Step interactions on Pt (111) vicinal surfaces determined by grazing incidence X-ray diffraction: Influence of the step orientation (original) (raw)
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By step fluctuation spectroscopy, using low-energy electron microscopy ͑LEEM͒, we investigate step energies and relaxation on clean Pt͑111͒ and Pd͑111͒ surfaces at temperatures above half the melting temperature T m. Some effort has been expended to develop accurate procedures for analyzing fluctuations observed as video recordings. The average step stiffnesses are about 210 meV/ nm and 265 meV/ nm for Pt and Pd, weakly temperature dependent, and in each case fairly isotropic with mainly a sixfold angular variation. Consequently, the step free energies are highly isotropic. At the lower temperatures, the relaxation rates of fluctuations with wave vector, q, vary as q 3. This is the unambiguous signature of step relaxation by surface diffusion over the terraces. It affords accurate determinations of the surface mass diffusion coefficients D s =5͑ϫ2 ±1 ͒ ϫ 10 −4 exp͑−1.2± 0.1 eV/ k B T͒ cm 2 / s for Pt͑111͒ and D s =3͑ϫ2.5 ±1 ͒ ϫ 10 −3 exp͑−1.1 5 ± 0.1 5 eV/ k B T͒ cm 2 /s for Pd͑111͒. At more elevated temperatures the measured rates vary approximately as q 2 in both cases. This corresponds to the surface process being short-circuited by a faster flow of bulk vacancies. Known bulk diffusion coefficients for Pt and Pd are consistent with this interpretation. An effective procedure is developed to separate bulk and surface contributions. There is the appearance of universality in the fluctuation processes, which approximates as an homologous dependence on T / T m. It is observed for Pt͑111͒ at 1400 K and above that neighboring steps react to form multisteps that retain capillary characteristics. The stiffnesses of multisteps formed from up to five associated steps have been determined by fluctuation spectroscopy and are employed to discuss the energetics of multistep formation. Clear evidence is found that the multistep free energy contains important contributions from internal degrees of freedom. The kinetics of multistep fluctuations are explained by the same diffusion coefficient D s determined from single steps.
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Physical Review B, 1989
Angular distributions and surface-phonon dispersion curves have been measured by He-atom scattering for clean Pt between T = 160 and 373 K and for Pt-H(l X 1) at 160 K along the (110) and (112) azimuths. Inelastic-scattering intensities over a wide range of wave vectors are also reported for the clean surface. The results are analyzed with use of the distorted-wave Born approximation for a model potential describing the He-atom-Pt-surface potential. A set of 14 force constants adjusted to the bulk-phonon dispersion data cannot explain the measured time-of-Aight spectra and the observed first-layer relaxation. However, by modifying at the surface the nearest-neighbor tangential force constants (aI'= 1.7ct"and a', =1. 3a,), the radial force constants (P|'=0. 39P,), and the three-body force constant (yI'=0. 037@,)where the superscripts~i and J. denote forces within the surface plane and between the first and second planes, respectivelya good fit to all the data is achieved. The new surface force constants are discussed in terms of electronic-charge redistribution within the first layer.
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