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Updated by Franck, April 22 2008 |
Dissociative hydrogen adsorption on palladium requires aggregates of three or more vacancies
The contrast is such that only the H vacancies are visible in the form of bright spots. The corresponding schematic diagrams show the H atoms and vacancies relative to the Pd(111) substrate. Letters A–D mark four vacancies near the centre in a.
In b, these vacancies have formed a ten-site 4V cluster, indicated by the triangle, with 6 H atoms rapidly moving inside.
In c the 4V cluster has been annihilated by adsorption of an H2 molecule, leaving two remaining vacancies. |
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Dissociative hydrogen adsorption on palladium requires aggregates of three or more vacancies
T. Mitsui, M.K. Rose, E. Fomin, D.F. Ogletree, and M. Salmeron
Nature 422, 705 (2003). LBNL-52007 |
STM images from a movie showing the formation and annihilation of a four-vacancy (4V) cluster. |
Hydrogen vacancies diffusion and cluster formation on Pd(111): Nature 422, 705 (2003)
In this movies, vacancies appear as bright bumps surrounded by dark rings, while the periodic background represents the 1x1 H adlayer.
Parameters: 31.0 K, 0.100 eV (A), 0.105 eV (B), 1.020 eV (C), 1.025 eV (D), 500 us timestep, total time 6 hours (120 x speed).
Monte Carlo simulations of hydrogen vacancy diffusion and aggregation in a 1x1 layer of hydrogen on Pd(111)
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Our model includes four energy barriers for vacancy diffusion between adjacent sites, one for each of the cases illustrated below. In the figure, we consider diffusion of the vacancy from left to right. The diffusion barrier depends on occupation of the sites marked '1' and '2'. Configurations (A) and (B) have relatively low barriers, which we estimate from the measured mobility of free H atoms and the rate of vacancy hopping within the 2 vacancy clusters. Configurations (C) and (D) have larger barriers that are estimated from the rates of diffusion for di-vacancy clusters and isolated vacancies, respectively. Using these energy parameters the simulation reproduces the observed stability of 2-V and 3-V ensembles confined to triangular regions. We find that this general behavior depends primarily on the occupation of site '1'. The occupation of site '2', in contrast, effects the rates but not the overall dynamics. The top site marked by an arrow in (A) is apparently effective at inhibiting nearby diffusion pathways even when site '2' is unoccupied. These observations strongly suggest that simple interactions dominate the striking dynamics of H vacancies. |
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