Kinetics and structure of O2 chemisorption on Ni(111)

The kinetics of O-2 chemisorption at low dose on Ni(111) and the nature of the chemisorption site have been studied at 300 and 500 K using time-of-flight scattering and recoiling spectrometry (TOF-SARS), low energy electron diffraction, and scattering and recoiling imaging code (SARIC) simulations. Variations in the TOF-SARS spectra with different crystal alignments during O-2 dosing provide direct information on the location of oxygen adatoms on the Ni(111) surface at very low coverages as well as site-specific occupation rates (S-fcc and S-hcp) and occupancies (theta (fcc) and theta (hcp)) A system of equations has been developed that relate the slopes of the scattering and recoiling intensities versus O-2 exposure dose to these probabilities and occupancies. The results identify three chemisorption stages as a function of oxygen exposure, each with its own specific occupation rates and occupancies. The first-stage is observed up to theta (1) = theta (fcc) similar to 0.21 ML with theta (hcp) = 0 constant S = S-fcc similar to 0.18 +/- 0.01, and coverage ratio ill = theta (hcp)/theta (fcc) similar to 0 for both 300 and 500 K. The second-stage is observed at coverages between theta (1) similar to 0.21 and theta (2) similar to 0.32 ML with constant S-fcc = -(0.05 +/- 0.01) and S-hcp = (0.16 +/- 0.02) at 300 K and S-fcc = (0.005 +/- 0.003) and S-hcp = (0.007 +/- 0.003) at 500 K, and coverage ratios w = theta (hcp)/theta (fcc) similar to 1 at 300 K and w = theta (hcp)/theta (fcc) similar to 0.10 at 500 K The third-stage, observed for theta > 0.32 ML, involves saturation coverage of the adsorption sites. SARIC simulations were used to interpret the spectra and the influence of oxygen chemisorption and vibrational effects. A method for determining the "effective Debye temperature Theta*(D)" that uses the experimental TOF-SARS intensity variations as a function of temperature and the simulated SARIC signals as a function of the mean square vibrational amplitude [mu (2)] has been developed. The result for this system is Theta*(D) = 314 +/- 10 K. (C) 2001 Elsevier Science B.V. All rights reserved.