When an electron beam bombards the residual oxygen-containing gases, such as H2O, CO2 and CO, these gaseous molecules will be dissociated as follows: H2O→Oad+H2,CO2→Oad+CO,CO→Oad+Cad, and the oxygen, and carbon atoms will coad-sorb on Ni(001) surface forming respectively many independent adsorption domains. The structure of them is either p(2×2) or c(2×2),depending upon the oxygen and carbon concentrations on the surface. The electron beam bombardment assists the nuclea-tion, growth, coalescence and ordering of these domains. When the oxygen and carbon atoms have occupied about one half of four-fold adsorption sites of the Ni(001) surface, the above- mentioned reactions will be in a state of equilibrium with following desorp-tion reactions: C*+Oad→CO,O*+Cad→CO, where * denotes the electron excited atoms, and the nickel oxide or carbide begins to nucleate. As the content of oxygen in the residual gases surpasses that of carbon, the nucleation of nickel oxide will be predominant. Once nickel oxide is growing, the desorp-tion reaction of carbon becomes effective and the already adsorbed carbon atoms will desorb or diffuse away and the oxygen concentration will increase quickly. As a consequence, the oxygen concentration inside the electron beam spots becomes much higher than that outside and the reverse is true for the carbon concentration. During the electron beam bombardment a change of carbon Auger peak shape, indicating diffrent bonding states between the carbon and substrate atoms, takes place. In the initial stage of adsorption the dissociative effect of an electron beam plays an important role, but during the growth of nickel oxide its thermal effect in enhancing diffusion becomes predominant.
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