S. Schwegmann et al., QUANTITATIVE STRUCTURE-ANALYSIS OF A DISORDERED SYSTEM - RHEED STUDY OF THE CO-INDUCED (1X2)-](1X1) STRUCTURE TRANSITION OF PT(110), Surface science, 334(1-3), 1995, pp. 55-76
Recent progress in the understanding of RHEED has enabled the interpre
tation of data from disordered structures. If the correlation length o
f the disorder is small, the strong forward scattering at high energie
s corresponds to an averaging over the disorder such that the multiple
scattering is mainly determined by the two-dimensional periodic struc
ture part of the surface layers. Hence, this structure part which is i
ndependent of the statistics of the disorder, can be extracted from ro
cking curves of the principal reflections with a similar accuracy as f
or ordered surfaces. As an application we present a structural study o
f the CO adsorption on Pt(110)(1 X 2) and its induced transition into
a disordered Pt (1 X 1) phase. We measured several sets of rocking cur
ves for different exposures. These experimental rocking curves were fi
tted using an automated evaluation scheme. In this way structure data
versus CO exposure (0-saturation) were determined for both the Pt and
the CO. We found strong evidence that the lift of the (1 x 2) missing-
row reconstruction begins with considerable changes of the Pt relaxati
ons (top interlayer spacing, pairing, buckling). Besides adsorption on
the first layer, adsorption on the second Pt layer takes place, too,
as soon as the (1 X 2) ordering decreases because the rearrangement of
the first layer atoms provides for additional vacant top sites in the
second layer.