping of the evolving valence electronic structure. The profound detail and mechanistic insights emerging from the pioneering experiments at the carbon K-edge are outlined here. Comparisons of the experimental methodology with other techniques employed to study similar reactions are drawn, where applicable and relevant. We show that femtosecond time-resolved X-ray transient absorption spectroscopy blazes a new trail in the study of nonadiabatic molecular dynamics. Despite table-top implementations being largely in their infancy, future chemical applications of the technique will set the stage for widely applicable, universal probes of photoinduced molecular dynamics with unprecedented temporal resolution.
关键词： time-resolved spectroscopy,photochemical reactions,high-harmonic generation,X-ray transient absorption spectroscopy,nonadiabatic molecular dynamics
更新于2025-09-23 15:21:21
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Theoretical Model of Exciton States and Ultra-fast Energy Transfer in Heliobacterial Type-I Homodimeric Reaction Center
摘要： A simple theoretical model of exciton dynamics was proposed to interpret the fast excitation energy transfer process in the Type-I homodimeric reaction center of Heliobacterium modesticaldum (hRC); this structure was recently identified and shown to resemble that of the plant/cyanobacterial photosystem I (PSI) reaction center. The exciton state model, which mainly relies on the geometries of 54 bacteriochlorophyll (BChl) g, 4 BChl-g′ and 2 chlorophyll (Chl) a on hRC and assumes constant site energy values for the pigments, reproduced the absorption spectrum of hRC rather well. The model also enabled numerical analysis of the exciton dynamics on hRC, which can be compared with the decay-associated spectra obtained by the laser spectroscopy experiments. The model indicates that the stronger transition-dipole moment on BChl-g contributes to the faster energy transfer due to the higher coherency of the delocalized exciton states on hRC compared to that on PSI that arranges Chl-a at almost homologous locations.
关键词： chlorophyll,photosystem I,bacteriochlorophyll,exciton dynamics,energy transfer,reaction center
更新于2025-09-23 15:21:21
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Density, Structure, and Stability of Citrate <sup>3–</sup> and H <sub/>2</sub> Citrate <sup>–</sup> on Bare and Coated Gold Nanoparticles
摘要： We simulate the packing of citrate3– and H2citrate– onto gold nanoparticles (AuNPs) to understand how citrate anions cap and stabilize AuNPs. We determine the molecular configurations of citrate on 4, 6, and 8 nm AuNP surfaces as a function of charge state and packing density and find that both the distribution of configurations and maximum packing density are independent of AuNP size. A combination of molecular dynamics simulations and in situ Fourier transform infrared spectroscopy (FTIR) is employed to compare the molecular configurations, stability, and density of citrate on 4 nm citrate-coated (cit-AuNPs) and within polycation-wrapped 4 nm cit-AuNPs. FTIR experiments indicate the presence of H2citrate– within polycation-wrapped cit-AuNPs with coordination between the H2citrate– layer and polycation layer in agreement with simulations. Intermolecular hydrogen bonding between terminal carboxylic-acid groups of H2citrate– stabilizes the anionic layer at the interface between cit-AuNPs and adsorbing charged molecules. The calculated total density of H2citrate– on AuNPs decreases from 3.3 × 10-10 mol/cm2 to 3.0 × 10-10 mol/cm2 upon adsorption of a polycation due to some displacement of dangling H2citrate– hydrogen bonded to the surface-bound layer. The density of the surface-bound layer is consistently 2.8 × 10-10 mol/cm2 with and without polycation adsorption. We provide all-atom level insight into the distribution and organization of experimentally derived binding modes of citrate on bare and coated cit-AuNPs. The citrate density and surface charge density are determined for all-atom and coarse-grained modeling of cit-AuNPs, their functionalization, and transformations in complex environments.
关键词： polycation adsorption,gold nanoparticles,FTIR spectroscopy,citrate,molecular dynamics simulations
更新于2025-09-23 15:21:21