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Diffusion of hydrogen atoms on a Si(111)-(7×7) reconstructed surface: Monte Carlo variational phase-space theory (1994)

Sorescu, Dan C. ; Thompson, Donald L. ; Raff, Lionel M.

College Park, Md. : American Institute of Physics (AIP)

The Journal of Chemical Physics 101 (1994), S. 1638-1647

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ISSN: 1089-7690

Source: AIP Digital Archive

Topics: Physics , Chemistry and Pharmacology

Notes: The diffusion of hydrogen atoms on a reconstructed Si(111)-(7×7) surface has been investigated using variational phase-space theory methods. The dimer–adatom-stacking (DAS) fault model of the reconstructed Si(111)-(7×7) surface proposed by Takayanagi et al. is employed to describe a four-layer lattice structure containing 292 atoms. The lattice potential is that developed by Bolding and Andersen; the gas–lattice interaction potential is described by a sum of Morse functions and bending terms between the hydrogen adatom and the Si atoms in the first and second layers. Canonical Markov walks with importance sampling are used to evaluate the flux across a set of dividing surfaces separating different adsorption sites. The minimum jump frequencies are then used as input to a set of coupled phenomenological kinetics equations that describe the diffusion rates of adatoms between adjacent adsorption sites. The diffusion coefficients D at different temperatures are computed from the slope of plots of the time variation of the root-mean-square displacements obtained from the solution of the rate equations. The results at 300, 500, and 800 K yield D=0.023 exp(−1.54 eV/kT) cm2/s. The calculated activation energy of 1.54 eV is in excellent agreement with the experimental results obtained by Reider et al. using an optical second-harmonic diffraction technique. The coordinates corresponding to the minimum energy diffusion path suggest that hydrogen-atom diffusion between atop sites occurs along paths that involve lattice penetration. Calculated upper limits for the tunneling rates at 300, 500, and 800 K show that tunneling processes make only a small contribution to the total diffusion rate.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1063/1.467785

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Statistical effects in the skeletal inversion of bicyclo(2.1.0) pentane (1994)

Sorescu, Dan C. ; Thompson, Donald L. ; Raff, Lionel M.

College Park, Md. : American Institute of Physics (AIP)

The Journal of Chemical Physics 101 (1994), S. 3729-3741

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ISSN: 1089-7690

Source: AIP Digital Archive

Topics: Physics , Chemistry and Pharmacology

Notes: A semiempirical potential-energy surface for bicyclo(2.1.0) pentane which includes bond stretching, bending, and torsional terms is reported. The bond dissociation energies have been estimated using the available thermochemical data and results of ab initio molecular orbital calculations performed at the fourth order Møller–Plesset (MP4) perturbation theory level using a 6-31G** basis set. The predicted equilibrium geometry of bicyclo(2.1.0) pentane and of the 1,3-cyclopentanediyl radical, the barrier for the ring inversion, and the fundamental frequencies of bicyclo(2.1.0) pentane are in fair-to-good agreement with the measured and ab initio calculated values. Using a projection method of the instantaneous Cartesian velocities onto the normal mode vectors and classical trajectory calculations, the skeletal inversion and the intramolecular energy flow in bicyclo(2.1.0) pentane are studied for different types of excitation. For random energization of the vibrational modes, the results of trajectory calculations agree with the predictions of statistical unimolecular theory. The same statistical behavior is supported by the results of power spectra calculated at different energization levels. The significant broadening and overlapping of the spectral bands, together with the disappearance of characteristic spectral features in the power spectra of the flap angle, indicate high intramolecular vibrational redistribution rates and global statistical behavior. The total intramolecular vibrational relaxation rates for the energy flow from the flap mode have been extracted from the time dependence of the average total normal-mode energy in this mode. For initial excitation of the flap mode in the range 30–60 kcal/mol, the calculated total intramolecular vibrational relaxation rates are found to be significantly larger than the microcanonical ring inversion rates. This result further supports the statistical character of the ring inversion in bicyclo(2.1.0) pentane.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1063/1.467557

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