Structure of oxygen adsorbed on Mo(100) studied by high-resolution electron energy-loss spectroscopy

Oxygen-modified Mo(100) surfaces with coverages from 0.8 to 1.4 monolayers (ML), prepared by high-temperature annealing, have been investigated using high-resolution electron energy loss spectroscopy (HREELS). The surface with 0.8 ML coverage, a (√5 × √5)-R26°33 minutes low-energy electron diffraction (LEED) pattern, produces a three-peak pattern associated with modes derived from oxygen located in a three-coordinate site. With increasing oxygen coverage up to 1.1 ML and a change in the LEED pattern to (2 × 1), the three-peak pattern persists which indicates that the three-coordinate O/Mo structural motif is preserved. As the oxygen coverage exceeds 1 ML, an additional loss peak due to oxygen located on an atop site appears at 985 cm^-1. At 1.4 ML the HREELS spectrum is very similar to the vibrational spectrum of MoO₂, consistent with the formation of this oxide phase. Time-dependent HREELS measurements at 300 K and coverages ≤ 1 ML reveal a gradual clustering of adsorbed oxygen and relocation from three-coordinate sites to atop sites.