The chemisorption of hydrogen on the (110) surface of iridium

Abstract

The chemisorption of hydrogen on Ir(110) was studied under ultra-high vacuum conditions with thermal desorption mass spectrometry, LEED, contact potential difference measurements and ultraviolet photoelectron spectroscopy. The Ir(110)-(1×2) reconstructed surface is stable in hydrogen at pressures from 10−9 to 10−5 Torr and surface temperatures from 130 to 1000 K, the conditions investigated. No streaks or spots were observed in the LEED pattern due to the presence of hydrogen. Absolute coverage measurements indicate the saturation density at 130 K on Ir(110) is (2.2±0.2)×1015 atoms cm−2. Thermal desorption measurements indicate that hydrogen obeys second order desorption kinetics and exhibits two features, β1 and β2 states, with intensities 2:1, respectively, which exchange isotopically with one another. However, β2 hydrogen obeys first order adsorption kinetics with an initial sticking probability S0 equal to unity, while β1 hydrogen has an S0 equal to 7×10−3 and obeys second order kinetics. Rate parameters for hydrogen desorption from Ir(110) were calculated as a function of fractional surface coverage. A sympathetic increase in the rate parameters up to at least half of saturation is observed for the β2 state of hydrogen, where Ed and νd assume the values 23 kcal mol and 1.5×10−2 cm2 s−1, respectively. The increase is attributed to adatom–adatom attractive interactions of hydrogen in the β2 state. For the β1 state, the energy of desorption, in kcal mol−1, is given by Ed=17−10ϑ from ϑ equal to 0.4–0.7, and the pre-exponential factor, νd, varies weakly as a function of coverage with an average value of (2÷×2)×10−7 s−1. The contact potential difference (CPD) increases linearly with coverage up to 0.20 eV and reaches a maximum at 0.30 eV for the β2 state of hydrogen for T?300 K. A continuous decrease in the CPD occurs over the β1 state up to saturation at 140 K and becomes lower than the clean surface contact potential for total fractional coverages above 0.9. Probable binding sites for the β1 and β2 states are inferred from the absolute coverage measurements and the CPD measurements. Ultraviolet (HeI,hv=21.2 eV) photoelectron spectra of hydrogen show a broad H(1s)level centered approximately at 6.1 eV below the Fermi level. A strong decrease in the d-band emission occurs near the Fermi level after adsorption of the β2 state which suggests preferred binding locations for this state of hydrogen on the Ir(110)-(1×2)surface.