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

Abstract

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-energyelectron-diffraction, Auger-electron and secondary-electron-emission spectroscopy, and workfunctionchange measurements {b, P). From all the results, three different stages of the oxygen interaction are distinguished:(i) dissociative chemisorption stage up to -3 L, (ii) incorporation of 0 adatoms into the selvedge between 3 and 20 L, and (iii) oxidation above 20 L, leading to the formation of y-Fe203. [1 langmuir {L)-:1 0 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 0 2p~P e + 3d charge-transfer transitions. The y-Fe203 -FeO phase transition at -570'C was also confirmed by monitoring an energy-loss peak due to the d -+d transition, which is spin allowed for an Fe + ion in FeO. It is shown that the Fe 3d", electrons play a major role in the chemisorption bond (0 adatoms located in the centered hollow or bridge site), and for the incorporation process the Fe 3d 2 electrons are also Z 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 M23VV Auger spectrum, whichthough not yet unambiguously explainedhas been tentatively assigned to an Auger transition from a doubly ionized Fe core level.

Observation of a high negative spin polarization at the Fe/MgO(100) surface oxidized at room temperature

“…The oxygen uptake curve at Ͻ50 L is similar to that reported for Fe͑100͒ single crystal surfaces. 9,15 Similar to previous studies, 9,15 three stages, i.e., I ͑0-3 L͒, II ͑3-20 L͒ and III ͑Ͼ20 L͒, are also seen in our result. The regions I, II, and III have been assigned to the dissociative chemisorption, oxygen incorporation, and the formation of a three-dimensional oxide, respectively.…”

“…The regions I, II, and III have been assigned to the dissociative chemisorption, oxygen incorporation, and the formation of a three-dimensional oxide, respectively. 15 Our result is however different from that of Sakisaka et al 15 at the high exposure range. They reported that the O/Fe Auger ratio, although it is about 1.3 at ϳ50 L, increases to 2.2 at 350 L. 15 They suggested that the surface oxide at 350 L would be Fe 2 O 3 .…”

“…15 Our result is however different from that of Sakisaka et al 15 at the high exposure range. They reported that the O/Fe Auger ratio, although it is about 1.3 at ϳ50 L, increases to 2.2 at 350 L. 15 They suggested that the surface oxide at 350 L would be Fe 2 O 3 . 15 However, because our Auger intensity ratio is low ͑Ͻ1.4 at 2200 L͒, the final oxide could be different from theirs.…”

“…This is consistent with result of the oxidation of a Fe͑100͒ single crystal surface. 15 At exposures Ͼ10 L, the asymmetry increases with exposure and becomes comparable to that for the clean Fe surface at 1700 L. The A value further increases by cooling the sample to 77 K. It is noted that the A-H curve exhibits a slope at H Ͼ 20 kOe. Figure 1͑c͒ shows the A-H curve measured at the exposure of 100 L with the incidence angle ͑ i ͒ of 15°to the surface normal.…”

Oxygen adsorption on Fe(110) surface revisited

“…Investigations of the initial stages of the interaction of oxygen with an Fe(100) surface have shown [24,25] that oxygen atoms adsorb in the fourfold coordinated hollow sites on Fe(100). These findings were confirmed by density-functional calculations [4,25].…”