Structure Analysis of Adsorbates on Single Crystal Surfaces using Photoelectron Diffraction

The structural determination of the Pt(111)c(5x√3)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. Index Abstract ................................................................................................................................................................... 2