Sticking Probabilities for CO Adsorption on Pt(111) Surfaces Revisited
J. Phys. Chem., (1995)DOI: 10.1021/j100016a062
Abstract
The coverage dependence of the sticking probaiblity for carbon monoxide
on R( 1 11) surfaces was investigated
by using the dynamic method originally devised by King and Wells.
The CO uptake was studied as a function
of CO beam flux, surface-to-doser distance, and surface temperature.
The sticking probability on the clean
surface was found to be quite high in all cases, about 0.8, and to
remain approximately constant at low
temperatures up to coverages close to 0.50 monolayers (ML), at which
point a ~ ( 4 x 2o)r dered layer forms
on the surface. This behavior is explained by a model originally proposed
by Kisliuk where the molecules
adsorb on a highly mobile extrinsic precursor state before migrating
to their final chemisorbed state. Above
0.50 ML the sticking probability then drops suddenly, presumably because
the chemisorption energy drops
as compressed CO layers start to form on the surface. The saturation
coverage varies at low temperatures
with CO beam flux because of the induced changes in the adsorption-desorption
dynamics at the high
coverages by the changes in CO impinging rates. The effect of inhomogeneities
in the spatial distribution of
the CO beam across the surface was studied by changing the distance
between the sample and the doser,
which was found to affect the overall shape of the CO uptake curves.
Finally, the adsorption kinetics was
determined as a function of surface temperature: the uptake was found
to change from precursor mediated
below 200 K to a more Langmuir type behavior around room temperature.