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  • American Institute of Physics (AIP)  (8)
  • 1
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 7828-7836 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Collisions between large neutral argon clusters and a pyrolytic graphite surface have been investigated at normal incidence by measuring angular distributions of density and angularly resolved time-of-flight distributions of scattered species (essentially monomers). These data have been taken for two surface temperatures (440 and 660 K) with cluster sizes N ranging from 400 to 8200 atoms per cluster, and at an incoming cluster energy of 72 meV/atom. A higher incident kinetic energy (115 meV/atom) has been investigated also for N=900 atoms per cluster. The experimental results have been analyzed by considering two components. First, an "ejection'' contribution, dominant at large scattering angles, has been assigned to the evaporation of monomers from cluster material having acquired a flow velocity parallel to the surface. The average value of this flow velocity increases sharply with the incident cluster velocity in the investigated range. Second, a thermal contribution, dominant close to the surface normal, has been assigned to the trapping-desorption of monomers on the graphite surface for the smallest values of N, and to direct evaporation from the cluster when N increases. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present a thermokinetic model together with new experimental results for the scattering of large argon clusters off a graphite surface. Both angular and time-of-flight distributions are shown for a large range of surface temperatures, incidence angles, and incident cluster sizes. A quantitative comparison between the proposed thermokinetic model and our measurements allows one to interpret most of the experimental results as due to thermal evaporation of very small fragments from their parent clusters gliding along the surface. The coefficient of tangential velocity conservation cF and the local temperature Tlocal of the evaporating fragments have been determined quantitatively. Although the investigated parameters were varied over a large range, Tlocal remains essentially constant around (140 ± 20) K. The coefficient cF turns out to be approximately (0.80 ± 0.05) independent of surface temperature and incident cluster size for all incidence angles larger than 40°. It increases, however, rapidly to 1.4 when the incidence angle is reduced to 20° suggesting a substantial transfer from normal to tangential kinetic energy. For high enough surface temperatures, incident cluster sizes, and incident angles, both experimental time-of-flight (TOF) and angular distributions show the appearance of a grazing exit angle component attributed to large cluster fragments leaving the surface before total evaporation.
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: We present a complete apparatus for molecule-surface interaction studies. Three UHV chambers are connected together allowing preparation, characterization, transfer, and experimental investigations with well-defined surface samples under ultrahigh-vacuum conditions. A chopped, supersonic molecular beam with well-controlled profile enters the main UHV chamber where it is scattered by the sample under study. Detection systems have been designed to measure angular and time-of-flight distributions and rotational populations of beam particles scattered off the surface sample giving access to the energy exchange between the internal degrees of freedom of the scattered molecules and the surface. The characteristics of our apparatus are reported and experimental tests for nitrogen and argon molecular beams scattered off a graphite surface are shown.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 8228-8234 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A simple four component lattice model is used to reproduce qualitatively phase behavior of nonaqueous microemulsions. In microemulsions where glycerol and propylene glycol replace water, 2(underbar)-3-2¯ phase transitions are observed as the glycerol content in the polar organic mixture increases. The model predicts such phase behavior and reproduces the effect of oil solvophobicity and the surfactant solvophilic–lipophilic balance. In addition, temperature insensitive phase behavior is observed in these nonaqueous mixtures and is predicted in the model. The results are used to propose possible molecular interaction mechanisms which would account for the microemulsion phase behavior. The model is compared with a similar model developed for water microemulsions and the differences are cast in light of the different molecular interactions which occur between propylene glycol, water, and nonionic surfactants.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 90 (1989), S. 2077-2089 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We report on our IR–IR double resonance experiment study of the excited stretching state of silane involving two or three vibrational quanta, and on the interpretation of our results by using the local mode model. The DR spectra were recorded by means of two separately tunable IR beams, one of which was used to pump selected lines inside the υ3 or 2υ3 IR bands, and the other (the probe) to observe transitions from the pumped vibration–rotation states. We characterized 7 out of the 14 vibrational states belonging to these manifolds for the first time, and the A1+F2 diad localized previously at 6362 cm−1 has been studied with much higher accuracy than before. Only the two highest states of the N=3 manifold are still unknown. The vibrational energies are very well reproduced by the local mode model ((approximately-equal-to)1 cm−1 rms deviation), and the harmonic interbond couplings have been determined more precisely than before. The rotational structures of the excited states are also well accounted for by Champion's theory of vibration–rotation coupling in Td, adapted to the description of stretching vibrations in terms of local modes. More high resolution (0.01 cm−1 or better) data are needed however to fully assess this point.
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  • 6
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present experimental results on the translational and rotational energy transfers due to the collisions of large (N2)n clusters with a highly oriented pyrolytic graphite sample at 750 ms−1 incident velocity, for two surface temperatures Ts (440 and 580 K), for average cluster sizes n from 200 to 1000 monomers, and for incidence angles θi from 30° to 70°. Angular distributions of flux and angularly resolved time-of-flight profiles of scattered particles are measured with a quadrupole mass spectrometer while rotational state distributions are determined by resonantly enhanced multiphoton ionization. For all incidences but the largest (70°) the translational distributions of scattered molecules recorded at detection angles θd from 20° to 70° are compatible with a simple thermokinetic model assuming that monomer evaporation occurs at a well-defined temperature Tloc from the cluster gliding on the solid surface. Fits of the translational data yield Tloc values between 250 and 500 K, increasing with Ts and decreasing with n and θi. On the other hand, the rotational distributions are always non-Boltzmann. Actually, they are well described as sums of two contributions, at 75 and 375 K, respectively. The relative weight of the cold contribution is found to increase with θd, with a slope increasing with θi. Possible interpretations of the observed differences between translational and rotational degrees of freedom of evaporated molecules are discussed. © 1999 American Institute of Physics.
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  • 7
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 110 (1999), S. 2579-2587 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present experimental results on the collision of large binary noble gas clusters with a hot graphite surface at an incident velocity of 430 ms−1. The mixed ArnXm clusters (X=Kr, Xe) with average sizes n+m between 1000 and 13 000 are obtained from pure argon clusters by the pickup technique. The surface scattering dynamics belongs to the thermal evaporation regime and large surviving binary fragments are detected at grazing angles. As expected from simple binding energy considerations, in all cases the surviving clusters are richer in the dopant species X. This enrichment has been measured as a function of incidence angle, incident cluster size, and dopant molar fraction x. For the lowest values of x (about 0.7%), the measured enrichment is the same for Kr and Xe, and decreases slowly with increasing size and incidence angle. On the other hand, when x is raised up to 15%, this enrichment decreases substantially for Xe, and much less for Kr. This unexpected behavior clearly shows that collision induced evaporation cannot be viewed as a mere distillation of an homogeneous binary mixture. The possible role of the incoming cluster structure is discussed in connection with the preparation of the binary clusters by pickup. © 1999 American Institute of Physics.
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  • 8
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A semiempirical dynamic zone structure model is presented to explain the behavior of the grazing exit angle component previously observed in experiment and simulation during the scattering of large van der Waals clusters from surfaces. The proposed model that is based on a simple energy balance and the Leidenfrost phenomenon does not only qualitatively reproduce the measured importance of the grazing exit angle component as a function of incident cluster size, incident velocity, angle of incidence, and surface temperature, but it also gives an order-of-magnitude estimate for the size of the large fragments scattered in this component and for the involved picosecond interaction times. Recent results obtained from both trajectory calculations and experiments show very good agreement with the predictions concluded from the proposed model. © 1995 American Institute of Physics.
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