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  • Articles  (1,678)
  • American Institute of Physics (AIP)  (1,678)
  • 2000-2004  (478)
  • 1995-1999  (1,200)
  • 1980-1984
  • Chemistry and Pharmacology  (1,678)
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  • Articles  (1,678)
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  • 1
    Electronic Resource
    Electronic Resource
    Calculations of the dynamic Debye–Scherrer diffraction patterns for small metal particles (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 2384-2394 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Dynamic diffraction pattern profiles are calculated for randomly oriented aggregates of gold and silver in the size range from 147 to 5083 atoms and at incident electron energies of 40 kV and 100 kV. The Debye–Scherrer diffraction patterns were obtained by combining a series of multislice calculations performed on model particles over a range of orientations. Calculations are performed for both fcc and icosahedral structures. The results show that corrections to the kinematical theory (Debye equation) are more important than predicted by the two-beam theory of Blackman. One calculation, a fcc 923-atom silver aggregate at 100 kV, showed a distortion to the (111) Bragg peak causing it to to shift its center to a higher scattering angle. Scattering corrections to the icosahedral results are less important than for an equivalent sized fcc aggregate and preserve the general diffraction features seen in kinematic calculations. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.469662
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  • 2
    Electronic Resource
    Electronic Resource
    Vibrational dynamics of a stepped metallic surface: Step-edge phonons and terrace softening on Ni(977) (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 9077-9089 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Inelastic helium atom scattering has been used to measure the surface and step localized phonons on a stepped metallic surface, Ni(977). These time-of-flight measurements were carried out both perpendicular and parallel to the step direction. Surface phonon dispersion data collected across the steps show backfolding of the surface Rayleigh mode, and, most importantly, dramatic softening as compared to the forces present at the smooth Ni(111) surface. This softening suggests significant relaxation perpendicular to the step edge. Single-phonon scattering data collected along the step direction reveals the presence of two new step-edge localized modes, as well as the Rayleigh mode for this direction of the crystal. The Rayleigh mode here does not exhibit the notable softening that was found for the other direction. Novel in- and out-of-phase scattering measurements, with respect to the terraces, lead us to assign the new step induced modes as the two transversely polarized vibrations which propagate along the direction of the step edge. An analytic one-dimensional lattice model is proposed which well represents the dispersion data for these two step modes; its use allows us to determine the effective local force field in the two transverse directions with respect to the step edge. The findings reported herein shed new light on such topics as interface stability, crystal growth, and charge redistribution in the vicinity of well-characterized extended surface defects. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468856
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  • 3
    Electronic Resource
    Electronic Resource
    Hydrogen on polycrystalline diamond films: Studies of isothermal desorption and atomic deuterium abstraction (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 992-1002 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Studies of hydrogen isothermal desorption and abstraction from polycrystalline diamond surfaces are presented. The surface H and D coverages (θH and θD) are measured in real time by mass analyzing the recoiled ions generated in a time-of-flight scattering and recoil spectroscopy (TOF-SARS) experiment. For surface temperatures (Ts) from 825 and 920 °C, isothermal H2 desorption is 1st order in θH with a measured activation energy, ET, of 69±6 kcal/mol and a pre-exponential factor, ν, of 1010.5±0.9 s−1. For H2 desorption from diamond, the estimated ΔET based on bond energy calculations is ≈88 kcal/mol, substantially higher than the experimentally measured ET. This difference suggests π-bonding of the surface after H2 desorption is involved. Using a simple bond order argument, the π-bonding contribution is estimated to be ≈21 kcal/mol. The abstraction and replacement of absorbed H by atomic deuterium (Dat) is explained by three first-order reactions. Under a constant Dat flux, the rate of abstraction of adsorbed H by Dat is 1st order in θH, with an "apparent activation energy'' (Ea) of 0.8±0.2 kcal/mol and ν=(1.3±0.2)10−3 s−1. The low Ea and 1st order kinetics imply that H is abstracted from the surface by a generalized Eley–Rideal or direct mechanism. Using the relative rate of Dat abstraction of H to Dat adsorption on clean diamond, we estimate an upper limit for the abstraction activation barrier of 16 kcal/mol. Under identical reaction conditions, the rate for Hat abstraction of D is ≈1/3 the rate for Dat abstraction of H. We interpret this isotope effect using momentum transfer arguments. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.469167
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  • 4
    Electronic Resource
    Electronic Resource
    Vibrational relaxation of a polyatomic solute in a polyatomic supercritical fluid near the critical point (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 8973-8976 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Vibrational lifetimes and absorption spectra of the asymmetric CO stretching mode (∼1990 cm−1) of W(CO)6 in supercritical CO2 are reported as functions of solvent density and temperature. Close to the critical temperature, the observables are density independent over a twofold range of density. Possible explanations are discussed for this unique behavior. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.472628
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  • 5
    Electronic Resource
    Electronic Resource
    Lateral interactions and corrugation in physisorption systems: CH4/Cu(100) (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 7808-7814 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present and analyze isothermal adsorption and desorption measurements for methane physisorbed on a Cu(100) surface to characterize the interplay between molecule–surface and molecule–molecule interactions. A 2D gas–liquid phase boundary for the methane layer is observed and partially mapped in coverage and temperature. Comparing our measured critical temperature of 66 K ±2 K to previous work on related systems implies an attractive well between physisorbed molecules of ∼130 K. This reduction compared to the gas phase well can be adequately understood in terms of dielectric screening of the dispersion interaction. By contrast, analyzing the low-coverage adsorption and desorption isotherms using a quasiequilibrium model and imperfect gas statistics, we deduce a lateral attractive intermolecular potential well of only 40–50 K. This drastic reduction of the attractive well from the gas phase value is not consistent with image charge screening, and is difficult to reconcile with the observed critical temperature. This discrepancy is discussed in terms of surface corrugation which dictates different positions for molecule–surface and molecule–molecule potential energy minima. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.472562
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  • 6
    Electronic Resource
    Electronic Resource
    A detailed test of mode-coupling theory on all time scales: Time domain studies of structural relaxation in a supercooled liquid (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 3723-3733 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The dynamics of supercooled salol (phenyl salicylate) was measured in the time domain using optical Kerr effect techniques. By combining several experimental setups, data spanning more than six decades in amplitude and time (∼100 fs to ∼1 μs) were observed. The data have a complex shape, ranging from high-frequency intramolecular oscillations at short times, to nearly exponential relaxation at long times. As predicted by mode-coupling theory (MCT), the data for some ranges of time appear as power laws. The slowest power law, the von Schweidler power law, has an almost constant exponent of ∼0.59 over the entire temperature range studied (247–340 K). Above the MCT Tc (T〉∼1.17 Tg, where Tg is the laboratory glass transition temperature) for t〉∼1 ps, the decays are shown to be in excellent agreement with the master curve predicted by ideal MCT when higher order terms are included. However, the data do not display the plateau predicted by ideal MCT. To discuss the data at all temperatures, the intermediate time scale portion of the data, 2〈t〈10 to 500 ps (depending on the temperature), is modeled as a power law that falls between the critical decay and the von Schweidler power law. This intermediate power law shows significant temperature dependence with an exponent that decreases to a value of ∼−1 below Tc. Calculations using extended MCT, for a full range of hopping times, demonstrate that the temperature dependence of the intermediate time scale data near and below Tc cannot be explained by extended MCT. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1287595
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  • 7
    Electronic Resource
    Electronic Resource
    Electron distribution in water (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 9206-9208 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The x-ray structure factor of water measured under ambient conditions with synchrotron radiation is compared with those predicted on the basis of partial structure factors describing the nuclear positions obtained by neutron diffraction and of different assumptions for the electron distribution. The comparison indicates that a charge of approximately 0.5 e is transferred from each hydrogen atom to the oxygen on the same molecule, implying an effective dipole moment of 2.9 D, in good agreement with theoretical estimates. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.481541
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  • 8
    Electronic Resource
    Electronic Resource
    Vibrational lifetimes and vibrational line positions in polyatomic supercritical fluids near the critical point (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 3747-3757 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Picosecond infrared pump–probe experiments are used to measure the vibrational lifetime of the asymmetric (T1u) CO stretching mode of W(CO)6 in supercritical CO2, C2H6, and CHF3 as a function of solvent density and temperature. As the density is increased at constant temperature from low, gaslike densities, the lifetimes become shorter. However, in all three solvents, it is found that within a few degrees of the critical temperature (Tr≡T/Tc(approximate)1.01), the lifetimes are essentially constant over a wide range of densities around the critical value (ρc). When the density is increased well past ρc, the lifetimes shorten further. At higher temperature (Tr=1.06) this region of constant vibrational lifetime is absent. Infrared absorption spectra of W(CO)6 and Rh(CO)2acac in supercritical CO2, C2H6, and CHF3 acquired for the same isotherms show that the vibrational spectral peak shifts follow similar trends with density. The peak positions shift to lower energy as the density is increased. Near the critical point, the peak positions are density independent, and then redshift further at densities well above ρc. It is shown that critical fluctuations play a dominant role in the observed effects. Theoretical calculations ascribe the density independence of the observables to the cancellation of various rapidly changing quantities near the critical point. The theory's calculation of density independence implicitly involves averages over all local densities and does not involve any form of solute–solvent clustering. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.474732
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  • 9
    Electronic Resource
    Electronic Resource
    The H+N2O→OH(2Π3/2,v′,N′)+N2 reaction at 1.5 eV: New evidence for two microscopic mechanisms (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 110 (1999), S. 11335-11345 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The product state-resolved dynamics of the photon-initiated reaction H+N2O→OH×(2Π3/2,v′,N′)+N2 has been studied using Doppler-resolved laser induced fluorescence (LIF) at a mean collision energy of 143 kJ mol−1 (≡1.48 eV). Nascent OH(v′=0,1) rovibrational population measurements indicate that only a small fraction of the available energy is channeled into the internal modes of the OH reaction products, as is consistent with previous work at other collision energies. State-resolved angular scattering distributions have been determined and are found to depend sensitively on product OH rovibrational quantum state. For the v′=0 products, the angular scattering distributions are forward-backward peaking at low N′, changing to sideways peaking at high N′. OH products born in the v′=1,N′=6 state possess forward-backward peaking angular scattering distributions, similar to the OH(v′=0) products born with intermediate N′. In addition to these findings, the experiments have allowed the precise determination of the OH quantum state-resolved distributions of kinetic energy releases and, hence, by energy balance, of internal energies accessed in the N2 co-products. The product state-resolved kinetic energy disposals are found to broaden somewhat, and to favor higher kinetic energy disposal, as the internal energy of the OH is increased. The internal energies accessed in the OH and N2 products are therefore (anti-)correlated. More interestingly, the kinetic energy distributions are bimodal, particularly for OH(v′=0) fragments born in high N′, and for those born in v′=1. This finding is attributed to the operation of two microscopic reaction mechanisms, which are probably associated with H atom attack at the two ends of the NNO target molecule. The results are discussed in the light of previous experimental and theoretical work. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479074
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  • 10
    Electronic Resource
    Electronic Resource
    Velocity map imaging studies of the Lyman α photodissociation mechanism for H atom production from hydrocarbons (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 4703-4706 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: H atoms produced in Lyman α photolysis of ethane, propane, and ethylene have been studied using velocity map imaging techniques. Two types of H atoms are identified, one formed along with an alkyl radical in the Rydberg state and the other by the subsequent decomposition of this Rydberg radical. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.477082
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