MOLECULAR ADSORPTION STATES OF O-2 ON W(111) AT LOW-TEMPERATURES (DOWN TO 5 K)

Adsorption of O-2 on W(111) at temperatures down to 5K is studied usin g molecular beam (MB) and thermodesorption spectroscopy (TD) methods. The low-temperature TD spectra for the molecularly adsorbed O-2 on W(1 11) contain three peaks at 25, 27 and 60 K corresponding respectively to multilayer condensation, physisorption and weak molecular chemisorp tion. The role of the substrate structure for the O-2 chemisorption is discussed on the basis of present results for W(111) and earlier data for different metal surfaces. Field-effect on the O-2-W(111) interact ion is studied using field ion appearance energy spectroscopy (FIAES) of O-2(+) ions emitted from the apex plane of a [111]-oriented W field emitter tip. Applying a thermionic cycle, field-modified binding ener gies for O2W/(111) are derived from the O-2(+) appearance energies mea sured at 79 K. FIAES data reveals a field-adsorbed O-2 layer as the or igin of O-2(+) field ions emitted from W(111) for field ion imaging.