The surface potential of oxygen on uranium

The contact potential difference between ultra-high vacuum-deposited films of gold and of uranium has been followed during the admission of oxygen. Taking the work function of gold as constant up to oxygen pressures of the order of 10 -3 torr, and varying in a known fashion at higher pressures, the work function of the uranium film could be deduced for all conditions of oxygen coverage from nude to saturation, followed by subsequent re-evacuation to 5 × 10 -8 torr. The outstanding result is that the surface potential is positive with respect to the clean metal at all coverages. In the region of initial adsorption, estimated surface dipole moments are so large as to indicate an ionic type of binding between oxygen and uranium. The surface potential changes during initial adsorption are interpreted in terms of penetration of oxygen atoms below the surface into two different types of interstitial hole in the uranium lattice, the larger being filled first. At higher pressures of oxygen, oxide formation is postulated, with the appearance first of uranium dioxide and then subsequently unidentified higher oxides, at the surface. The separation of a semiconducting oxide phase at the surface in fact explains the observation that the surface potential is at all stages positive, since an additional phase-boundary is introduced. An incidental result from the measurements is the average work function of vacuum-deposited gold films on aged polycrystalline tungsten, 4.714 ± 0.004 eV.