Electron-energy-loss-spectroscopy study of oxygen chemisorption and initial oxidation of Fe(100)

The initial stages of the interaction of oxygen with an Fe(100) surface have been investigated at 300 K mainly by electron-energy-loss spectroscopy (EELS) with in situ combined low-energy-electron-diffraction, Auger-electron and secondary-electron-emission spectroscopy, and workfunction---change measurements (Δφ). From all the results, three different stages of the oxygen interaction are distinguished: (i) dissociative chemisorption stage up to {~}3L, (ii) incorporation of O adatoms into the selvedge between 3 and 20 L, and (iii) oxidation above 20 L, leading to the formation of γ-Fe₂{O}₃. [1 langmuir (L)≡{10}^{{-}6}$ Torr sec.] Of special interest is the change in surface electronic properties from that characteristic of the metal to that of the oxide observed by EELS. An energy-loss peak characteristic of the chemisorbed oxygen was observed at 6 eV below 3 L, being ascribed to the transition between the bonding and antibonding orbitals, and the EELS spectrum in the oxide phase was characterized by the peaks due to the O^2- 2p→Fe³⁺ 3d charge-transfer transitions. The γ-Fe₂{O}₃-FeO phase transition at ~570^∘C was also confirmed by monitoring an energy-loss peak due to thed{→}d$ transition, which is spin allowed for an ${Fe}²⁺$ ion in FeO. It is shown that the Fe $3{d}yz,zx$ electrons play a major role in the chemisorption bond (O adatoms located in the centered hollow or bridge site), and for the incorporation process the Fe $3{d}_{{z}²}$ electrons are also involved in bonding by the symmetry breaking. A direct-recombination process following the $3p{→}3d$ transition is proposed for the high-energy-side peak of the Fe ${M}2,3VV$ Auger spectrum, which---though not yet unambiguously explained---has been tentatively assigned to an Auger transition from a doubly ionized Fe core level.