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Caltech Authors (169.428 recursos)

Repository of works by Caltech published authors.

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Mostrando recursos 1 - 20 de 349

  1. Defining the baseline of the REE stable isotope variations in solar system materials: Earth

    Hu, J. Y.; Tissot, F. L. H.; Yokochi, R.; Ireland, T. J.; Dauphas, N.
    Mass-dependent fractionations (MDFs) of stable isotopes record critical information regarding the origin and evolution of planetary materials [1]. Studies of MDF of refractory lithophile elements (RLEs) can provide insights into condensation/evaporation and planetary accretion processes in the early solar system. For example, the lighter calcium isotope composition observed in carbonaceous meteorites compared to that of the bulk silicate Earth, enstatite and ordinary chondrites [2, 3] may be due to the contribution of refractory dust [4, 5], which has a light Ca isotope composition [6, 7]. In contrast, titanium, another RLE with a similar chemical behavior in the early solar system, was found to have uniform isotope compositions among different groups...

  2. ^(36)Cl-^(36)S in Allende CAIs: Implication for the origins of ^(36)Cl in the early solar system

    Tang, H.; Liu, M-C.; McKeegan, K. D.; Tissot, F. L. H.; Dauphas, N.
    Chlorine-36 (t_(1/2)=0.3 Myr) decays to either ^(36)Ar (98%, β-) or ^(36)S (1.9%, ε and β+). This radionuclide can be produced by either charged particle irradiation [1,2] or stellar nucleosynthesis [3]. Evidence for the prior existence of ^(36)Cl in the Early Solar System (ESS) comes from radiogenic excesses of ^(36)Ar [4,5] and/or ^(36)S [6-9] in secondary phases (e.g., sodalite and wadalite) of ESS materials such as Ca, Al-rich inclusions (CAIs) and chondrules. However, the inferred initial ^(36)Cl/^(35)Cl ratios vary over three orders of magnitude among different chondrite constituents (5×10^(-6)-9×10^(-3)) [6-9]. Interestingly, although the initial ^(36)Cl/^(35)Cl ratios inferred in previous studies vary widely, all secondary phases...

  3. Uranium isotope variations in group II refractory inclusions

    Tissot, F. L. H.; Dauphas, N.; Grossman, L.
    The ^(235)U/^(238)U ratio shows little variability in most terrestrial and meteoritic bodies (≤1 ‰) [1-3]. In contrast, large excesses of ^(235)U, up to 3.5 ‰, have been found in a few Calcium-Aluminum rich inclusions (CAIs) and have been interpreted as evidence of live ^(247)Cm (t_(1/2) = 15.6 My) in the early solar system (SS) [4]. Though this is a plausible explanation, it relies on four points with high Nd/U ratio that define a “pseudochron”, so more work is needed to determine the cause of U isotope variations in CAIs. Here, we report some preliminary results on the identification, characterization and U isotopic analysis of 12 fine-grained, group II, CAIs from the Allende...

  4. Introducing Teflon-HPLC

    Tissot, F. L. H.; Ireland, T. J.; Yokochi, R.; Dauphas, N.
    With increasingly ambitious sample return missions and instrumentation of ever-increasing sensitivity and precision, column chromatography appears to be the neglected step-child of isotope geochemistry and little improvement has been brought to it in the past few decades. Traditional column chromatography (i.e., open-system, gravity driven) techniques suffer from significant limitations pertaining to the overall length of column, resin size and diffusion effects, which can severely compromise separation efficiencies. Furthermore, some fine-scale separations still require complicated multi-step, highly time-consuming protocols (e.g. Ni-Mg, [1]). High-performance liquid chromatography (HPLC), while overcoming many of these limitations (e.g. a closed-system setup; the ability to pressurize the system,...

  5. ^(238)U/^(235)U in marine carbonates as a tracer of Precambrian paleoredox conditions

    Chen, C.; Tissot, F. L. H.; Dauphas, N.; Bekker, A.; Halverson, G. P.; Veizer, J.
    The timing and magnitude of the oxygenation of Earth’s ocean is still a matter of intense debate. Previous work suggested that the uranium isotope variations recorded in ancient marine sediments, such as shales and carbonates, could provide valuable insights into paleoredox conditions [e.g., 1-11]. In this work, we study U concentration and isotopic composition of a large number of Precambrian carbonates to place constraints on long-term variations in oceanic redox conditions.

  6. Establishing the liquid phase equilibrium of angrites to constrain their petrogenesis

    Tissot, F. L. H.; Collinet, M.; Dauphas, N.; Grove, T. L.
    Angrites are a series of differentiat-ed meteorites, extremely silica undersaturated and with unusally high Ca and Al contents [1]. They are thought to originate from a small planetesimal parent body of ~ 100-200 km in radius ([2-3]), can be either plutonic (i.e., cumulates) or volcanic (often referred to as quenched) in origin, and their old formation ages (4 to 11 Myr after CAIs) have made them prime anchors to tie the relative chronologies inferred from short-lived radionuclides (e.g., Al-Mg, Mn-Cr, Hf-W) to the absolute Pb-Pb clock [4]. They are also the most vola-tile element-depleted meteorites available, displaying a K-depletion of...

  7. Developments in PF-HPLC (pneumatic-fluoropolymer high performance liquid chromatography)

    Hu, J. Y.; Tissot, F. L. H.; Yokochi, R.; Ireland, T. J.; Dauphas, N.
    Return missions are providing unique opportunities to deepen our knowledge of the formation and evolution of the solar system. The six Apollo missions have been critical in shaping our understanding of the Earth-Moon history [1], and the recent Genesis (solar wind; e.g., [2]), Stardust (cometary dust from Wild 2; e.g., [3,4]) and Hayabusa (dust from S-type asteroid from Itokawa; e.g., [5]) missions brought in a wealth of data.

  8. I-Xe studies of aqueous alteration in the Allende CAI Curious Marie

    Pravdivtseva, O.; Meshik, A.; Tissot, F. L. H.; Dauphas, N.
    The Allende fine-grained inclusion Curious Marie is a unique CAI. It is depleted in uranium but contains large ^(235)U excess [1], providing new evidence that ^(247)Cm was alive in the Early Solar System, as has been previously suggested [2], and leading to an updated (^(247)Cm/^(235)U)initial ratio of (1.1±0.3)×10^(-4).

  9. The REE isotopic compositions of the Earth

    Hu, J. Y.; Dauphas, N.; Tissot, F. L. H.; Yokochi, R.; Ireland, T. J.
    Lanthanides are a group of 14 naturally occurring elements with atomic numbers ranging from 57 (La) to 74 (Lu), which are also known as rare earth elements (REE). REEs are ubiquitous in minerals and rocks. The chemical properties of REEs vary as smooth functions of their atomic numbers, a phenomenon known as the contraction of the lanthanides. This is the main control behind REE fractionation in minerals and rocks. The relative abundance of REEs is usually presented as the REE pattern by normalizing the concentrations in the sample to those in reference materials such as chondrites and shales.

  10. Learning recurrent representations for hierarchical behavior modeling

    Eyjolfsdottir, Eyrun; Branson, Kristin; Yue, Yisong; Perona, Pietro
    We propose a framework for detecting action patterns from motion sequences and modeling the sensory-motor relationship of animals, using a generative recurrent neural network. The network has a discriminative part (classifying actions) and a generative part (predicting motion), whose recurrent cells are laterally connected, allowing higher levels of the network to represent high level phenomena. We test our framework on two types of data, fruit fly behavior and online handwriting. Our results show that 1) taking advantage of unlabeled sequences, by predicting future motion, significantly improves action detection performance when training labels are scarce, 2) the network learns to represent...

  11. Reactive intermediates in Cu MOR zeolites for alkane oxidation

    Vanelderen, Pieter; Hadt, Ryan G.; Kirschhock, Christine; Schoonheydt, Robert A.; Solomon, Edward I.; Sels, Bert F.
    The Cu^(2+)-O-Cu^(2+) core in ZSM-5 zeolite is, thus far, the only well-characterized copper intermediate in the methane to methanol reaction. This recent discovery has had a broad impact in the field of heterogeneous catalysis and bioinorg. chem. Next to its identification, its formation pathway both from N_2O and O_2 was studied in detail recently. O_2 activation proceeds through the formation of a µ-η^2:η^2-peroxodicopper(II) site. Remarkably, almost at the same time, it was reported that also pMMO enzyme is also able to form such a peroxo dicopper site. Studying the activation mechanism of N_2O is important in view of N_2O decompn....

  12. Coherent vibrational imaging for living cells

    Lu, Wei; Wei, Min
    Innovations in optical spectroscopy and microscopy have revolutionized our understanding in biol. systems at subcellular levels. While fluorescence microscopy with versatile fluorescent probes is widely popular for biomedical research, vibrational microscopy with rich chem. information is less explored. Here, I present two chem. imaging strategies addressing two fundamental challenges in optical bio-imaging, resp. First, we devised a live-cell Bioorthogonal Chem. Imaging platform suited for probing the metabolic dynamics of small bio-mols., which cannot be effectively labeled by bulky fluorophores. This scheme couples the emerging stimulated Raman scattering (SRS) microscopy with small vibrational tags including stable isotopes and triple bonds. Second, we developed a super-multiplex optical imaging technique...

  13. Illuminating small bio-molecules: Stimulated Raman scattering imaging of vibrational tags

    Lu, Wei; Wei, Min
    Innovations in spectroscopy principles and microscopy technol. have significantly impacted modern biol. and medicine. While most of the contemporary bio-imaging modalities harness electronic transition, nuclear spin or radioactivity, vibrational spectroscopy has not been widely used yet. Here we will discuss an emerging chem. imaging platform, stimulated Raman scattering (SRS) microscopy, which can enhance the otherwise feeble spontaneous Raman eight orders of magnitude by virtue of stimulated emission. When coupled with stable isotopes (e.g., deuterium and 13C) or bioorthogonal chem. moieties (e.g., alkynes), SRS microscopy is well suited for probing in vivo metabolic dynamics of small bio-mols. which cannot be labeled by bulky fluorophores. Phys. principle of...

  14. Photosensitized, energy transfer-mediated organometallic catalysis through electronically excited nickel(II)

    Welin, Eric; Le, Chip; Arias-Rotondo, Daniela; McCusker, James; MacMillan, David
    Transition metal catalysis has traditionally relied on organometallic complexes that can cycle through a series of groundstate oxidn. levels to achieve a series of discrete yet fundamental fragment- coupling steps. The viability of excited- state organometallic catalysis via direct photoexcitation has been demonstrated extensively over the past 100 years. Although the utility of triplet sensitization by energy transfer has long been known as a powerful activation mode in org. photochem., it is surprising to recognize that photosensitization mechanisms to access excited- state organometallic catalysts have lagged far behind. Here, we demonstrate excited- state organometallic catalysis via such an activation pathway: Energy transfer from an iridium sensitizer...

  15. Selective partial oxidation of light alkanes using iodine oxides and halides

    Schwartz, Nichole; Fortman, George; Kalman, Steven; Fu, Ross; Nielsen, Robert; Boaz, Nicholas; Goddard, William; Groves, John; Gunnoe, T.
    While natural gas is a plentiful energy resource, methods to effectively convert light alkanes to readily transported liqs. are energy and capital intense. Com. available and inexpensive halide and iodine oxide salts can selectively functionalize methane, ethane and propane in high yields in trifluoroacetic acid solvent at temps. ∼200 °C. For the conversion of methane, yields > 40 % have been demonstrated with > 97 % selectivity for Me trifluoroacetate. Under photolytic conditions, the conversion of methane to Me trifluoroacetate also proceeds with high yield and selectivity (∼50 % and ∼95 %, resp.) Ethane and propane can also be functionalized photolytically with similar yields and...

  16. Solar Army: Incorporating real-time research into outreach efforts

    Schuttlefield Christus, Jennifer; DeBoever, Michelle
    The Solar Army is the international outreach effort for the NSF- funded Center for Chem. Innovation Solar Fuels based at the California Institute of Technol. The CCI Solar Fuel research focuses on one of the "holy- grails" of 21st Century chem. - the efficient and economical conversion of solar energy into stored chem. fuel. CCI Solar investigators are targeting the crit. science underpinning the solar- driven decompn. of water into hydrogen and oxygen. The Solar Army utilizes three outreach kits to educate participants on solar energy conversion and provide students with an opportunity to participate in real- time research searching for solar water- splitting catalysts....

  17. Calculating electronic g-tensors with density matrix renormalization group wavefunctions

    Sayfutyarova, Elvira; Chan, Garnet
    I will present a state interaction spin- orbit coupling method using d. matrix renormalization group wavefunctions. This is used for the calcn. of g- tensors, the primary parameter characterizing the complex open- shell electronic structure of metalloenzyme active sites. The proposed method's performance on the example of transition metal complexes, mimicking active centers of metalloenzymes, will be discussed.

  18. Optical, morphological, and electrochemical multimodal characterization for integrated BiVO4 photoanodes

    Liu, Guiji; Eichhorn, Johanna; Haber, Joel; Gregoire, John; Sharp, Ian; Toma, Francesca Maria
    Photoelectrochem. water splitting is a promising route for efficient conversion of solar energy to chem. fuel. However, the development of an efficient photoanode remains the primary materials challenge in the establishment of a scalable technol. for solar water splitting. The typical photoanode architecture consists of a semiconductor light absorber coated with a metal oxide that serves a combination of functions, including corrosion protection, electrocatalysis, light trapping, hole transport, and elimination of deleterious recombination sites. In addn., the functional behavior of photocatalytic systems strongly depends on the presence of structural defects and heterogeneity over different length scales. Indeed, charge trapping at interfaces and/or (photo) corrosion processes can affect...

  19. DFT simulation of edge halogenated nanosheets as an ORR catalyst in fuel cells

    Humphrey, Nicholas; Rodriguez, Roberto; Arias, Gabriel; Yu, Ted; Goddard, William
    Catalytic fuel cells have become increasingly attractive due to the harsh demand on fossil fuels. However, they are expensive due to the cost and scarcity of precious metals such as platinum. Recently, edge halogenated graphene- nanosheets have been discovered to be a low cost Oxygen Redn. Reaction (ORR) catalyst for alk. fuel cells. A better understanding of these ORR catalysts is needed to understand why the preferred product is water as opposed to the peroxide for undoped graphene, and how to utilize these catalysts for ORR in an acidic environment. This study uses the PBE flavor d. functional theory (DFT) with the SeqQUEST code to...

  20. Study to improve electrolytes in sodium batteries

    Higgwe, Tamunotelema; Walker, Dejuante; Thai, Edward; Yu, Ted
    With increasing need for batteries and energy storage sodium batteries are of increasing demand. Lithium batteries are very popular but focus is being placed on sodium due to its higher abundance and subsequently, its lower cost. Sodium is also less poisonous to the human body than lithium for use in ingestible battery applications. The main focus of the study is to improve the electrolyte. We test the electrolyte in a coin cell battery with sodium as both the anode and cathode. Five different solvents mixts. for the electrolytes were tested in our lab: ethylene carbonate (EC)/propylene carbonate (PC), EC/PC/DEC (di-Et...

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