oxygen coverage for the dry oxygen exposure at 300 K while another referred to a c(2 x 2). On the contrary, well-defined, ordered oxygen adlayers were observed on the Fe(110) surface. For instance, an adlayer structure of c(2 x 2) was observed at 0.25 ML coverage while c(3 x 1) was observed at 0.33 ML. Three stages of oxygen adsorption on the iron surfaces were determined by experimental studies: 1) dissociative chemisorption of the O2 molecule, 2) formation of oxygen thin film with oxygen starting to penetrate sub-surface layers,