KINETICS OF THE DISSOCIATIVE ADSORPTION OF O2 ON PT(111) - A TREELS STUDY

Isothermal dynamic and steady state HREELS measurements were taken during the dissociative adsorption of O2 on Pt(111) for different temperatures and different initial coverages produced by Pulses of an O2-molecular beam. The time resolution of the experiment allows one to observe the dissociative adsorption process in the temperature range between 86 and 130 K in detail by monitoring the evolution of loss intensities which are characteristic of O2 and O, simultaneously. The steady state data yield detailed information on the sticking Of O2 as a function of O coverage. Possible O2-O distributions which are almost stable at T < 100 K fulfil the relation theta(O2)/0.44 + theta(O)/0.25 less-than-or-equal-to 1. Detailed analysis of data derived from isothermal dynamic measurements unambiguously demonstrate the existence of a non-equilibrium state with two different adsorption rates at the same coverages Of O2 and O at the same temperature. This is interpreted in terms of a time-delayed, O2-induced surface reconstruction.