809 research outputs found

    Reply to Catalán : double-proton-transfer dynamics of photo-excited 7-azaindole dimers

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    The letter by Catalán (1) is concerned with the nature of double-proton transfer in dimers of 7-azaindole (7-AI), and the pertinent issue is whether or not the reaction is concerted. The subject is not new, and, for organic reactions, it has been discussed in the literature for decades. It is now understood that the concerted/consecutive mechanism has to be defined based on the timescale of the vibration motions and the family of coherent trajectories involved (ref. 2 and references therein). For double-proton transfer in isolated dimers, this timescale criterion has been invoked, and, as supported by a variety of time-resolved experiments in several groups and also by theory (ref. 3 and references therein), the conclusion is that the reaction is not concerted on the timescale of the vibrations involved; Catalán and some researchers (see ref. 3) assert that the two protons move in exact concert, maintaining the C_(2h) symmetry at all times

    Acceleration of Large-Scale Electronic Structure Simulations with Heterogeneous Parallel Computing

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    Large-scale electronic structure simulations coupled to an empirical modeling approach are critical as they present a robust way to predict various quantum phenomena in realistically sized nanoscale structures that are hard to be handled with density functional theory. For tight-binding (TB) simulations of electronic structures that normally involve multimillion atomic systems for a direct comparison to experimentally realizable nanoscale materials and devices, we show that graphical processing unit (GPU) devices help in saving computing costs in terms of time and energy consumption. With a short introduction of the major numerical method adopted for TB simulations of electronic structures, this work presents a detailed description for the strategies to drive performance enhancement with GPU devices against traditional clusters of multicore processors. While this work only uses TB electronic structure simulations for benchmark tests, it can be also utilized as a practical guideline to enhance performance of numerical operations that involve large-scale sparse matrices

    Nonchaotic Nonlinear Motion Visualized in Complex Nanostructures by Stereographic 4D Electron Microscopy

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    Direct electron imaging with sufficient time resolution is a powerful tool for visualizing the three-dimensional (3D) mechanical motion and resolving the four-dimensional (4D) trajectories of many different components of a nanomachine, e.g., a NEMS device. Here, we report a nanoscale nonchaotic motion of a nano- and microstructured NiTi shape memory alloy in 4D electron microscopy. A huge amplitude oscillatory mechanical motion following laser heating is observed repetitively, likened to a 3D motion of a conductor’s baton. By time-resolved 4D stereographic reconstruction of the motion, prominent vibrational frequencies (3.0, 3.8, 6.8, and 14.5 MHz) are fully characterized, showing evidence of nonlinear behavior. Moreover, it is found that a stress (fluence)−strain (displacement) profile shows nonlinear elasticity. The observed resonances of the nanostructure are reminiscent of classical molecular quasi-periodic behavior, but here both the amplitude and frequency of the motion are visualized using ultrafast electron microscopy

    Chirped imaging pulses in four-dimensional electron microscopy: femtosecond pulsed hole burning

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    The energy and time correlation, i.e. the chirp, of imaging electron pulses in dispersive propagation is measured by time-slicing (temporal hole burning) using photon-induced near-field electron microscopy. The chirp coefficient and the degree of correlation are obtained in addition to the duration of the electron pulse and its energy spread. Improving temporal and energy resolutions by time-slicing and energy-selection is discussed here and we explore their utility in imaging with time and energy resolutions below those of the generated ultrashort electron pulse. Potential applications for these imaging capabilities are discussed

    Revisiting Multiplication Area Models for Whole Numbers

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    We argue that there are two conceptually different area models for whole number multiplication: area-to-area model and lengths-to-area model depending on how multiplication operation is conceived: action on / change of an initial quantity, or operation / coordination of two quantities. These models complement each other to promote students’ conceptual understanding of whole number multiplication and help students extend their understanding of whole number multiplication to fraction multiplication

    Revisiting Multiplication Area Models for Whole Numbers

    Get PDF
    We argue that there are two conceptually different area models for whole number multiplication: area-to-area model and lengths-to-area model depending on how multiplication operation is conceived: action on / change of an initial quantity, or operation / coordination of two quantities. These models complement each other to promote students’ conceptual understanding of whole number multiplication and help students extend their understanding of whole number multiplication to fraction multiplication

    Water-wire catalysis in photoinduced acid–base reactions

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    The pronounced ability of water to form a hyperdense hydrogen (H)-bond network among itself is at the heart of its exceptional properties. Due to the unique H-bonding capability and amphoteric nature, water is not only a passive medium, but also behaves as an active participant in many chemical and biological reactions. Here, we reveal the catalytic role of a short water wire, composed of two (or three) water molecules, in model aqueous acid–base reactions synthesizing 7-hydroxyquinoline derivatives. Utilizing femtosecond-resolved fluorescence spectroscopy, we tracked the trajectories of excited-state proton transfer and discovered that proton hopping along the water wire accomplishes the reaction more efficiently compared to the transfer occurring with bulk water clusters. Our finding suggests that the directionality of the proton movements along the charge-gradient H-bond network may be a key element for long-distance proton translocation in biological systems, as the H-bond networks wiring acidic and basic sites distal to each other can provide a shortcut for a proton in searching a global minimum on a complex energy landscape to its destination

    Ultrafast Electron Microscopy Integrated with a Direct Electron Detection Camera

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    In the past decade, we have witnessed the rapid growth of the field of ultrafast electron microscopy (UEM), which provides intuitive means to watch atomic and molecular motions of matter. Yet, because of the limited current of the pulsed electron beam resulting from space-charge effects, observations have been mainly made to periodic motions of the crystalline structure of hundreds of nanometers or higher by stroboscopic imaging at high repetition rates. Here, we develop an advanced UEM with robust capabilities for circumventing the present limitations by integrating a direct electron detection camera for the first time which allows for imaging at low repetition rates. This approach is expected to promote UEM to a more powerful platform to visualize molecular and collective motions and dissect fundamental physical, chemical, and materials phenomena in space and time.ope

    Atomic-Scale Imaging in Real and Energy Space Developed in Ultrafast Electron Microscopy

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    In this contribution, we report the development of ultrafast electron microscopy (UEM) with atomic-scale real-, energy-, and Fourier-space resolutions. This second-generation UEM provides images, diffraction patterns, and electron energy spectra, and here we demonstrate its potential with applications for nanostructured materials and organometallic crystals. We clearly resolve the separation between atoms in the direct images and the Bragg spots/Debye−Scherrer rings in diffraction and obtain the electronic structure and elemental energies in the electron energy loss spectra (EELS) and energy filtered transmission electron microscopy (EFTEM)
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