Interaction of O and Cu on W(110): II. The presegregation regime

Abstract

Adsorption of oxygen on Cu 1/W(110) and Cu 2/W(110), as well as adsorption of Cu on O x/W(110) (0 ⩽ x ⩽ 0.85) was studied. It is concluded that at 90 K. O adsorption occurs on top of Cu 1/W(110) or Cu 2/W(110), with O diffusing to the W surface without desorption on heating to 300 K, at which point incipient segregation occurs. Cu can also be adsorbed on top of O 0.6/W(110). On heating Cu multilayer formation starts in this case at ~ 200 K. For O adsorption on Cu 1/W(110) the O dipole moment is nearly the same as on clean W(110), while it is much larger on Cu 2/W(110). A monolayer of Cu on O x/W(110) shows metallic behavior, and in particular screens the W-O dipole potential almost completely up to x = 0.6, so that φ is nearly that of Cu 1/W(110). Similar screening was also noted for a Cu monolayer on CO/W(110). The sticking coefficient s of O 2 on Cu 1]/W(110) is unity over a wide coverage range at all temperatures investigated, while it is only 0.5 on Cu 2/W(110) and 0.55 on W(110). This behavior is tentatively attributed to soft transverse vibrational modes on Cu 1/W(110) arising from W/Cu size mismatch which permits efficient energy exchange between incoming O 2 molecules and the surface. As noted in paper I there is also adsorption of O 2 on Cu 1/O 0.6/W(110). The sticking coefficient for this process is 0.58 at 90 K up to maximum θ o = 0.2. UPS measurements for Cu n /W(110) where 0.5⩽ n ⩽ 8 are presented, and show a gradual transition to Cu bulk DOS. UPS measurements for O/Cu 1/W(110), O/Cu 2/W(110) and Cu 1/O 0.6/W(110) are also presented and discussed. In particular it is noted that the spectrum for Cu 1/O 0.6/W(110) seems to resemble that of Cu[in2 /W(110), i.e. appears more Cu-like than that of O/Cu 1/W(110) or Cu 1/W(110).