Kinetics and structure of O2 chemisorption on Ni(1 1 1)

The kinetics of O2 chemisorption at low dose on Ni(1 1 1) 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 O2 dosing provide direct information on the location of oxygen adatoms on the Ni(1 1 1) surface at very low coverages as well as site-specific occupation rates (Sfcc and Shcp) and occupancies (θfcc and θhcp). A system of equations has been developed that relate the slopes of the scattering and recoiling intensities versus O2 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 θ1=θfcc∼0.21 ML with θhcp=0, constant S=Sfcc∼0.18±0.01, and coverage ratio w=θhcp/θfcc∼0 for both 300 and 500 K. The second-stage is observed at coverages between θ1∼0.21 and θ2∼0.32 ML with constant Sfcc=−(0.05±0.01) and Shcp=(0.16±0.02) at 300 K and Sfcc=(0.005±0.003) and Shcp=(0.007±0.003) at 500 K, and coverage ratios w=θhcp/θfcc∼1 at 300 K and w=θhcp/θfcc∼0.10 at 500 K. The third-stage, observed for θ>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 ΘD*” 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 〈u2〉 has been developed. The result for this system is ΘD*=314±10 K.