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Caltech Authors (157.532 recursos)

Repository of works by Caltech published authors.

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Mostrando recursos 1 - 20 de 23

  1. Tunable far infrared laser spectroscopy of van der Waals bonds: Extended measurements on the lowest Sigma bend of ArHCl

    Busarow, Kerry L.; Blake, Geoffrey A.; Laughlin, K. B.; Cohen, R. C.; Lee, Y. T.; Saykally, R. J.
    A tunable far infrared laser system has been used to measure the vibration–rotation spectrum of the lowest Sigma bending state of ArHCl near 24 cm^−1 in a cw planar jet operating with a terminal jet temperature near 3 K. Over 60 transitions have been observed for both 35Cl and 37Cl isotopes with resolution of the quadrupole hyperfine structure. An improved set of molecular parameters was determined, including B, D, H, and eqQ for both upper and lower states. Very narrow linewidths (approximately 300 kHz) resulting in high resolution and sensitivity make this technique a powerful new method for the detailed...

  2. Electronic spectroscopy of trans-azomethane by electron impact

    Mosher, Oren A.; Foster, Michael S.; Flicker, Wayne M.; Beauchamp, J. L.; Kuppermann, Aron
    The electron impact excitation of trans-azomethane (i.e., trans-dimethyl diazine CH3–N–N–CH3) has been studied by both trapped electron (TE) and differential electron scattering (DES) techniques. The nature of the excited state in each of several transitions has been identified by the energy and angular dependences of the excitation cross section. Two previously unreported singlet-->triplet transitions are observed with maxima at 2.75 and 4.84 eV. Theoretical calculations on the parent compound, trans-diimide (H–N=N–H), suggest that these are the χ 1Ag-->1 3Bg (produced by excitation of an electron from an n + molecular orbital to a pi* molecular orbital) and the χ 1Ag-->1...

  3. Quantitative analysis of semiconductor alloy composition during growth by reflection-electron energy loss spectroscopy

    Nikzad, Shouleh; Ahn, Channing C.; Atwater, Harry A.
    Determination of alloy composition during epitaxial growth of GexSi1–x alloys has been demonstrated using reflection-electron energy loss spectroscopy (REELS) at reflection high-energy electron diffraction (RHEED) energies. Measurements of inelastic scattering intensities from Si K (1840 eV) and Ge L2,3 (1217 eV) core losses were performed using a conventional RHEED gun together with an electron energy loss spectrometer in a molecular beam epitaxy system. Comparison of ex situ composition measurements by Rutherford backscattering and energy dispersive x-ray spectroscopy in a transmission electron microscope indicate excellent agreement with composition determination by REELS, demonstrating the capability of REELS as a quantitative in situ...

  4. The synthesis of D-erythro- and D-threo-α-amino-β,γ-dihydroxy-n-butyric acids

    Niemann, Carl; Nichols, Peter L., Jr.
    We repeated the synthesis of Fischer and Feldmann [2] and obtained, as they did, the amino acid with [α]D24 = -13.7° in good yield. However, in addition another α-amino-β,γ-dihydroxy-n-butyric acid, with [α]D24 = 16.0°, was isolated from the reaction mixture. It is the purpose of this communication to show that the amino acid with [α]D24 = -13.7° is not a mixture of disastereoisomers, but is D-threo-α-amino-β,γ-dihydroxy-n-butyric acid, and that the amino acid with [α]D24 = 16.0° is D-erythro-α-amino-β,γ-dihydroxy-n-butyric acid.

  5. Study on the precision of the multiaperture scintillation sensor turbulence profiler (MASS) employed in the site testing campaign for the Thirty Meter Telescope

    Els, S. G.; Schöck, M.; Seguel, J.; Tokovinin, A.; Komilov, V.; Riddle, R.; Skidmore, W.; Travouillon, T.; Vogiatzis, K.; Blum, R.; Bustos, E.; Gregory, B.; Vasquez, J.; Walker, D.; Gillett, P.
    The multiaperture scintillation sensor (MASS) has become a device widely employed to measure the altitude distribution of atmospheric turbulence. An empirical study is reported that investigates the dependence of the MASS results on the knowledge of the instrumental parameters. Also, the results of a side-by-side comparison of two MASS instruments are presented, indicating that MASS instruments permit measurements of the integrated seeing to a precision better than 0.05 arc sec and of the individual turbulence layer strength Cn2(h)dh to better than 10^−14 m^(1/3).

  6. Analytical carbon-oxygen reactive potential

    Kutana, A.; Giapis, K. P.
    We present a reactive empirical potential with environment-dependent bond strengths for the carbon-oxygen (CO) system. The distinct feature of the potential is the use of three adjustable parameters characterizing the bond: the strength, length, and force constant, rather than a single bond order parameter, as often employed in these types of potentials. The values of the parameters are calculated by fitting results obtained from density functional theory. The potential is tested in a simulation of oxidative unzipping of graphene sheets and carbon nanotubes. Previous higher-level theoretical predictions of graphene unzipping by adsorbed oxygen atoms are confirmed. Moreover, nanotubes with externally...

  7. Optical cavity modes in gold shell colloids

    Penninkhof, J. J.; Sweatlock, L. A.; Moroz, A.; Atwater, H. A.; van Blaaderen, A.; Polman, A.
    Core-shell colloids composed of a dielectric core surrounded by a metal shell show geometric cavity resonances with optical properties that are distinctly different than those of the collective plasmon modes of the metal shell. We use finite-difference time domain calculations on silica colloids with a core diameter of 456 nm, surrounded by a 38 nm thick Au shell, to study the temporal evolution of the mode field intensity inside the cavity upon pulsed excitation. Calculations using Mie theory and the T-matrix method are used to analytically determine the dipolar cavity resonance spectrum, which is found superimposed on the broad collective...

  8. The Particulate Methane Monooxygenase from Methylococcus capsulatus (Bath) Is a Novel Copper-containing Three-subunit Enzyme: isolation and charactization

    Nguyen, Hiep-Hoa T.; Elliott, Sean J.; Yip, John Hon-Kay; Chan, Sunney I.
    The particulate methane monooxygenase (pMMO) is known to be very difficult to study mainly due to its unusual activity instability in vitro. By cultivating Methylococcus capsulatus (Bath) under methane stress conditions and high copper levels in the growth medium, membranes highly enriched in the pMMO with exceptionally stable activity can be isolated from these cells. Purified and active pMMO can be subsequently obtained from these membrane preparations using protocols in which an excess of reductants and anaerobic conditions were maintained during membrane solubilization by dodecyl beta-D-maltoside and purification by chromatography. The pMMO was found to be the major constituent in...

  9. Constitutive expression of the neuron-restrictive silencer factor (NRSF)/REST in differentiating neurons disrupts neuronal gene expression and causes axon pathfinding errors in vivo

    Paquette, Alice J.; Perez, Sharon E.; Anderson, David J.
    The neuron-restrictive silencer factor (NRSF; also known as REST for repressor element-1 silencing transcription factor) is a transcriptional repressor of multiple neuronal genes, but little is known about its function in vivo. NRSF is normally down-regulated upon neuronal differentiation. Constitutive expression of NRSF in the developing spinal cord of chicken embryos caused repression of two endogenous target genes, N-tubulin and Ng-CAM, but did not prevent overt neurogenesis. Nevertheless, commissural neurons that differentiated while constitutively expressing NRSF showed a significantly increased frequency of axon guidance errors. These data suggest that down-regulation of NRSF is necessary for the proper development of at...

  10. Application of Ion Cyclotron Resonance to the Study of Inelastic Excitation by Low-Energy Electrons

    Ridge, Douglas P.; Beauchamp, J. L.
    The geometry of the resonance cell employed for ion cyclotron resonance spectroscopy (1-3) is ideally suited for studying inelastic excitation by low-energy electrons. It has been shown that the electron beam traverses a parabolic potential well between the trapping electrodes, the depth of which is approximately half the applied trapping voltage (3,4). Low-energy electrons generated by impact excitation of an atomic or molecular energy level can be trapped in the resonance cell if their final translational energy is insufficient to escape the preset depth of the potential well. These electrons can be drifted from the source of the resonance region...

  11. Infrared photochemistry of ethylene clusters

    Casassa, Michael P.; Bomse, David S.; Beauchamp, J. L.; Janda, Kenneth C.
    Infrared irradiation of ethylene clusters formed in supersonic molecular beams, using a low power cw CO2 laser, results in the photodissociation of a large fraction of the van der Waals molecules. Under such conditions, infrared absorption intensity exhibits first-order power dependence and is readily detected as loss in molecular beam intensity. Intramolecular energy transfer rates, determined by measuring spectral linewidths, are shown to vary with the vibrational mode initially excited. Ethylene clusters containing one quantum of vibrational energy corresponding to the nun7 fundamental in the monomer (949 cm^–1) have a vibrationally predissociative lifetime of 0.33 psec. In comparison, the relaxation...

  12. Germanium quantum dots: Optical properties and synthesis

    Heath, James R.; Shiang, J. J.; Alivisatos, A. P.
    Three different size distributions of Ge quantum dots (>~200, 110, and 60 Å) have been synthesized via the ultrasonic mediated reduction of mixtures of chlorogermanes and organochlorogermanes (or organochlorosilanes) by a colloidal sodium/potassium alloy in heptane, followed by annealing in a sealed pressure vessel at 270 °C. The quantum dots are characterized by transmission electron microscopy, x-ray powder diffraction, x-ray photoemission, infrared spectroscopy, and Raman spectroscopy. Colloidal suspensions of these quantum dots were prepared and their extinction spectra are measured with ultraviolet/visible (UV/Vis) and near infrared (IR) spectroscopy, in the regime from 0.6 to 5 eV. The optical spectra are...

  13. Tunable far infrared laser spectroscopy of van der Waals bonds: Vibration–rotation–tunneling spectra of Ar–H2O

    Cohen, R. C.; Busarow, Kerry L.; Laughlin, K. B.; Blake, Geoffrey A.; Havenith, M.; Lee, Y. T.; Saykally, R. J.
    The first high resolution spectra of a rare gas–H2O cluster have been observed using a tunable far infrared laser to probe the vibration–rotation–tunneling levels of Ar–H2O formed in a continuous planar supersonic jet. The high sensitivity of this spectrometer facilitated extensive measurements of two perpendicular subbands which are assigned to transitions from the ground state to the upper component of a hydrogen exchange tunneling doublet (c-type) at 21 cm^−1, and to vb1 =1+ (b-type) at 25 cm^−1, the lower tunneling component of a bending vibration which is perpendicular to the tunneling coordinate. The tunneling splitting is shown to be in...

  14. Stochastic sensitivity analysis in chemical kinetics

    Costanza, Vicente; Seinfeld, John H.
    The stochastic sensitivity analysis problem in chemical kinetics is defined as determining the probability density function (pdf) of the concentrations given probability density functions for the parameters and initial conditions. The joint concentration parameter pdf is found to satisfy the equation ([partial-derivative]p/[partial-derivative]t)+div(Fp) = 0, where the system dynamics are given by ? = F(x). The properties of the solution of this equation are studied, and the approach is applied to analyze the sensitivity of the kinetics of the photolysis of a mixture of carbon monoxide, nitrogen dioxide, nitric oxide, and water in air to uncertainties in the initial concentrations of...

  15. The interaction of ions with nonpolar neutrals: The collision broadening of ion cyclotron resonance lines of ions in hydrogen and methane

    Ridge, D. P.; Beauchamp, J. L.
    The motion of ions in an ion cyclotron resonance cell is considered, and measurements of ion cyclotron resonance linewidths are described. The connection between cyclotron resonance linewidths and kinetic parameters is developed. Mobilities and collision frequencies of CH5 + , C2H5 + , C3H7 + , C4H9 + , and Na + in methane and H + , H3 + , H30 + , CH5 + , C2H5 + , C3H7 + , and Na + in hydrogen are determined from the linewidth measurements. Resulting mobilities of H + , H3 + , and Na + in H2 are found...

  16. Mutational analysis of the coordinate expression of the yeast tRNAArg-tRNAAsp gene tandem

    Reyes, Vicente M.; Newman, Anna P.; Abelson, John
    tRNA genes occur in the yeast genome as highly dispersed and independent transcriptional units. The 5'-tRNAArg-tRNAAsp-3' gene tandem, separated by a 10-base-pair spacer sequence, thus represents a rare case of tight clustering. Previous in vitro studies did not reveal any primary transcript from the tRNAAsp gene, but rather a dimeric precursor containing both gene sequences plus spacer, which undergoes a series of maturation steps. This seems anomalous since the tRNAAsp gene contains the sequences necessary for its own transcription. We found that site-directed mutation of the highly conserved C at position 56 to a G in the tRNAArg gene suppresses...

  17. Diversity Analysis of Distributed Space-Time Codes in Relay Networks with Multiple Transmit/Receive Antennas

    Jing, Yindi; Hassibi, Babak
    The idea of space-time coding devised for multiple-antenna systems is applied to the problem of communication over a wireless relay network, a strategy called distributed space-time coding, to achieve the cooperative diversity provided by antennas of the relay nodes. In this paper, we extend the idea of distributed space-time coding to wireless relay networks with multiple-antenna nodes and fading channels. We show that for a wireless relay network with M antennas at the transmit node, N antennas at the receive node, and a total of ℛ antennas at all the relay nodes, provided that the coherence interval is long enough,...

  18. Code Design for Multihop Wireless Relay Networks

    Oggier, Frédérique; Hassibi, Babak
    We consider a wireless relay network, where a transmitter node communicates with a receiver node with the help of relay nodes. Most coding strategies considered so far assume that the relay nodes are used for one hop. We address the problem of code design when relay nodes may be used for more than one hop. We consider as a protocol a more elaborated version of amplify-and-forward, called distributed space-time coding, where the relay nodes multiply their received signal with a unitary matrix, in such a way that the receiver senses a space-time code. We first show that in this scenario,...

  19. Improved electrical properties of wafer-bonded p-GaAs/n-InP interfaces with sulfide passivation

    Nakayama, Keisuke; Tanabe, Katsuaki; Atwater, Harry A.
    Sulfide-passivated GaAs and InP wafers were directly bonded to explore the efficiency of sulfide passivation on the bonded interfacial properties. We find that the bonded GaAs/InP interfaces after sulfide passivation contain sulfur atoms and a decreased amount of oxide species relative to the pairs bonded after conventional acid treatment; however, the residual sulfur atoms have no effect on the bonding strength. The electrical properties of the bonded p-GaAs/n-InP heterojunctions were studied for different acceptor concentrations in p-GaAs. A reduced interfacial trap state density enhances the tunnel current flow across the depletion layer in the sulfide-passivated case. A directly bonded tunnel...

  20. A synthetic Escherichia coli predator–prey ecosystem

    Balagaddé, Frederick K.; Song, Hao; Ozaki, Jun; Collins, Cynthia H.; Barnet, Matthew; Arnold, Frances H.; Quake, Stephen R.; You, Lingchong
    We have constructed a synthetic ecosystem consisting of two Escherichia coli populations, which communicate bi-directionally through quorum sensing and regulate each other's gene expression and survival via engineered gene circuits. Our synthetic ecosystem resembles canonical predator–prey systems in terms of logic and dynamics. The predator cells kill the prey by inducing expression of a killer protein in the prey, while the prey rescue the predators by eliciting expression of an antidote protein in the predator. Extinction, coexistence and oscillatory dynamics of the predator and prey populations are possible depending on the operating conditions as experimentally validated by long-term culturing of...

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