Oxygen-induced surface phase transformation of Pd(1 1 1): sticking, adsorption and desorption kinetics

Adsorption and desorption of oxygen on Pd(1 1 1) were studied by high-flux molecular beam adsorption, LEED, TDS and scanning tunnelling microscopy (STM) between 300 and 623 K sample temperature for oxygen coverages ΘO up to 1 ML. While adsorption below ΘO=0.25 is precursor mediated and proceeds without changes of the Pd substrate, it is activated for ΘO>0.25 and induces a massive change of the surface structure. STM reveals the formation of a new surface phase which consists of islands with a local oxygen coverage of 1 ML but less Pd atoms than the bulk-terminated (1 1 1) layer. Its formation and decay require activated mass transport of Pd and O atoms over mesoscopic distances. Due to island growth of this phase the oxygen sticking decreases linearly between ΘO=0.25 and 1 ML. For ΘO>0.25 ML the TPD rate maxima are shifted towards higher temperature with increasing initial coverage, indicating autocatalytic desorption kinetics. Desorption occurs preferentially from a dilute chemisorbed phase on Pd(1 1 1) terraces, with the islands of the high oxygen-density phase acting as a reservoir for O. The experimental TPD data can be well described by a simple mathematical model considering phase equilibrium during desorption.