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  • 1
    Electronic Resource
    Electronic Resource
    Laser spectroscopy of crossed molecular beams: The dissociation energy of BaI from energy-balance measurements (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 8544-8556 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Through application of energy-balance arguments to the crossed-beam reaction Ba(1S0)+HI(X 1Σ+) →BaI(X 2Σ+) +H(2S1/2), a lower limit for the BaI bond dissociation energy is determined to be D00(BaI) (approximately-greater-than)76.8±1.7 kcal/mol (3.33±0.07 eV). Based on the upper bound of D00(BaI) (approximately-less-than)78.5±0.5 kcal/mol, as determined from earlier predissociation studies [M. A. Johnson, J. Allison, and R. N. Zare, J. Chem. Phys. 85, 5723 (1986)], we recommend a BaI bond strength of 77.7±2.0 kcal/mol (3.37±0.09 eV). This dissociation energy is more than 5 kcal/mol higher than the previously accepted value of D00(BaI) as derived from mass spectrometric measurements.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459292
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  • 2
    Electronic Resource
    Electronic Resource
    Fluorine atom abstraction by Si(100) II. Model (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 5190-5204 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A model is developed to describe the kinetics of the three scattering channels—unreactive scattering and dissociative chemisorption via single atom abstraction and two atom adsorption—that are present in the interaction of F2 with Si(100). The model provides a good description of the non-Langmuirian coverage dependence of the probabilities of single atom abstraction and two atom adsorption, yielding insight into the dynamics of the gas–surface interaction. The statistical model is based on the premise that the two dissociative chemisorption channels share a common initial step, F atom abstraction. The subsequent interaction, if any, of the complementary F atom with the surface determines if the overall result is single atom abstraction or two atom adsorption. The results are consistent with the orientation of the incident F2 molecular axis with respect to the surface affecting the probability of single atom abstraction relative to two atom adsorption. A perpendicular approach favors single atom abstraction because the complementary F atom cannot interact with the surface, whereas a parallel approach allows the F atom to interact with the surface and adsorb. The fate of the complementary F atom is dependent on the occupancy of the site with which it interacts. The model distinguishes between four types of dangling bond sites on the Si(100)(2×1) surface, based on the occupancy of the site itself and that of the complementary Si atom in the Si surface dimer. The results show that the unoccupied dangling bond sites on half-filled dimers are about twice as reactive as those on empty dimers, which is consistent with an enhanced reactivity due to a loss of a stabilizing π interaction between the two unoccupied dangling bonds on a dimer. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.481092
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  • 3
    Electronic Resource
    Electronic Resource
    Fluorine atom abstraction by Si(100). I. Experimental (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 3679-3695 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In the interaction of low energy F2 with Si(100) at 250 K, a dissociative chemisorption mechanism called atom abstraction is identified in which only one of the F atoms is adsorbed while the other F atom is scattered into the gas phase. The dynamics of atom abstraction are characterized via time-of-flight measurements of the scattered F atoms. The F atoms are translationally hyperthermal but only carry a small fraction (∼3%) of the tremendous exothermicity of the reaction. The angular distribution of F atoms is unusually broad for the product of an exothermic reaction. These results suggest an "attractive" interaction potential between F2 and the Si dangling bond with a transition state that is not constrained geometrically. These results are in disagreement with the results of theoretical investigations implying that the available potential energy surfaces are inadequate to describe the dynamics of this gas–surface interaction. In addition to single atom abstraction, two atom adsorption, a mechanism analogous to classic dissociative chemisorption in which both F atoms are adsorbed onto the surface, is also observed. The absolute probability of the three scattering channels (single atom abstraction, two atom adsorption, and unreactive scattering) for an incident F2 are determined as a function of F2 exposure. The fluorine coverage is determined by integrating the reaction probabilities over F2 exposure, and the reaction probabilities are recast as a function of fluorine coverage. Two atom adsorption is the dominant channel [P2=0.83±0.03(95%, N=9)] in the limit of zero coverage and decays monotonically to zero. Single atom abstraction is the minor channel (P1=0.13±0.03) at low coverage but increases to a maximum (P1=0.35±0.08) at about 0.5 monolayer (ML) coverage before decaying to zero. The reaction ceases at 0.94±0.11(95%, N=9) ML. Thermal desorption and helium diffraction confirm that the dangling bonds are the abstraction and adsorption sites. No Si lattice bonds are broken, in contrast to speculation by other investigators that the reaction exothermicity causes lattice disorder. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479677
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  • 4
    Electronic Resource
    Electronic Resource
    Soft-landed ion diffusion studies on vapor-deposited hydrocarbon films (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 2222-2234 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Cesium and hydronium ions were deposited with a "soft-landing" ion beam (1 eV) on n-hexane and 3-methyl-pentane vapor-deposited thin films on a Pt (111) surface at 27 K. Dielectric properties and ion migration were studied during the ion deposition and during a temperature ramp up to the desorption temperature of the molecular films. The ions were found to migrate through amorphous versions of these films as expected by simple viscosity models near 90 K with ion mobilities of about 10−18 m2 V−1 s−1. No, or very limited, diffusion was observed through crystalline films. The n-hexane films crystallize during the ion motion. Analysis of this permits the estimation that average diffusional motion for a neutral hexane during crystallization is about 1 molecular diameter. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479494
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  • 5
    Electronic Resource
    Electronic Resource
    Product rotational distributions and specific opacity functions for the reaction Ba +HI → BaI (v=0,4,8,12,16,18) +H (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 7220-7225 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The reaction Ba+HI→BaI(v)+H was studied under beam-gas, single-collision conditions with an average center-of-mass collision energy of 13 kJ mol−1. BaI (v) rotational distributions were recorded for v=0, 4, 8, 12, 16, and 18 by means of selectively detected laser-induced fluorescence of the BaI C 2Π–X 2Σ+ band system. Each rotational distribution exhibits a maximum toward its high energy end and the range of rotational states becomes narrower as product vibration increases. Because the kinematic constraint causes almost all reagent orbital angular momentum to appear in product rotation, the principle of angular momentum conservation provides the means for determining specific opacity functions from the rotational distributions and the reagent relative velocity distribution. The specific opacity functions are narrow functions of the impact parameter. The peak values decrease smoothly from approximately 4.5 A(ring) for v=0 to 1.5 A(ring) for v=18, indicating a strong correlation between impact parameter and product vibrational state such that Ba+HI collisions with small impact parameter produce BaI with large vibrational excitation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.463547
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  • 6
    Electronic Resource
    Electronic Resource
    Immobility of protons in ice from 30 to 190 K (1999)
    [s.l.] : Macmillan Magazines Ltd.
    Nature 398 (1999), S. 405-407 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] The anomalously fast motion of hydronium ions (H3O+) in water is often attributed to the Grotthuss mechanism, whereby protons tunnel from one water molecule to the next. This tunnelling is relevant to proton motion through water in restricted geometries, such as ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/18848
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