THE ADSORPTION OF OXYGEN ON NIAL(001) AT 300 K AND 20 K

The adsorption of oxygen on NiAl at 300 K and 20 K has been studied by means of high resolution electron energy loss spectroscopy (EELS), lo w energy electron diffraction (LEED) and Auger electron spectroscopy ( AES). The adsorption of oxygen at 300 K leads to formation of a thin f ilm of amorphous aluminum oxide (a-Al2O3). After oxygen adsorption at 300 K and annealing of the oxygen-saturated surface to 1200 K, a well- ordered thin theta-Al2O3 film is formed. After annealing of the oxygen -saturated surface to 1400 K, an alpha-like Al2O3 oxide is grown. Even at 20 K oxidation of the NiAl(001) surface is found. However, at high er exposure (greater than or equal to 5 L) oxygen physisorbs and the E EL spectrum exhibits characteristics of resonance scattering. The reso nance energy for physisorbed oxygen on the amorphous Al oxide layers f ormed at 20 K has a value of 7.0 eV. The frequency of O-O stretching v ibration for physisorbed O-2 is 193 meV. The line shape of this mode i s asymmetric with a pronounced tail on the high energy side. The asymm etric line shape corresponds to multiple losses due to the overlap of the O-O stretching mode with low frequency modes. After physisorption of oxygen at 20 K on the alpha-like Al oxide layers, the resonance ene rgy is shifted to 9.0 eV, a value close to that of oxygen in the gas p hase.