Structure analysis of adsorbates on single crystal surfaces using photoelectron diffraction

The structural determination of the Pt(111)c(5x3)rect.-CO phase formed by 0.6 ML of
adsorbed CO has been undertaken using scanned-energy mode photoelectron diffraction
utilising the two distinct components of the C 1s photoemission peak. Earlier assignments of
CO to atop and bridge sites have been confirmed as well as the respective 2:1 ratio of these
assignments. Additionally, quantitative local structural details have been obtained. In particular,
the Pt-C chemisorption bond lengths for the atop and bridging sites are 1.86 ± 0.02 Å and 2.02
± 0.04 Å respectively. These values are similar to those obtained in previous studies for the 0.5
ML coverage c(4 x 2) phase involving an atop:bridge occupation ratio of 1:1. The results also
indicate a definite tilt in the atop CO species of 10.7º +1.5º/-3.1º consistent with earlier
investigations using electron-stimulated desorption ion angular distribution, LEED, Monte
Carlo simulations and IR.
The local structure of benzene adsorbed on Si(001) has also been investigated using scanned
energy photoelectron diffraction. The standard butterfly (SB), tilted (T), tight bridge (TB),
pedestal (P), twisted bridge (TB), and diagonal bridge butterfly (DDB) models were optimized
and compared with the lowest R-factors being achieved for SB and TB models (0.2337 and
0.2641 respectively). Further optimization was performed for a mixed overlayer (0.25 ML)
consisting of SB and TB structures in various proportions. A significant improvement in the Rfactor
was achieved for a combined model in which 58 ± 35 % of the overlayer is composed of
the SB structure.
Using the structural data for the CO/Pt(111), and benzene/Si(001) adsorption systems,
comparative simulations have been undertaken to explore the effect of using vertically and
horizontally polarized radiation on PhD modulation amplitudes and more importantly the
sensitivity of each method to various structural parameters.
It has been shown theoretically that perpendicularly polarized photoelectron diffraction
(PPPhD) yields modulation functions with intensities often being several times those observed
in PhD. The new technique is shown to be more sensitive when the parameters involve mainly
lateral displacements. The sensitivity of PhD on the other hand exceeds that of PPPhD only
when dealing with bond lengths involving mainly vertical displacements. Parameters involving
similar vertical and lateral displacements show similar sensitivities for both methods. Despite
potential weaknesses such as a reduced signal to noise ratio and the sensitivity of PPPhD to the
sample positioning, the potential gains of this technique especially when considering systems in
which the adsorbates lie across the substrate such as benzene adsorbed on Si(001), make it ripe
for experimental validation.