Surface X-Ray Diffraction of 5-I-Uracil/Ag(111) and 2-S-Uracil/Ag(111)

Abstract

We present surface X-ray diffraction experiments on ordered large organic monolayers, As model systems we have chosen two uracil derivatives, namely 5-I-uracil (IU) and 2-S-uracil (SU) which on Ag(111) form a uniaxially incommensurate and a commensurate superstructure, respectively. These are characterized by transformation matrices (263/1.78 7 1 ) for IU and ( 4 2 5 3 ) for SU. Although the measured reflection intensities are extremely weak (about 2 to 4 orders of magnitude weaker than those near the antiphase condition along the Ag crystal truncation rods), about 40 independent inplane reflections (hk0) could be measured in each case allowing the development of structure models. In the case of IU adsorption a surface reaction takes place where a 5-5'-di-uracil molecule is formed after separation of the I atom from the pyrimidine rings. The formation of ordered structures needs mild thermal activation (T - 400 K). The I atoms form a separate I/ Ag(111)-(√3 × √3) phase. In the case of SU no reaction is observed and the unit cell contains four molecules. The arrangement of the SU molecules in the surface unit cell is similar to the arrangement in bulk SU. The commensurability of the SU superstructure is explained by the strong S-Ag interaction.