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Mostrando recursos 1 - 20 de 49.378

  1. Label-Free Bond-Selective Imaging by Listening to Vibrationally Excited Molecules

    Wang, Han-Wei; Chai, Ning; Wang, Pu; Hu, Song; Dou, Wei; Umulis, David; Wang, Lihong V.; Sturek, Michael; Lucht, Robert; Cheng, Ji-Xin
    We report the realization of vibrational photoacoustic (VPA) microscopy using optical excitation of molecular overtone vibration and acoustic detection of the resultant pressure transients. Our approach eliminates the tissue scattering problem encountered in near-infrared spectroscopy and enables depth-resolved signal collection. The 2nd overtone of the CH bond stretch around 8300  cm^(−1), where blood interference is minimal, is excited. We demonstrate 3D VPA imaging of lipid-rich atherosclerotic plaques by excitation from the artery lumen, and lipid storage in live Drosophila larvae, with millimeter-scale penetration depth.

  2. DNA/RNA nucleotides and nucleosides: direct measurement of excited-state lifetimes by femtosecond fluorescence up-conversion

    Peon, Jorge; Zewail, Ahmed H.
    Fluorescence decay times of the nucleosides: adenosine, guanosine, cytidine and thymidine, and of the corresponding nucleotides, were determined using the technique of fluorescence up-conversion with femtosecond time resolution. The excited-state lifetimes of these nucleic acid molecules all fall in the sub-picosecond time scale, confirming the presence of an ultrafast internal conversion channel for both the nucleotides and the nucleosides; the nucleotides lifetimes are longer than those of the nucleosides by up to 20%. The ultrafast internal conversion is biologically relevant to the stability of DNA, and our results support the sub-picosecond repopulation of the ground state, consistent with transient absorption...

  3. Chemistry at the Uncertainty Limit

    Zewail, Ahmed H.
    Spatial and temporal resolution with, until recently, an unimaginable precision—about 0.01 Å and 10 fs, respectively—are today possible thanks to femtochemistry. However, this atomic-scale resolution raises fundamental questions from quantum mechanics: the Uncertainty Paradox. How is it that this is not an obstacle, and what are the future directions? are the themes of this Essay.

  4. Kinetics modeling of dynamics: the case of femtosecond-activated direct reactions

    Møller, Klaus B.; Zewail, Ahmed H.
    A comparison between a kinetics model and the familiar picture of femtosecond dynamics for direct bond breakage is presented. The kinetics model involves a series of consecutive first-order reactions between transient configurations of the molecule as the bond stretches, before it breaks. We show that in the limit where a large enough number of transient configurations are included, the kinetics model reproduces the molecular response expected from the dynamical picture in a classical, one-trajectory sense.

  5. Avulsion cycles and their stratigraphic signature on an experimental backwater-controlled delta

    Ganti, Vamsi; Chadwick, Austin J.; Hassenruck-Gudipati, Hima J.; Lamb, Michael P.
    River deltas grow in large part through repeated cycles of lobe construction and channel avulsion. Understanding avulsion cycles is important for coastal restoration and ecology, land management, and flood hazard mitigation. Emerging theories suggest that river avulsions on lowland deltas are controlled by backwater hydrodynamics; however, our knowledge of backwater-controlled avulsion cycles is limited. Here, we present results from an experimental delta that evolved under persistent backwater hydrodynamics achieved through variable flood discharges, shallow bed-slopes, and subcritical flows. The experimental avulsion cycles consisted of an initial phase of avulsion setup, an avulsion trigger, selection of a new flow path, and...

  6. Constraints on the star formation efficiency of galaxies during the epoch of reionization

    Sun, G.; Furlanetto, S. R.
    Reionization is thought to have occurred in the redshift range of 6 < z < 9, which is now being probed by both deep galaxy surveys and CMB observations. Using halo abundance matching over the redshift range 5 < z < 8 and assuming smooth, continuous gas accretion, we develop a model for the star formation efficiency f⋆ of dark matter haloes at z > 6 that matches the measured galaxy luminosity functions at these redshifts. We find that f⋆ peaks at ∼30 per cent at halo masses M ∼ 10^(11)–10^(12) M⊙, in qualitative agreement with its behaviour at lower redshifts. We...

  7. C. elegans Stress-Induced Sleep Emerges from the Collective Action of Multiple Neuropeptides

    Nath, Ravi D.; Chow, Elly S.; Wang, Han; Schwarz, Erich M.; Sternberg, Paul W.
    The genetic basis of sleep regulation remains poorly understood. In C. elegans, cellular stress induces sleep through epidermal growth factor (EGF)-dependent activation of the EGF receptor in the ALA neuron. The downstream mechanism by which this neuron promotes sleep is unknown. Single-cell RNA sequencing of ALA reveals that the most highly expressed, ALA-enriched genes encode neuropeptides. Here we have systematically investigated the four most highly enriched neuropeptides: flp-7, nlp-8, flp-24, and flp-13. When individually removed by null mutation, these peptides had little or no effect on stress-induced sleep. However, stress-induced sleep was abolished in nlp-8; flp-24; flp-13 triple-mutant animals, indicating...

  8. Ultrafast Electron Diffraction and Structural Dynamics: Transient Intermediates in the Elimination Reaction of C_2F_4I_2

    Ihee, Hyotcherl; Goodson, Boyd M.; Srinivasan, Ramesh; Lobastov, Vladimir A.; Zewail, Ahmed H.
    We give a full account of our earlier report on the structural dynamics in the elimination reaction of C_2F_4I_2, as studied with the newly constructed third-generation apparatus for ultrafast electron diffraction (UED3) at Caltech (Ihee, H.; Lobastov, V. A.; Gomez, U., Goodson B. M.; Srinivasan, R.; Ruan, C.-Y.; Zewail, A. H. Science 2000, 291, 458). Improvements in experimental stability, sensitivity, resolution, and versatility provided by UED3 permitted the reaction to be probed with spatial and temporal resolution of ∼1 ps and ∼0.01 Å, respectively, and with a sensitivity to chemical change of ∼1%. In addition to the improvements in apparatus...

  9. Ultrafast Dynamics of Porphyrins in the Condensed Phase: I. Free Base Tetraphenylporphyrin

    Baskin, J. Spencer; Yu, Hua-Zhong; Zewail, Ahmed H.
    With femtosecond resolution, using fluorescence up-conversion and transient absorption, we have carried out measurements on free base tetraphenylporphyrin (H_2TPP) in benzene solution, pumping with ∼1300 cm^(-1) of excess vibrational energy in each of the Soret, Q_y, and Q_x bands, and also pumping the lowest vibrational band of Q_y. From these studies, made for different excitations and at different detection wavelengths, we provide a model for describing the elementary intramolecular processes in the Soret, Q_y, and Q_x electronic manifolds, with the following order of time scales and couplings:  electronic (femtosecond), vibrational (femtosecond−picosecond), and singlet−triplet (nanosecond). These dynamical electronic and vibrational relaxation...

  10. Ultrafast Dynamics of Porphyrins in the Condensed Phase: II. Zinc Tetraphenylporphyrin

    Yu, Hua-Zhong; Baskin, J. Spencer; Zewail, Ahmed H.
    Femtosecond spectroscopic studies of zinc tetraphenylporphyrin (ZnTPP) in benzene and dichloromethane are reported, combining both fluorescence up-conversion and transient absorption measurements. The purpose is to investigate the initial electronic and vibrational relaxation of the S_1 and S_2 excited states, in a system in which interference from solvent rearrangement is insignificant as evidenced by the small Stokes shift in the fluorescence. Excitation of the low-lying singlet excited state (S_1) results in nanosecond relaxation, while excitation to S_2, the Soret band, leads to multiple electronic and vibrational relaxation time scales of S_2 and S_1 populations, from hundreds of femtoseconds to tens of...

  11. Ultrafast decay and hydration dynamics of DNA bases and mimics

    Pal, Samir Kumar; Peon, Jorge; Zewail, Ahmed H.
    The ultrafast decay of the excited-state of DNA bases, reported here and elsewhere [Peon & Zewail, Chem. Phys. Lett. 348 (2001) 255], makes very difficult the probing of the full hydration dynamics in DNA. Here we present studies of the natural DNA base mimic, 2-aminopurine (2AP), whose excited-state decay is much longer, in water and, for comparison, in bulk ethanol. Guided by these solvation results, which directly reflect the role of orientational and translational diffusion, we reexamine the ultrafast decay rates for the natural nucleotides and nucleosides. The results are important for the mechanism of energy disposal in DNA.

  12. Unbounded entanglement in nonlocal games

    Mančinska, Laura; Vidick, Thomas
    Quantum entanglement is known to provide a strong advantage in many two-party distributed tasks. We investigate the question of how much entanglement is needed to reach optimal performance. For the first time we show that there exists a purely classical scenario for which no finite amount of entanglement suffices. To this end we introduce a simple two-party nonlocal game H, inspired by Lucien Hardy’s paradox. In our game each player has only two possible questions and can provide bit strings of any finite length as answer. We exhibit a sequence of strategies which use entangled states in increasing dimension d and succeed with probability 1 - O(d^(-c))...

  13. Evidence for non-self-similarity of microearthquakes recorded at a Taiwan borehole seismometer array

    Lin, Yen-Yu; Ma, Kuo-Fong; Kanamori, Hiroo; Song, Teh-Ru Alex; Lapusta, Nadia; Tsai, Victor C.
    We investigate the relationship between seismic moment M_0 and source duration t_w of microearthquakes by using high-quality seismic data recorded with a vertical borehole array installed in central Taiwan. We apply a waveform cross-correlation method to the three-component records and identify several event clusters with high waveform similarity, with event magnitudes ranging from 0.3 to 2.0. Three clusters—Clusters A, B and C—contain 11, 8 and 6 events with similar waveforms, respectively. To determine how M_0 scales with t_w, we remove path effects by using a path-averaged Q. The results indicate a nearly constant t_w for events within each cluster, regardless...

  14. Femtosecond Charge Transfer Dynamics of a Modified DNA Base: 2-Aminopurine in Complexes with Nucleotides

    Fiebig, Torsten; Wan, Chaozhi; Zewail, Ahmed H.
    As a fluorescent isomer of adenine, 2-aminopurine (Ap) is a powerful probe of DNA dynamics and DNA-mediated charge transfer processes. Here, we report studies with femtosecond resolution of the excited-state dynamics of Ap in various solvents and in bimolecular complexes with nucleotides. Using time-resolved transient absorption and fluorescence up-conversion methods we identify charge transfer as the origin for the quenching of the Ap fluorescence by all four DNA nucleotides. The direction of the redox process is, however, dependent on the base, and from the rates we deduce the nature of the transfer, hole versus electron transfer. The pH and the...

  15. Multidimensional Detection of Nitroorganic Explosives by Gas Chromatography-Pyrolysis-Ultraviolet Detection

    Hodyss, Robert; Beauchamp, J. L.
    We describe a new methodology for the trace detection of organic explosives containing nitro functionalities. Conventional gas chromatography separates the components of an explosive mixture. Effluent from the gas chromatograph is pyrolyzed by passage over a heated Nichrome wire. Nitric oxide produced on pyrolysis of a nitroorganic compound is then detected by ultraviolet absorption spectroscopy between 180 and 240 nm, using a deuterium lamp as the light source. Nitric oxide exhibits a sharply banded, characteristic spectrum in this region, enabling detection of nitroorganics. The system is tested using the explosive simulants nitrobenzene and 2,4-dinitrotoluene, and with the nitramine explosive tetryl....

  16. Biological Water: Femtosecond Dynamics of Macromolecular Hydration

    Pal, Samir Kumar; Peon, Jorge; Bagchi, Biman; Zewail, Ahmed H.
    The unique features of a macromolecule and water as a solvent make the issue of solvation unconventional, with questions about the static versus dynamic nature of hydration and the physics of orientational and translational diffusion at the boundary. For proteins, the hydration shell that covers the surface is critical to the stability of its structure and function. Dynamically speaking, the residence time of water at the surface is a signature of its mobility and binding. With femtosecond time resolution it is possible to unravel the shortest residence times which are key for the description of the hydration layer, static or...

  17. Femtosecond dynamics of solvated oxygen anions. II. Nature of dissociation and caging in finite-sized clusters

    Kim, Nam Joon; Paik, D. Hern; Zewail, Ahmed H.
    Ultrafast dissociation and recombination dynamics of (O_2)^−_n, n=3–10 was studied using femtosecond,time-resolvedphotoelectron spectroscopy. The observed transients of nascent fragment anions, following 800 nm fs pulse excitation, exhibit a biexponential rise with two distinct time constants. The time constants, which vary with the number of solvent O_2 molecules, clearly show the solvation effect in two different dissociation pathways. Consistent with the bifurcation picture in the preceding paper, the direct subpicosecond dissociation(τ_1=110-620 fs, depending on n) is governed by electron recombination and kinematics of the half-collision. The second pathway is indirect (τ_2=0.7–8.0 ps, for O^−_6 to O^−_(20)) and controlled by intramolecular vibrational-energy redistribution. In the...

  18. Electrokinetically Based Approach for Single-Nucleotide Polymorphism Discrimination Using a Microfluidic Device

    Erickson, David; Liu, Xuezhu; Venditti, Roberto; Li, Dongqing; Krull, Ulrich J.
    In this work, we describe and implement an electrokinetic approach for single-nucleotide polymorphism (SNP) discrimination using a PDMS/glass-based microfluidic chip. The technique takes advantage of precise control of the coupled thermal (Joule heating), shear (electroosmosis), and electrical (electrophoresis) energies present at an array of probes afforded by the application of external electrical potentials. Temperature controllers and embedded thermal devices are not required. The chips can be easily and inexpensively fabricated using standard microarray printing methods combined with soft-lithography patterned PDMS fluidics, making these systems easily adaptable to applications using higher density arrays. Extensive numerical simulations of the coupled flow and...

  19. An Electrochemical Pumping System for On-Chip Gradient Generation

    Xie, Jun; Miao, Yunan; Shih, Jason; He, Qing; Liu, Jun; Tai, Yu-Chong; Lee, Terry D.
    Within the context of microfluidic systems, it has been difficult to devise pumping systems that can deliver adequate flow rates at high pressure for applications such as HPLC. An on-chip electrochemical pumping system based on electrolysis that offers certain advantages over designs that utilize electroosmotic driven flow has been fabricated and tested. The pump was fabricated on both silicon and glass substrates using photolithography. The electrolysis electrodes were formed from either platinum or gold, and SU8, an epoxy-based photoresist, was used to form the pump chambers. A glass cover plate and a poly(dimethylsiloxane) (PDMS) gasket were used to seal the...

  20. Femtosecond dynamics of solvated oxygen anions. I. Bifurcated electron transfer dynamics probed by photoelectron spectroscopy

    Paik, D. Hern; Kim, Nam Joon; Zewail, Ahmed H.
    The ultrafast dissociationdynamics of O−_6⋅X (X=O_2, N_2, Xe, or N_2O) was investigated by femtosecondphotoelectron spectroscopy. The transients, monitoring nascent O−_2, exhibit biexponential rises with two distinct time constants—the fast component (τ_1∼200 fs) corresponds to the joint rate constant for electron recombination and direct dissociation of the O−_4 core perturbed by solvent molecules, whereas the slow component (τ_2=2.0–7.7 ps, depending on the solvent) corresponds to the process for the liberation of O−_2, which is governed by vibrational predissociation and intramolecular vibrational-energy redistribution. These observations are consistent with the mechanism proposed in the earlier communication of this work [Paik et al., J. Chem. Phys....

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