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Repository of works by Caltech published authors.

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Mostrando recursos 1 - 14 de 14

  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. 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).

  4. 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...

  5. 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...

  6. 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...

  7. 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...

  8. 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...

  9. 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...

  10. 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,...

  11. 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,...

  12. 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...

  13. Efficient multiphoton dissociation of CF3I+ in the metastable X-tilde 2E1/2 excited state using cw infrared laser radiation

    Thorne, L. R.; Beauchamp, J. L.
    Evidence for the multiphoton dissociation of an electronic metastable state of CF3I+ with CO2 laser radiation is presented. Only those ions possessing sufficient internal excitation before irradiation can be dissociated. Photodissociation results primarily from resonant absorption by the nu1 vibrational mode of CF3I+ in the higher of the two, spin–orbit levels of the ground electronic state Xbar^2 E1/2. Only the lowest energy decomposition channel is observed; this yields CF + 3 and I. Spin–orbit and vibrational relaxation compete with photodissociation. At CF3I pressures below 2×10^−7 Torr, the collisional relaxation rate is proportional to CF3I pressure kc = (3.1±0.4)×10^−9 cm^3 molecule^−3...

  14. Vibrational overtone initiated unimolecular dissociation of HOCH_2OOH and HOCD_2OOH: Evidence for mode selective behavior

    Matthews, Jamie; Fry, Julie L.; Roehl, Coleen M.; Wennberg, Paul O.; Sinha, Amitabha
    The vibrational overtone induced unimolecular dissociation of HMHP (HOCH2OOH) and HMHP-d2 (HOCD2OOH) into OH and HOCH2O (HOCD2O) fragments is investigated in the region of the 4nuOH and 5nuOH bands. The unimolecular dissociation rates in the threshold region, corresponding to the 4nuOH band, exhibit measurable differences associated with excitation of the OH stretch of the alcohol versus the peroxide functional group, with the higher energy alcohol OH stretching state exhibiting a slower dissociation rate compared to the lower energy peroxide OH stretch in both HMHP and HMHP-d2. Predictions using the Rice–Ramsperger–Kassel–Marcus theory give rates that are in reasonably good agreement with...

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