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SUMMARY:The use of H atom band structures in the first principles determin
 ation of H atom surface diffusion - John Ellis\, SMF Group\, Cavendish Lab
 oratory
DTSTART:20121108T160000Z
DTEND:20121108T170000Z
UID:TALK39107@talks.cam.ac.uk
CONTACT:Stephen Walley
DESCRIPTION:First principles calculations of adatom potential energy surfa
 ces using density functional theory are being performed with ever more acc
 uracy with the aim of achieving ‘chemical accuracy’ in the prediction 
 of rates of surface processes. For activated diffusion in the quantum case
 \, relatively little attention has been given as to how to actually use th
 ese potential energy landscapes to predict diffusion rates with correspond
 ing accuracy: typically the rate for activated diffusion is given by the p
 roduct of a Boltzmann factor and the barrier energy\, and the classical pr
 e-exponential value kBT/h (see for example [1]). The advent of helium spin
 -echo (HeSE) quasielastic scattering experiments makes possible measuremen
 ts on atomic length scales\, guaranteeing that terrace\, as opposed to ove
 r-step-edge\, diffusion is being measured. The measurements are at a suffi
 ciently short timescale (typically 1ps to 1ns) to require little extrapola
 tion in the evaluation of the pre-exponential factor and so deliver diffus
 ion measurements in equilibrium situations of unparalleled accuracy and re
 liability. Calculations of the momentum dependence of hydrogen atom energy
  states in the atom-surface potential (atomic band structures) have been u
 sed for some time in the interpretation of the vibrational spectra of surf
 ace adsorbed hydrogen [2]. Here we extend this concept and use these band 
 structures to determine the rate of hydrogen diffusion with a formalism th
 at does not make the arbitrary distinction between ‘over the barrier’ 
 and tunnelling states. We compare HeSE data and published field emission f
 luctuation data with predictions for the Cu(111)\, Pt(111)\, Ru(001) and N
 i(111). The high temperature\, activated diffusion is predicted with remar
 kable accuracy\, but at low temperatures significant discrepancies are fou
 nd due to the transition to incoherent atom wave propagation. I show\, how
 ever\, how the bandstructure formalism gives a good framework in which to 
 discuss and include calculations of such effects.\n\n[1] P.G. Sundell and 
 G. Wahnström\, Phys. Rev. B 70\, 081303 (2004)\n[2] M.J. Puska and R.M. N
 ieminen\, Surf. Sci. 157\, 413 (1985)\n[3] T.-S. Lin and R. Gomer\, Surf. 
 Sci. 255 41 (1991)\n\n
LOCATION:Mott Seminar Room\, Cavendish Laboratory\, Department of Physics
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