The chemisorption of oxygen on the (110) surface of iridium∗

Abstract

The chemisorption of oxygen on Ir(110) has been investigated under ultra-high vacuum conditions with thermal desorption mass spectrometry, contact potential difference measurements, Auger electron spectroscopy, and LEED. Oxygen may adsorb in three distinct chemical states: a molecularly chemisorbed species that is stable below 100 K, a dissociatively chemisorbed species, and a surface oxide that forms rapidly above 700 K. The oxide layer saturates at a coverage of 0.25 ML (1 ML = 9.6 × 10 14 atoms/ cm 2) and orders to form a (1 × 4) LEED pattern. Different LEED patterns of dissociatively chemisorbed oxygen are observed on clean and oxidized Ir(110). A p(2 × 2) pattern forms on the clean surface near 0.25 ML coverage whereas a c(2 × 2) pattern forms on the oxidized surface near 0.5 ML coverage. Oxygen desorbs molecularly from Ir(110) with an activation energy of 45–70 kcal/mole, decreasing continuously with increasing coverage. The adsorption kinetics are described by a second-order precursor model for surface temperatures between 300–700 K. Oxygen chemisorption is not activated since the initial sticking probabilities on the clean and the oxidized Ir(110) surfaces are equal to 0.28 and 0.4, irrespective of the surface temperature. The dipole moment and polarizability of dissociatively chemisorbed oxygen change at 0.25 ML coverage on the clean surface and at 0.5 ML coverage on the oxidized surface. Although the dipole moment for any coverage is independent of temperature, the polarizability is inversely proportional to temperature. The activation energy for the dissociation of molecularly chemisorbed oxygen is 8 kcal/mole.