Low temperature adsorption and condensation of O 2, H 2O and NO on Pt(111), studied by core level and valence band photoemission

Abstract

The chemisorption and condensation of O2, H2O and NO on Pt(111) was studied between 20 and 90 K by photoelectron spectroscopy. All oxygen derived levels (O1s, O2s and valence band) were measured using light energies of 1253.6, 120 and 21.2 eV. The rarely investigated O2s spectra turn out to be very useful for the analysis of different adsorbed species. Oxygen physisorbs and condenses at 20 K in neutral form. At 90 K, a strong Ols chemical shift (5.2 eV) is observed, the O2s level splitting into a bonding and antibonding state has almost vanished and the valence band completely changed, consistent with chemisorption of the ionic molecular species O22− or O2− as proposed previously. The adsorption of this species is thermally activated. The valence band spectra of water in the submonolayer region and the thick ice layer differ considerably due to the strong hydrogen bonding in the condensed layer. Chemisorbed monomeric NO is observed in the submonolayer regime even at 20 K. Monomer chemisorption is not activated. The species chemisorbed at 20 K reduces the work function considerably (− 420 meV for saturation at ~ 0.5 ML) consistent with predictions for terminal (on-top) NOδ+ Chemisorption at 80 K is monomeric, too. However, the O1s peak shape and the different work function change indicate a mixture of NO, probably on different sites. On top of the chemisorbed layer, NO condenses at 20 K in the form of (NO)2 dimers as concluded from the appearance of strong shake-up peaks in the O1s and the valence band spectra.