We have used scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and X-ray photoelectron spectroscopy (XPS) to investigate the thermal stability of Pd(111) islands and thin films on TiO2(110)-(1 × 2). Two new nano-structures were observed to form on the surface of the Pd by annealing to 973 K. Atomic resolution STM images show a “pinwheel” super-structure. One domain has a unit cell with respect to the Pd(111) whereas the other domain has a unit cell. Coexisting with this phase is a structure consisting of zigzag rows that run along the close-packed directions of the Pd(111) islands. This has a rectangular unit cell incommensurate with both the substrate TiO2(110) and the Pd(111) islands. STM shows these two structures merge with no noticeable domain barriers or steps, suggesting a close relationship between the two. LEED shows several distinct, overlapping patterns that can be identified with the surface structures observed; TiO2(110)-(1 × 2), Pd(111)-(1 × 1), the hexagonal pinwheel structure, and the rectangular zigzag unit cell. XPS at normal and grazing emission show the encapsulating layer to be composed of TiO x with Ti predominantly in ∼2+ or ∼3+ oxidation states. The proposed models are structurally consistent with the LEED and STM data and have stoichiometries of TiO and TiO1.4, chemically consistent with the XPS spectra. The STM images of the zigzags bear a strong similarity to structures seen for annealed Pt islands on TiO2(110)-(1 × 1) and TiO x supported on Pt(111), while the pinwheel structure is similar to annealed Cr on Pt(111). We discuss the similarities of our structures to those seen before for these related systems.

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We have employed variable-temperature scanning tunneling microscopy ͑STM͒ to follow the kinetics of reoxidation of a nonstoichiometric TiO 2 ͑110͒ single-crystal surface. The surface is seen to grow during reoxidation between 573 and 1000 K and at pressures from 5ϫ10 Ϫ8 to 2ϫ10 Ϫ6 mbar O 2 . The reaction proceeds by combination of gas-phase O 2 with mobile interstitial Ti 3ϩ from the bulk. The surface is observed to grow new layers of TiO 2 in a cyclic process, resulting in the formation of alternate layers of (1ϫ1) and cross-linked (1ϫ2). The growth rate was calculated by measuring the area of new material grown on consecutive STM images of the same area. The rate shows a linear dependence upon oxygen pressure, with an Arrhenius plot indicating an activation barrier of ϳ25Ϯ4 kJ mol Ϫ1 ͑0.25Ϯ0

The nuclear response function in the random-phase-approximation theory may be substantially altered at high excitation energy due to coupling of 1p-1h to 2p-2h or higher excitations. In the continuum region of quasi-elastic scattering, this coupling can be included by inserting into the random-phase-approximationGreen's function a complex self-energy for the p-h states, which depends only on their energy, spin (S), and isospin (X). This allows the full response to be expressed directly as an integral over the random-phase-approximation response. The self-energies, which can be determined from empirical single-particle spreading widths, are large in the spin and isospin channels, but small in the T,S =0,0 channel, giving rise to substantial damping in all channels except 0,0. We apply this method to calculate damping of the random-phase-approximation surface response, and discuss the consequences for cross sections and spin observables in quasi-elastic (p, p ) and (n, p) reactions.