Journal of Electron Spectroscopy and Related Phenomena

Low temperature adsorption of O2 on Pt(111)

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The oxygen interaction with the Pt(111) surface has been studied by X-ray photoelectron (XPS), ultraviolet photoemission (UPS) and thermal desorption (TDS) spectroscopies with the crystal temperature T, ranging from 25 to ~100 K. In the O2 Pt(111) system, both the atomic state and a molecular chemisorbed state have been well characterized. The present investigation aims at the detection of a physisorbed state. This physisorbed state is thermally converted into the molecularly chemisorbed species demon strating that it is an intrinsic precursor to molecular chemisorption. By comparison with adsorption at Ts > 45 K, the O2 adsorption at Ts = 25 K produces a considerably different O(1s) spectra, with a main peak at 535.8 eV and a weak shake-up satellite structure at 545.0 eV. The binding energy of the main O(1s) peak is within 2 eV of gas phase O2, after correction for the Pt(111) work function. This implies that this state is weakly interacting with or physisorbed on the Pt(111) surface. The large shifts in the XPS spectra between physisorbed and molecularly adsorbed O2 allow an easy separation of the two species. In contrast, the variations in the O2 derived features of the UPS spectra are largely obscured by the overlapping of the Pt 5d bands. TDS peaks are observed at 30, 45, ~130 and ~750 K. These are associated with desorption from multilayer, physisorbed, molecularly chemisorbed and atomic (associative) states, respectively. © 1990.