Adsorbed molecular shuttlecocks: An NIXSW study of Sn phthalocyanine on Ag(1 1 1) using Auger electron detection

Normal incidence X-ray standing wave (NIXSW) spectroscopy has been used to determine the orientation of Sn phthalocyanine (SnPc) molecules in a highly ordered, but incommensurate, monolayer on the Ag(1 1 1) surface. Our sample preparation procedure differs from that used in previous work on this system [C. Stadler, S. Hansen, F. Pollinger, C. Kumpf, E. Umbach, T.-L. Lee, J. Zegenhagen, Phys. Rev. B 74 (2006) 035404] and leads to a different unit cell with basis vector lengths of ∼15.0 Å and 15.3 Å (γ = 98°) which is oriented at an angle of ∼5° to the underlying Ag(1 1 1) lattice. Structural parameters extracted from Sn MNN NIXSW spectra indicate that SnPc, a buckled, 'shuttlecock' phthalocyanine, adsorbs in a Sn-down geometry with the Sn atom approximately 2.3 Å above the Ag(1 1 1) surface plane. Despite the incommensurate nature of the overlayer, we find a surprisingly high coherent fraction for standing wave data taken for the (1¯11) reflection and argue that this arises from the small domain size of the superstructure.