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  • 1
    Electronic Resource
    Electronic Resource
    Hydrogen motion on a rigid Cu surface: The calculation of the site to site hopping rate by using flux–flux correlation functions (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 2083-2098 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We use the quantum flux–flux correlation function theory to calculate the rate coefficient for site-to-site hopping by a single hydrogen atom absorbed on a rigid Cu(100) surface. We investigate hydrogen dynamics during barrier crossing and determine the time scales on which the hydrogen atom crosses or recrosses the barrier, as well as the time scale on which double jumps occur. We define two kinds of transition state theory rate coefficients: one (Miller and Tromp) which assumes that only the short time dynamics contributes to the rate coefficient and another which includes the effect of the earliest recrossing. We examine numerically the accuracy of these approximations and compare them to other transition state theory calculations and to our "exact'' calculations. The simulations are also used to study the contribution of multiple jumps to the diffusion coefficient, to calculate the isotope effect on the rate coefficient and to determine the role of dimensionality in modeling surface diffusion. We find that the motion of the adsorbed atom perpendicular to the surface influences strongly the migration dynamics because the energy is very rapidly transferred back and forth between motion parallel and perpendicular to the surface. In particular this energy exchange process enhances the frequency of recrossing events and diminishes the frequency of the multiple jumps. We also make an extensive comparison between classical and quantum simulations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458585
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  • 2
    Electronic Resource
    Electronic Resource
    Multiphoton dissociation of a diatomic molecule: Laser intensity, frequency, and pulse shape dependence of the fragment momentum distribution (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 88 (1988), S. 5496-5505 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We study by an exact method the infrared multiphoton dissociation of a rotationless diatomic molecule and calculate the fragment relative momentum distribution as a function of the laser intensity and frequency, using either a square or smoothly varying pulse shape. The distribution has peaks due to multiphoton transitions. The nature of the peak structure depends on the laser intensity and whether the laser frequency is comparable to (i.e., within 20%) the ν=0 to ν=1 transition frequency (ω10) of the diatomic: If it is the distribution has bands spaced by the photon energy which contain peaks due to transitions from many bound states; if the laser frequency is not comparable to ω10, the distribution consists of isolated peaks spaced by the photon energy, which result from multiphoton transitions from the ground vibrational state. Changing the pulse shape from smoothly varying to square adds additional structure to the distributions. At sufficiently high intensities (1015 W/cm2) the high momentum peaks increase in intensity and the low momentum peaks are suppressed as the laser intensity is increased (this effect is often referred to as peak switching). At high laser intensities and frequencies comparable to ω10, classical mechanical calculations of the fragment momentum distribution give a smoothed out approximation to the quantum results and display a shifting similar to peak switching. Classical mechanics is unable to reproduce the quantum results at low intensities or at frequencies not comparable to ω10.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.454561
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  • 3
    Electronic Resource
    Electronic Resource
    Quantum simulation of hydrogen migration on Ni(100): The role of fluctuations, recrossing, and multiple jumps (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 3251-3267 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We examine the mobility of a hydrogen atom adsorbed on a Ni(100) in a temperature range (200–400 K) where the motion consists of jumps between lattice sites. We view these jumps as isomerization reactions and calculate their rate constants by using the flux–flux correlation function theory. We examine in detail the effect of lattice fluctuations and lattice distortion on the jumping rates and test the accuracy of several short time approximations which provide an extension of the transition state theory to quantum systems. We find that the magnitude of the diffusion coefficient is affected by multiple jumps and that recrossing effects are significant. By comparing the present quantum results to those obtained previously by classical simulations, we find that in this temperature range the quantum effects are small (i.e., at most a factor of 6) and originate mostly from the differences in the magnitudes of the thermodynamic quantities appearing in the rate coefficient expression. Numerical experiments show that it is possible to calculate the transition state rate coefficient in quantum systems by using a free particle approximation to calculate the short time evolution of the flux–flux correlation function. This approximation provides significant computer time savings and will permit calculations for quantum systems with a very large number of degrees of freedom.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459795
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  • 4
    Electronic Resource
    Electronic Resource
    A theoretical study of I2 vibrational motion after excitation with an ultrashort pulse (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 5693-5699 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We calculate the population created by a short pump pulse exciting the I2 molecule to the bound region of the B state, followed by excitation with a short probe pulse to the state E (or F). The nuclear state produced by the pump oscillates in the well of the B state and the probe is absorbed to populate the E (or F) state only when the wave function passes through the Franck–Condon region of the B→E (or B→F) transition. Because of this, the population on the E (or F) state oscillates with the delay time between the pump and the probe. The calculations agree with the experiment in the case when the probe excites the E state. When the F state is excited the theory predicts a doublet structure which is not observed; moreover, in some cases the experiment and theory differ at the shortest delay times. We discuss the dependence of the LIF signal on the pulse width and the initial state, the long time behavior of the LIF signal, and illustrate the role of the population transients on the B states at early times, during or immediately after the pump pulse acted on the molecule.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459563
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  • 5
    Electronic Resource
    Electronic Resource
    Adsorbate migration on a solid surface: The connection between hopping dynamics and the atom-surface interaction energy (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 2087-2098 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The migration of an adsorbed atom at moderate temperatures is described in terms of uncorrelated jumps between lattice sites which lead to diffusion. It is widely believed that a jumping rate coefficient and therefore a diffusion coefficient can be defined only if energy exchange with the moving lattice or collisions with randomly distributed impurities give the motion of the adsorbate a random character. In this paper we examine systematically a suggestion of Haug, Wanhstrom, and Metiu, who conjectured that coupling between the adsorbate motion along the surface and its motion perpendicular to it can provide the necessary randomization and, in particular, make possible the definition of a hopping rate coefficient. We calculate the flux–flux correlation functions needed for describing the dynamics of single and double jumps by using a set of simple, but reasonably realistic, adsorbate-surface interactions. In all these calculations the lattice atoms are held fixed. We show that in spite of this, the correlation functions converge and rate constants can be defined for many of the potentials. We study in detail those features of the potential energy surface (PES) that lead to convergence and also how the shape of the PES influences the amount of recrossing (i.e., the accuracy of the transition state theory) and multiple jumping. Our results indicate that it is possible to develop a correction to the transition state theory which includes the effect of thermal fluctuations and calculates the recrossing correction by holding the lattice atoms fixed. This saves substantial computer time.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459034
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  • 6
    Electronic Resource
    Electronic Resource
    CH3ONO predissociation by ultrashort laser pulses: Population transients and product state distribution (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 2317-2327 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We use a two-dimensional potential energy surface for CH3ONO to model the behavior of the molecule interacting with an ultrashort laser pulse. All the results presented here are obtained by solving accurately the time-dependent Schrödinger equation; when possible we also develop a simple model to explain the exact observations. We calculate the lifetimes of the predissociative resonances, the transient populations induced by semi-infinite pulses, and the time evolution of the product state distribution after excitation with a very short pulse. The behavior of the excited state population created by the pulse is explained in terms of the interference between the amplitudes representing photon absorption at different times when the pulse–molecule interaction is not zero. The time evolution of the final state distribution is explained in terms of interference between the wave packets at different times from the interaction region.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457973
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  • 7
    Electronic Resource
    Electronic Resource
    A time-dependent interpretation of the absorption spectrum of CH3ONO (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 1-13 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We use time-dependent quantum theory to interpret the absorption spectrum of CH3ONO in terms of the nuclear motion on the upper potential surface. The model uses one excited potential energy surface and two nuclear coordinates: the NO stretch and the CH3O–NO bond. The latter bond breaks upon excitation leading to dissociation. The spectrum consists of a broad band and two progressions corresponding to predissociation resonances. The band width is inverse proportional to the time scale on which the NO bond length increases to adjust to the longer equilibrium bond length of the upper potential energy surface. The progression of intense narrow resonances corresponds to the NO stretching motion. The other progression is due to oscillations along the reaction coordinate of the wave function temporarily trapped in the predissociation well. Our calculations show that important dynamic information can be obtained by "smearing off'' the high resolution spectrum to generate a series of low resolution versions which reveal the time scales on which various spectral features develop in the spectrum. We also show that time-dependent theory can be used efficiently to calculate the lifetime of relatively long lived resonances.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458463
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  • 8
    Electronic Resource
    Electronic Resource
    Erratum: Molecular dynamics simulations of energy flow at a solid surface: New methods using a smaller number of atoms [J. Chem. Phys. 90, 1229 (1989)] (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 1385-1385 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457686
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  • 9
    Electronic Resource
    Electronic Resource
    Surface damage caused by bombardment with low-energy (10–30 eV) argon (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 2735-2742 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We apply a recently developed combined molecular dynamics–local Langevin equation method to the simulation of the scattering of Ar by the (100) face of a face-centered cubic solid. The kinetic energies of the Ar are chosen to be low compared to the typical energies used in sputtering. We find that even at low energies, a significant amount of surface damage is inflicted by the Ar, leading to ejection of metal atoms into the gas phase, the formation of dislocations, and the production of isolated atoms trapped on the surface. We study both the probability that such events occur and individual trajectories which display the dynamic processes through which sputtering takes place or defects are created.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456983
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  • 10
    Electronic Resource
    Electronic Resource
    Molecular dynamics simulations of energy flow at a solid surface. New methods using a small number of atoms (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 90 (1989), S. 1229-1236 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present a number of new methods for the treatment of the dynamics of a strongly perturbed, finite sized, anharmonic system embedded in a structured, infinite system. All of the methods divide the full system in three classes of particles: Those that are treated by molecular dynamics; those that obey some form of stochastic molecular dynamics; and, those that are fixed. The methods are all simpler than the well-known generalized Langevin equation technique, and can be applied easily to much more complex processes than is possible for the GLE. A detailed illustration is provided of the energy flow through a solid lattice following a sudden disturbance of one surface atom. We compare the different methods and establish the size of the region of moving atoms needed to describe the lattice response over various time scales for the different dynamical methods.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456128
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