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The surface structure of TiO 2 ͑110͒ has been reexamined using scanning tunneling microscopy ͑STM͒, especially in terms of its temperature dependency. A dramatic topographic change was observed around 725 K, which is most likely due to the oxygen desorption into the gas phase and the simultaneous diffusion of Ti into the bulk. The atomically resolved STM images show a strong dependency on the tip composition and allow a double-bridging oxygen vacancy to be identified. The nucleation and growth of Pd on the TiO 2 ͑110͒ surface have also been studied at the nucleation stage with atomic resolution. Both dimer and tetramer Pd clusters have been observed; however no single Pd atoms were detected. These results support the ''classical'' nucleation model which assumes that only monomers are mobile, whereas dimers are stable nuclei. A marked preferential nucleation and growth of Pd clusters at step edges have also been observed. In addition to topographic structures, the local electronic properties of the Pd clusters have been studied to relate cluster structure to electronic composition. ͓S0163-1829͑97͒06243-7͔

In this work the effect of aggregation and oxidation on the optical absorption of eumelanin oligomeric sheets is investigated by applying quantum mechanics and atomistic simulation studies to a simplified eumelanin structural model that includes 1-3 sheets of hexameric oligomer sheets. The oligomeric hypothesis is supported by AFM characterizations of synthetic eumelanins, formed by auto-oxidation or electrochemical oxidation of dihydroxyindole (DHI). Comparison of calculated absorption spectra to experimental spectra demonstrates a red shift in absorption with oxidation and stacking of the eumelanin and validates the theoretical results.

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Spectroscopic simulations of a leading structural model for melanin, the pigment responsible for coloration and photoprotection in humans and animals, were done. We performed density functional theory (DFT) calculations using both the local density approximation (LDA) and the generalized gradient approximation (GGA) on a recent structural model for eumelanin based on higher oligomers of the monomer of neutral 5,6-indolequinone and its reduced forms, semiquinone and hydroquinone. This paper reports on our semiempirical spectroscopic simulations for the monomer and dimer energy-minimized structures. Our second study (part II) extends this approach to higher oligomers (tetramers through hexamers) and points out that the known optical spectra of eumelanins can be adequately explained on this basis.