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  • 1
    Electronic Resource
    Electronic Resource
    Dynamics of precursor-mediated chemisorption (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 8428-8440 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have developed theoretical models of precursor-mediated (nondissociative) molecular chemisorption and used the stochastic classical trajectory method to simulate experiments capable of determining whether a precursor is present. The simulations employ empirical many-dimensional potentials and include the full effects of surface vibrations, coupling among molecular degrees of freedom, and coupling between the molecule and surface. We find that coupling between molecular rotational and translational modes strongly affects the experimentally observable quantities. As a result, reasoning based on the usual one-dimensional picture of a precursor is unreliable. The most notable effect of a precursor is the strong rotational polarization it induces in desorbing molecules. We discuss the origins of rotational polarization and conclude that with no precursor, polarization will be weak and opposite in sign to that of the precursor case. The sticking probability as a function of incident energy and surface temperature can also distinguish whether precursor mediation is important but, again, inferences from one-dimensional models can be misleading. We also find that equilibrium diffusion rates for precursor molecules are faster than for chemisorbed molecules. However, the equilibrium process is relatively slow and until it is complete, chemisorbed molecules may well move farther across the surface than precursors.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.460076
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  • 2
    Electronic Resource
    Electronic Resource
    Washboard model of gas–surface scattering (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 680-686 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The conventional "hard cube'' and "soft cube'' models of gas–surface scattering are extended to incorporate the effects of surface corrugation. The proposed "washboard'' model invokes the hard cube assumption of conservation of tangential momentum, but with respect to the local surface tangent at the point of impact of the molecule with the corrugated surface. Expressions are derived in the form of convenient single variable quadratures for the angular scattering distribution, the mean velocity and kinetic energy as a function of scattering angle, and the trapping probability. The model is applied to the scattering of argon atoms from smooth and corrugated faces of platinum. Results are compared to those from multidimensional stochastic trajectory simulations employing a realistic interaction potential and moving surface atoms. The washboard model is shown to have a far wider range of validity than the cube models, and to describe properly the transition from singly peaked near-specular scattering from smooth surfaces to doubly peaked "rainbow'' scattering from highly corrugated surfaces.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458421
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  • 3
    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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  • 4
    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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  • 5
    Electronic Resource
    Electronic Resource
    The coverage dependence of the sticking probability of Ar on Ru(001) (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 9266-9276 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The coverage dependence of the sticking probability of argon on a cold Ru(001) surface is studied experimentally by thermal-desorption spectrometry, and simulated by classical molecular dynamics with an empirical pairwise-additive potential-energy function. Experimentally, a dramatic linear increase in sticking as a function of Ar coverage is observed between 0 and 0.7 monolayer for a 300 K thermal beam of Ar incident normal to the surface; at higher coverages the sticking probability gradually saturates to approach unity beyond 2 monolayers. The linear regime is reproduced with near-quantitative accuracy by the simulations, with a simple perfect islanding model. The origin of the enhanced sticking lies primarily in efficient energy transfer to the adsorbate, due to perfect mass matching and the deformability of the overlayer. In the simulations at incident energies above 50 kJ mol−1 the sticking on the monolayer diminishes to almost zero, and collision-induced desorption is observed. Additionally, several interesting dynamical effects which have experimental relevance emerge from the simulations, such as prolonged impacts and sticking mediated by attractive interactions at the edge of adsorbate islands.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.461207
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  • 6
    Electronic Resource
    Electronic Resource
    On the nature of trapping and desorption at high surface temperatures. Theory and experiments for the Ar–Pt(111) system (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 1516-1527 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We report molecular dynamics calculations and molecular beam experiments on trapping and desorption as a function of surface temperature and initial gas conditions for the Ar–Pt(111) system. The trapping process involves very rapid equilibration of the normal component of incidence velocity but extremely slow accommodation of the parallel component. At high surface temperatures for which the residence time of Ar is sufficiently short (e.g., roughly 40 ps at 273 K), trapped atoms desorb before their incident parallel velocity is thermalized. Thus trapping in the usual sense of complete equilibration with the surface does not occur; instead these quasitrapped atoms are characterized by full accommodation of only the normal velocity component. In both simulations and experiments there is a range of temperatures for which the desorbing flux associated with quasitrapping is distinguishable from the flux due to direct inelastic scattering. These results are qualitatively reproduced by a simple model for the time-varying velocity distribution of quasitrapped atoms, which treats the different components of velocity independently.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.460695
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  • 7
    Electronic Resource
    Electronic Resource
    Molecular dynamics with electronic transitions (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 1061-1071 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A method is proposed for carrying out molecular dynamics simulations of processes that involve electronic transitions. The time dependent electronic Schrödinger equation is solved self-consistently with the classical mechanical equations of motion of the atoms. At each integration time step a decision is made whether to switch electronic states, according to probabilistic "fewest switches'' algorithm. If a switch occurs, the component of velocity in the direction of the nonadiabatic coupling vector is adjusted to conserve energy. The procedure allows electronic transitions to occur anywhere among any number of coupled states, governed by the quantum mechanical probabilities. The method is tested against accurate quantal calculations for three one-dimensional, two-state models, two of which have been specifically designed to challenge any such mixed classical–quantal dynamical theory. Although there are some discrepancies, initial indications are encouraging. The model should be applicable to a wide variety of gas-phase and condensed-phase phenomena occurring even down to thermal energies.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459170
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  • 8
    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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  • 9
    Electronic Resource
    Electronic Resource
    Vibrational relaxation on metal surfaces: Molecular-orbital theory and application to CO/Cu(100) (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 3939-3949 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A nonempirical theory of vibrational relaxation at metal surfaces via nonadiabatic coupling to conduction electrons is presented. Using a single determinant Hartree–Fock (HF) description of the electronic states of the system, an expression for the lifetime of an excited vibration is obtained. Under certain additional assumptions, all the quantities necessary to calculate the lifetime can be obtained from the results of ab initio HF calculations on cluster models of the adsorbate-metal system. As a practical test of this procedure, the lifetime of the excited v=1 vibrational state of CO on Cu(100) is calculated using clusters of 6, 10, and 14 copper atoms. Results ranging between 1.1 and 3.5 ps are obtained, with our preferred procedure yielding 1.7 ps for the largest cluster, in good agreement with experiment. Extensions of this approach may also be valuable for treating other nonadiabatic phenomena at metal surfaces.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.461896
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  • 10
    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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