Study of the initial oxidation of Cr(100) at room temperature

Abstract

The adsorption of oxygen on Cr(100) at room temperature has been studied with a number of complementary techniques. Data on adsorption kinetics, binding energies and work function changes were obtained by means of photoemission measurements. Some results dealing with LEED and AES are also reported. On the basis of the mesurements it is concluded that the surface is reconstructed even at the lowest coverages. The activation energy for oxygen incorporation increases progressively with coverage because of the surface bond strength of the reaction product. Oxygen is then chemisorbed on top and an electric field is set up at the surface. For θ ≅1.1 monolayer the activation energy reaches a saturation value E ≅ 0.90 eV. The amount of on top chemisorbed oxygen as well as the corresponding field remain then constant in the range 1.1 < θ < 2. In this region oxide growth continues under constant field conditions according to a model proposed by Mott and Fehlner for low temperature oxidation. It is found that the electric field must be taken into account in any attempt to interpret binding energy changes. When θ reaches approximately 2.3 monolayers the adsorption rate drops to very low values and this is in sharp contrast with the behaviour of the Cr(111) surface where θ ≅ 4 at saturation. Our results are consistent with the less open structure ofCr(100) when compared with Cr(111).