Normal-incidence x-ray standing-wave determination of the adsorption geometry of PTCDA on Ag(111): Comparison of the ordered room-temperature and disordered low-temperature phases

Normal incidence x-ray standing wave (NIXSW) experiments have been performed for monolayers of 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) adsorbed on the Ag(111) surface. Two phases were analyzed: the low-temperature phase (LT phase), which is disordered and obtained for deposition at substrate temperatures below 150 K, and the ordered phase, which is obtained for deposition at room temperature (RT phase). From the NIXSW analysis the vertical bonding distances to the Ag surface were obtained for the averaged carbon atoms and the two types of chemically different oxygen atoms in the terminal anhydride groups. For the LT phase, we find about 2% (0.05 A degrees) and 8% (0.21 A degrees) smaller averaged bonding distances for the C and O atoms, respectively, compared to the RT phase. In both phases, the planar geometry of the free molecule is distorted; in particular, the carboxylic O atoms are closer to the surface by 0.20 A degrees (RT) and 0.31 A degrees (LT) with respect to the averaged C distance. The difference between the vertical bonding distances of the carboxylic and anhydride O atoms is found to be 0.32 (RT) and 0.33 A degrees (LT). These structural parameters of the two phases are compared to those of PTCDA monolayers adsorbed on Au(111) and Cu(111) surfaces and are discussed in the frame of current bonding models.