Interaction of oxygen with a Pd(110) surface. I. Structures and coverages

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Abstract
Adsorption and desorption of oxygen on a Pd(110) surface have been investigated by low energy electron diffraction(LEED),nuclear reaction analysis (NRA), and Rutherford backscattering(RBS). Exposure of Pd(110) to O2 at 100 K and subsequent heating, produces the following surface phases in sequence: pseudo‐(2×1), c(2×6), and c(2×4). Exposure at 280 K produces only the c(2×4) phase. Above 400 K the (2×3)‐1D and c(2×4) form in sequence upon O2 exposure. The phase transition (2×3)‐1D to c(2×4) is accompanied by the continuous movement of the third order beams in (2×3)‐1D to the half order positions. The transition between the c(2×4) and (2×3)‐1D is reversible above 400 K, and depends only on the oxygen coverage. NRA indicates that the saturation coverage of oxygen with exposure to O2 at 100 K is close to 1 ML (1 ML=9.4×1014 O atoms cm−2 ). The c(2×4) and (2×3)‐1D phases have surface coverages of 0.50±0.05 and 0.23±0.05 ML, respectively. The RBS results show that the surface exposed at low temperature, the (2×3)‐1D and the c(2×4) phases all involve reconstruction of the Pd surface. In the low temperature exposed surface, more than a monolayer of Pd atoms are displaced laterally from their bulk‐like locations in the surface region, indicating that there are displacements in at least the second layer. In the c(2×4) phase more than 0.6 ML of Pd atoms are laterally displaced. Models for the two phases are tentatively proposed.