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  • 1
    Electronic Resource
    Electronic Resource
    Scattering and trapping dynamics of gas-surface interactions: Theory and experiments for the Xe-graphite system (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 10339-10349 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We report on molecular beam experiments and molecular dynamics simulations of xenon scattering with incident energies E=0.06−5.65 eV from graphite. The corrugation felt by an atom interacting with the surface is found to be influenced by both surface temperature, Ts, and E. Angular distributions are significantly broadened when Ts is increased, clearly indicating corrugation induced by thermal motion of the surface also at the highest E employed. Direct scattering dominates for high E, while trapping becomes important for kinetic energies below 1 eV. The coupling between atom translation and surface modes in the normal direction is very effective, while trapped atoms only slowly accommodate their momentum parallel to the surface plane. The very different coupling normal and parallel to the surface plane makes transient (incomplete) trapping-desorption unusually pronounced for the Xe/graphite system, and atoms may travel up to 50 nm on the surface before desorption takes place. The nonlocal and soft character of the Xe-graphite interaction compared to interactions with close packed metal surfaces explains the observed high trapping probabilities and the lack of structural corrugation effects at high kinetic energies. Experimental results and simulations are in good agreement for a wide range of initial conditions, and we conclude that the model contains the most essential features of the scattering system. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.477689
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  • 2
    Electronic Resource
    Electronic Resource
    Classical trajectory study of argon–ice collision dynamics (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 110 (1999), S. 5380-5391 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Classical trajectory simulations have been used to study Ar–ice Ih collisional energy transfer, trapping coefficients and scattering distributions for initial Ar kinetic energies between 0.1 and 2.0 eV, incident angles between 0 and 70° and surface temperatures between 0 and 300 K. Collisional energy transfer is extremely efficient due to substantial transfer of energy from the Ar atom to the ice surface over typically 2–4 gas-surface encounters, and the rapid dissipation of this energy away from the collision center, preventing energy transfer back to the Ar atom. This leads to large trapping coefficients over this range of Ar collision energies, incident angles and surface temperatures. Scattered gas atoms lose most of their initial kinetic energy and have broad angular distributions. The large trapping coefficients obtained for the Ar–ice collisions are expected to be found for similar reactions under stratospheric conditions (e.g., HCl–ice, HOCl–ice and ClONO2–ice). © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.478433
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  • 3
    Electronic Resource
    Electronic Resource
    Scattering and trapping dynamics of gas-surface interactions: Vibrational excitation of CF3Br on graphite (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 10350-10360 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present results from molecular beam experiments and classical trajectory calculations of CF3Br scattering from graphite. Direct inelastic scattering dominates for initial translational energies Etr=0.6–3.5 eV and surface temperatures Ts=500–1170 K. An increase in the CF3Br vibrational temperature is observed in the scattered flux using the method of electron impact-induced fragmentation. The vibrational excitation depends on Etr and Ts, and a maximum vibrational temperature increase of 254±15 K is reached for Etr=3.5 eV and Ts=830 K. The vibrational excitation, angular distributions, and average translational energies are semi-quantitatively reproduced by classical trajectory calculations, indicating that the vibrational excitation can be explained by an electronically adiabatic "mechanical" process. The calculations suggest that a large fraction of the incident molecules experience multiple collisions with the surface. These transiently trapped molecules are slowly vibrationally excited while moving long distances, and are not thermalized even after 100 ps on the surface. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.477690
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  • 4
    Electronic Resource
    Electronic Resource
    Collision dynamics of large water clusters (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 5888-5897 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Classical trajectory calculations of (H2O)n+(H2O)n collisions are carried out for n=125 and n=1000. We investigate energy redistribution and fragmentation behavior for relative collision velocities up to 3000 ms−1, impact parameters up to 4 nm, and initial cluster temperatures of 160 and 300 K. Three main scattering channels are identified; coalescence, stretching separation, and shattering collisions. For small impact parameters, low collision velocities produce coalesced clusters while high velocities yield shattering behavior. Large impact parameters combined with high velocities result in stretching separation collisions. A decreased internal temperature influences the dynamics by increasing the stability of the collision complex. The results for (H2O)125 and (H2O)1000 are comparable, although the smaller size allows individual molecules to have a larger influence on the overall behavior. We find good agreement between the cluster simulations and experimental data for water drops in the micrometer range concerning the transition between coalescence and stretching separation, which shows that the clusters in some respects resemble "macroscopic" objects. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.475999
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  • 5
    Electronic Resource
    Electronic Resource
    Collision complex model of molecules scattering from corrugated surfaces (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 4239-4250 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A collision complex that gives almost quantitative agreement with a large set of data for inelastic scattering of atoms and molecules from surfaces is presented. In the model, a scattering molecule and a small part of the surface form a collision complex, that decomposes in a unimolecular fashion after statistical redistribution of energy. Both molecular translation and rotation are included in the model, and the surface is represented by a small number of harmonic oscillators. The surface is considered as locally flat at the place of impact, and surface corrugation is represented by a Gaussian distribution of local normal directions. Analytical solutions of simple integrals clearly illustrate the functional dependence on the principal parameters: translational energy, scattering angle, surface temperature, the relative size of the surface directly interacting with a scattering molecule, and the active degrees of freedom. Angular distributions for atoms, diatomic and polyatomic molecules scattering from metals, graphite and liquid surfaces are shown to be in good agreement with experimental results at thermal translational energies, and at least up to 0.5 eV. The model provides a simple and useful way to interpret and inter-relate experimental results, and makes it possible to evaluate the total information content in experimental data. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.469471
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  • 6
    Electronic Resource
    Electronic Resource
    Evaporation model of cluster scattering from surfaces (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 100 (1994), S. 3911-3924 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present classical trajectory calculations of ArnNem (n+m=111, 859) clusters scattering from a rigid surface. The dynamics of energy transfer and cluster decomposition during surface scattering is investigated for incident velocities of 100–700 m/s. The initial translational energy is at impact effectively transferred into internal degrees of freedom of the cluster. The overall energy transfer efficiency is very high but not complete, leaving too much energy in translation. No fragmentation takes place below 200 m/s. At incident velocities below 450 m/s, evaporation of small fragments from the heated cluster takes place in thermal equilibrium with the vibrational degrees of the cluster. This thermal evaporation is also the dominating ejection channel up to 700 m/s. Above 450 m/s, the formation of a compressed zone at impact opens up a new channel with ejection of fast fragments parallel to the surface plane. This effect becomes increasingly important at higher velocities. An evaporation model where fragmentation of the heated cluster takes place as isotropic and thermal ejection of small fragments is concluded to account for the major fragmentation processes observed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.466326
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  • 7
    Electronic Resource
    Electronic Resource
    Open source for excited species of alkali atoms and ions using diffusion through a thin metal foil (1994)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 65 (1994), S. 2034-2043 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A novel type of source for excited species of cesium is described. It employs an effusive flux to the back of a metal foil, with the excited-state emission from the other side of the covered foil. The foil is kept at a temperature of 1000–1900 K by direct heating with an electric current. From the foil, various excited species of ions and atoms, some of which are rather short lived, are emitted at large densities. Only the species observed at a distance of at least 40 mm from the source emitter are of interest here. Excited species are observed by both steady-state and pulsed field ionization, at current densities up to 10−4 A cm−2, which is several percent of the total beam flux. Positive and negative excited ions can also be observed in the flux from the source, by their ability to emit electrons with energy-independent efficiency at impact on surfaces. The corresponding flux densities are up to 10−5 and 10−4 A cm−2, respectively. Doubly excited atoms can be observed as Cs2+ after field ionization in a mass spectrometer. They also produce ions after energy transfer to small molecules.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1144809
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  • 8
    Electronic Resource
    Electronic Resource
    Source for excited states of alkali atoms and clusters using diffusion through a thin graphite foil (1992)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 63 (1992), S. 1966-1968 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A molecular beam source used for the production of excited and ionized clusters of Cs and other materials is described. This source is closed, with the emitting graphite foil as an integral part of the source enclosure. The foil is kept at 1100–1500 K by radiation heating. From the foil, excited clusters (Cs)*n with broad distributions of n, and excited Rydberg atoms Cs* are emitted. These highly excited states can be field ionized at field strengths of less than 400 V/cm. At a Cs reservoir temperature of 400 K, total field ionized flux densities up to 1015 ions cm−2 s−1 (3×10−4 A cm−2) are found.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1143312
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  • 9
    Electronic Resource
    Electronic Resource
    A Monte Carlo trajectory study of angular distributions in inelastic scattering of NO from graphite surfaces (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 86 (1987), S. 5825-5829 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A Monte Carlo classical trajectory study of NO/graphite surface scattering was carried out. NO was modeled as a rigid rotor interacting with a smooth vibrating surface through two Lennard-Jones 12-6 potential terms. The study simulated the experiments performed on the same system, first published by Frenckel et al. (1982). Angular distributions were calculated and found to generally reproduce the measured scattering lobes best at a simulated surface mass of 18 amu. Two potential well depths were used and at the larger of these all measured lobes could be quite well reproduced.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.452512
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  • 10
    Electronic Resource
    Electronic Resource
    A classical trajectory study of inelastic scattering of NO from graphite surfaces: Rotational energy distributions (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 6963-6971 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The inelastic scattering of NO molecules from graphite surfaces is studied by classical trajectory methods. The experimental results from Frenkel et al. (1982), Segner et al. (1983), and Häger and Walther (1984) are analyzed. A model using a small isolated part of the graphite surface in interaction with the NO molecule gives results in good agreement with experiment. The parameter values in the model are fixed at the values previously found to reproduce the angular distributions well [Nyman and Pettersson (1987)]. For this system, the experimental results give a "rotational cooling'' such that the rotational temperature of the inelastically scattered molecules becomes smaller than the surface temperature. This effect is reproduced accurately by the calculations, giving a rotational temperature of 250 K, independent of the surface temperature above 300 K. The main factor controlling this inelastic rotational cooling is the low initial value of the normal component of the total angular momentum. A "rotational rainbow'' structure is found in the calculations in many cases, primarily at high surface temperatures. The final energy distributions are shown to be mainly statistical by application of a unimolecular decomposition picture, similar to the common RRK type model used for gas phase reactions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.455322
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