Jump processes in surface diffusion - Bilkent University - Faculty of ...

58 G. Antczak, G. Ehrlich / Surface Science Reports 62 (2007) 39–61
Fig. 60. Schematic of different types of jump processes for an adatom on
W(211) [126]. β R denotes rebound transition.
Fig. 58. Normal Arrhenius plot for β double jumps of W adatom on
W(211) [126].
Fig. 61. Rate of rebound jumps, obtained as the difference between the
sum of single plus double jumps and total jump rate extrapolated from low
temperatures [129].
Fig. 59. Arrhenius plot for the sum of all jump rates measured for W adatom
on W(211). For temperatures ≥310 K, rate falls below linear plot [126].
such a rebound transition, and it is therefore not detected in
the normal diffusion measurements, but rebounds were found in
molecular dynamics simulations in 1989 by DeLorenzi [127],
and a few years later by Sanders and DePristo [128] and Ferrón
et al. [113].
A way to measure such rebounds has recently been
discovered. Antczak [129] has pointed out that the sum of the
measured jump rates has to be subtracted from the straightline
Arrhenius plot obtained by extrapolating the sum from
low temperatures. The rebound rate so obtained is shown
in Fig. 61; it has an activation energy of 1.03 ± 0.06 eV
and a frequency prefactor of 1.40(×10.3 ±1 ) × 10 16 s −1 , and
thus lies midway between single and double jumps. What
is surprising is that rebounds were not detected in twodimensional
diffusion on W(110) at all, suggesting it may be an
effect tied to the channelled structure of W(211) that is involved
in these transitions. However, they have been seen previously in
simulations of self-diffusion on the Cu(111) plane [113].
It should be noted that long jumps are not limited to the
movement of single atoms. As is apparent from Fig. 62,
long jumps were observed by Wang [130,131] for clusters of
iridium atoms on Ir(111), and for the large organic molecules
decacyclene and hexa-tert-butyl-decacyclene by Schunack
et al. [132]. However, information about the kinetics of these
processes is limited and crying for more work.
Finally, it should be said that theoretical predictions about
long jumps have been very important in pointing to their
contributions in diffusion. However, even the limited number
of experiments done so far have already revealed that these
transitions are rather more varied and more significant than
predicted, especially at low temperatures.
4. Summary
This survey should make it clear that a whole variety of
different jumps contributes to surface diffusion on metals. Atom
exchange processes are of course very much affected by the
chemistry of the interacting partners, but have so far only
been observed in experiments with fcc metals. On channelled
surfaces the direction favoured in diffusion is still puzzling.