The structure of ultrathin Cu-phthalocyanine (Cu-Pc) films on the (1 x 2)-Au(I 10) surface has been studied. The overlayer deposition has been monitored in real time by helium atom scattering (HAS) and low energy electron diffraction (LEED). Throughout the monolayer regime the Cu-Pc molecules are systematically observed to line-up edge-to-edge along the [110] direction of the Au substrate, yielding a commensurate 5-fold periodicity (14.4 angstrom). Cu-Pc chains deconstruct the 2-fold Au missing row order in the early stage of deposition. A set of higher order periodicities (5-, 7-, and 3-fold) are progressively observed along [001] with increasing CuPc deposition, the 3-fold phase appearing at the monolayer saturation coverage. The corresponding molecular orientation has been studied by variable polarization absorption spectroscopy (XAS), whereas the Au substrate structure has been determined by out-of-plane surface X-ray diffraction. The (5 x 5) phase is found to be rather corrugated, and it exhibits a high degree of long-range order yielding the most prominent diffraction pattern. In the (5 x 5) phase, the Cu-Pc chains are found to lift the underneath missing row reconstruction, being separated by residual Au rows. Similarly, in the more compressed 3-fold monolayer phase, the Cu-Pc molecules were formerly found to lie within a shallow (1 x 3) An reconstruction [Cossaro, A.; et al. J. Phys. Chem. B 2004, 108, 14671]. From comparison of the different deposition stages, as measured in real time by HAS, we can draw a comprehensive picture of the system evolution. In fact, the observed periodicities at different coverage are always formed by an array of Cu-Pc chains in shallow troughs that are equally spaced by a number of uncovered Au rows, as dictated by the Cu-Pc coverage. The growth of Cu-Pc arrays in the submonolayer range is thus driven by an interchain repulsion mechanism.
