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  • 1
    Electronic Resource
    Electronic Resource
    Direct inelastic scattering of N2 from Ag(111). IV. Scattering from high temperature surface (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 5793-5801 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have measured the rotational state distribution and the angular momentum alignment and orientation of N2 scattered from Ag(111) at 540 K. Using resonance enhanced multiphoton ionization (REMPI), we are able to probe the scattered flux as a function of the exit angle θexit. For a modestly glancing incident beam (θi =30°) and incident translational energy, Ei =0.3 eV, the angular momentum alignment (tumbling vs helicoptering) at both quasispecular detection (θexit=35°) and superspecular detection (θexit=50°) is only weakly dependent upon the surface temperature. However, the angular momentum orientation (clockwise vs counterclockwise rotation) is strongly affected by the surface temperature. Raising the surface temperature from Ts =90 K to Ts =540 K causes the orientation to decrease substantially. Stochastic trajectory calculations were carried out in conjunction with the experiments. They reveal that at low temperature there is an averaging over two important initial conditions: the two-dimensional impact parameter and the molecular orientation geometry. At high temperature there is also an averaging over the instantaneous positions and momenta of the surface atoms. Hence, a given two-dimensional impact parameter and molecular orientation geometry results in a greater range of final J states, angular momentum polarizations, and velocities (exit angles) at high temperature than at low temperature. The resulting "smearing'' accounts for the changes in rotational state distribution and polarization as a function of exit angle observed at high temperature. The major effect of averaging over the positions of the surface atoms (thermal roughening) upon the orientation of the scattered N2 is to increase the exit angle averaging rather than to increase the in-plane forces.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457532
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  • 2
    Electronic Resource
    Electronic Resource
    Direct inelastic scattering of N2 from Ag(111). III. Normal incident N2 (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 6947-6955 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have probed the angular momentum orientation of N2 scattered from cold Ag(111) when the N2 approaches the surface along the surface normal. Using resonance enhanced multiphoton ionization (REMPI) and pulsed molecular beam techniques, we are able to probe the flux backscattered along the surface normal. In accordance with the restrictions on cylindrically symmetric systems, the molecules backscattered along the surface normal have no angular momentum orientation nor does the entire scattered flux integrated over all exit angles. However, for detection away from the surface normal, we observe substantial angular momentum orientation; the degree and direction of orientation depends upon the rotational state being probed. Molecular dynamics calculations reproduce the experimental results semiquantitatively. The calculations show that for N2 incident along the surface normal, the exit angle is largely determined by the two-dimensional impact parameter of the molecule within the crystal unit mesh. However, the final rotational state, orientation, and alignment are determined largely by the molecular orientation geometry of the N2 during the collision. In essence, we have found a dynamical process which can partially differentiate between the two hidden initial conditions in a gas–surface collision: the two-dimensaional impact parameter and the molecular orientation geometry.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.455320
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  • 3
    Electronic Resource
    Electronic Resource
    Dynamics of cluster scattering from surfaces (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 90 (1989), S. 3831-3837 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have carried out stochastic trajectory simulations of the scattering of argon clusters, Arn, n=5 to 26, from a Pt(111) surface. At incident energies of 0.1 and 0.5 eV per atom, the clusters fragment almost completely into individual atoms. Some atoms remain trapped on the surface, but the majority scatter. For the larger clusters, the angular distributions of the scattered atoms peak near the surface tangent, and are almost independent of the incident angle. The mean kinetic energy of atoms is largest for those scattered near the surface tangent, and decreases as the scattering angle approaches the surface normal. These trends are in qualitative accord with the results of experimental studies of cluster scattering from surfaces. A multiple binary collision mechanism involving atom–atom collisions within the cluster as well as atom–surface collisions is responsible for this behavior.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456662
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  • 4
    Electronic Resource
    Electronic Resource
    Studies of chemical reactivity in the condensed phase. IV. Density dependent molecular dynamics simulations of vibrational relaxation in simple liquids (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 6687-6696 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Molecular dynamics simulations of the photodissociation/recombination process for iodine in liquid xenon at several densities are reported in this paper. These simulations were performed to aid in the understanding and interpretation of recent picosecond experimental investigations on model chemical reaction systems. From these calculations, it was found that geminate recombination occurs primarily within a few picoseconds at all densities considered. This is in agreement with previous molecular dynamics simulations with significantly smaller systems, and with the current interpretation of experimental results. Simulated iodine ground electronic state vibrational relaxation times range from about 1 ns at the lowest density to approximately 250 ps at the highest density reported here. In addition, the functional form of the decay of the average iodine vibrational energy was observed to be nearly independent of density. This result is discussed in terms of simple gas phase isolated binary collision models. Various force correlation functions projected onto the iodine vibrational coordinate were also examined, and indicate that the iodine molecule significantly perturbs the local solvent environment. These force correlation functions may be helpful when assessing the usefulness of liquid phase theories of vibrational relaxation of highly excited molecules. Finally, the simulation results on iodine vibrational relaxation are compared with the available experimental data. These comparisons indicate that the molecular dynamics calculations overestimate the rate of vibrational relaxation over the lower third of the iodine ground electronic state potential surface, and that the efficiency of V–TR transfer, relative to V–V transfer, may have been underestimated. The sensitivity of these results to several system parameters are analyzed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.455341
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  • 5
    Electronic Resource
    Electronic Resource
    Direct inelastic scattering of N2 from Ag(111). II. Orientation (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 2572-2582 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The orientation of the angular momentum of N2 scattered from clean Ag(111) is determined by resonance-enhanced multiphoton ionization. The orientation is the net helicity or handedness of the sense of rotation, i.e., clockwise vs counterclockwise. The orientation of the scattered N2 is measured along a direction perpendicular to the scattering plane. The degree and sign of the orientation is found to depend strongly on the final rotational quantum number J and on the final scattering angle. The results require that there are forces acting in the plane of the surface during the scattering. The observed behavior can be reproduced qualitatively by a conventional hard-cube, hard-ellipsoid model to which a tangential friction has been introduced to account for the in-plane forces. This produces a splitting of the rotational rainbow peak which leads to changes of sign of the orientation as a function of rotational quantum number. Thus, orientation measurements provide a unique probe of in-plane gas–surface forces.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.455052
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  • 6
    Electronic Resource
    Electronic Resource
    Stochastic trajectory studies of small argon cluster scattering from Pt(111) (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 88 (1988), S. 3376-3384 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have carried out stochastic trajectory calculations of the scattering of small argon clusters, n=1,2,3, and 5, from a Pt(111) surface at incident energies of 0.1 and 0.5 eV per atom. We employed a 6 by 6 by 2 layer slab of platinum atoms with periodic boundary conditions imposed in the x and y (surface plane) directions. We applied local friction and white random forces in the z (surface normal) direction to the bottom surface layer to maintain the proper temperature and to account for energy transfer with the bulk. We assumed Lennard-Jones interactions for Ar–Ar and Ar–Pt with realistic parameters. We have found that the scattering of small clusters from surfaces exhibits a very different behavior from the scattering of individual atoms or molecules. At the collision energies considered, most clusters fragment into atoms upon impact with the surface, but a surprising number survive either partially or totally intact. Angular distributions of the fragmented monomers are much broader than those of surviving clusters. The average energy of the fragmented monomers increases with scattering angle, the reverse of the "hard-cube'' trend for atoms scattering from surfaces. In addition, cluster scattering is associated with an enhanced trapping probability and enhanced initial lateral mobility of the trapped species in comparison to individual atom scattering. A sequential binary collision model invoking gas–surface and gas–gas collisions is suggested to account for these results.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.453933
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  • 7
    Electronic Resource
    Electronic Resource
    Precursor-mediated adsorption and desorption: a theoretical analysis (1988)
    s.l. : American Chemical Society
    Langmuir 4 (1988), S. 256-268 
    Details
    ISSN: 1520-5827
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/la00080a004
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  • 8
    Electronic Resource
    Electronic Resource
    Trajectory studies of hyperthermal Xe scattering from GaAs(110) (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 87 (1987), S. 1808-1816 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present the results of stochastic classical trajectory simulations of the scattering of a nonreactive gas, xenon, from a semiconductor surface of known structure and electronic and vibrational properties, GaAs(110). The range of incident energies considered is 1 to 8 eV, in order to make direct comparison with results of molecular beam experiments reported in the accompanying paper. We employed a 48-atom three-layer slab of GaAs, with periodic boundary conditions in two dimensions and frictional and stochastic forces in the third (surface normal) dimension. Pairwise additive Lennard-Jones potentials describe the gas–surface interaction. The calculations reproduce the large energy exchange and surprising structural sensitivity observed experimentally. Energy transfer is dominated by an initial binary interaction of the Xe with a single Ga or As atom. The repetitive collision nature of this binary encounter produces angular scattering patterns very similar to those obtained for scattering of light atoms, including "rainbow maxima.'' The mechanism of energy deposition may have implications to "gentle'' sputtering by neutral atom bombardment and to collision induced excitation of electron–hole pairs.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.453193
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  • 9
    Electronic Resource
    Electronic Resource
    Dynamics of gas–surface interactions: Scattering and desorption of NO from Ag(111) and Pt(111) (1985)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 83 (1985), S. 2594-2606 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Empirical potential energy surfaces have been constructed to describe the nondissociative interaction of NO with the (111) faces of Ag and Pt. Stochastic trajectory simulations employing these interaction potentials accurately reproduce experimental angular and velocity scattering distributions. Measured rotational energy distributions of scattered molecules, including the observed "rotational rainbow'' features, are also reproduced quantitatively. Arrhenius prefactors for desorption are computed to be large (1016 s−1), and the translational and rotational "temperatures'' of desorbed molecules are found to be lower than the surface temperature, in agreement with experiment. Sticking probabilities, desorption rates, and the rotational energy of desorbed and scattered molecules are all found to be strongly influenced by the dependence of the attractive region of the gas-surface potential on molecular orientation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.449253
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  • 10
    Electronic Resource
    Electronic Resource
    Molecular dynamics of nonequilibrium infrequent events: Laser-induced desorption from surfaces (1986)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 85 (1986), S. 7423-7433 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: An obvious shortcoming of classical trajectory simulations of chemical reactions is their limitation to short time scale processes. While procedures to extend simulations to long time scales have been developed for reactions occurring under equilibrium conditions, there are no analogous techniques for nonequilibrium situations. We propose such a method which, although not rigorous, is simple, practical, and appears to give results of useful accuracy. We illustrate the method by simulating infrared laser-induced desorption of an atom from a solid surface. At vanishingly low laser powers, desorption rate constants agree with those calculated by standard equilibrium infrequent event methods. They also agree at high laser powers with the values obtained from direct simulation in the fast time scale limit. At intermediate powers they are in accord with the results of a master-equation treatment described in the Appendix. We have examined the dependence of laser-induced desorption on the laser frequency and intensity and on the surface temperature. Enhanced desorption is observed at relatively low intensities, and a nonthermal selective desorption mechanism is indicated.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.451331
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