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The initial interaction of oxygen with a Cu(ll0) surface occurs in two stages, character&d by (2 X 1) and ~$6 X 2) overlayer structures and then a third stage where the cf6 X 2) structure remains but further oxygen uptake is registered only with e~psometry. The fust stage is nonactivated and is accornp~~~ by a work function increase of 370-420 meV, depending on sample temperature. The subsequent appearance of the extra features associated with the c(6 X 2) structure in the second stage is accompanied by a decrease in work function of -100 meV and is characterised by an apparent activation energy of -18 M mol-r . The adsorbed oxygen can be removed at all coverages by gas-phase CO. The reaction appears to foHOw Langmuir-Hiishelwood kinetics with an apparent activation energy of -25 M mol-' . fntroductionThe interaction of oxygen with the (110) surface of copper results in the initial formation of a (2 X 1) surface structure which is assumed to correspond to 0.5 monolayers [l--4]. Further oxygen exposure produces a ~(6 X 2) structure f5]. The first stage of the interaction is known to be accompanied by an increase in work function. Thus Ertl and Kiippers [6] have reported an increase of 0.2 eV after an exposure of 6 L and Delchar f7] a m~~urn increase of 0.67 eV after 2 X 10" L at room temperature. Delchar established that a change of 0.40 eV had already been reached after about 100 L. On the (100) surface a similar reaction sequence has been observed. The formation of the initial (u/z X 42) R45" structure is accompanied by an increase in the work function as might be expected, but the formation of the subsequent (d2 X 242) R4.5" structure by a decrease [8,9]. This has been 285