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Ultraviolet photoelectron spectroscopy (UPS) and electron spectroscopy by deexcitation of metastable noble gas atoms (MDS) were used to follow the continuous oxidation of Cs films. While UPS has a finite information depth, MDS probes only the properties of the outermost atomic layer. Small doses of O2 cause the formation of a ‘‘monolayer’’ of Cs11O3 in which the O2− ions are incorporated below the surface, while the surface itself consists merely of metallic Cs atoms. Further uptake of oxygen leads to a continuous transformation of the subsurface layer into Cs2O2, while a small concentration of adsorbed O atoms builds up on the surface which act as nucleation centers for subsurface oxidation. Finally, the surface layer is completely oxidized into CsO2 which then retards further oxygen penetration into the bulk. The chemical state of the surface can be directly correlated with the associated change of the work function. A monolayer of Cs adsorbed on a Cu(110) surface exhibits quite different properties.

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Deexcitation of metastable He atoms has been used to probe the existence of a partially occupied 4s-derived level close to the Fermi level of K atoms adsorbed on a Cu(llO) surface, even at very low coverages. The width of this level decreases from about 3 eV at low coverage to 2 eV at high coverage, reflecting the transition from the chemisorbed to the metallic state of the adsorbate.PACS numbers: 79.20. Rf, 68.40. + e Adsorption of alkali-metal atoms on metal surfaces has been quite extensively studied in the past. 1 The simplest picture for the formation of the chemisorption bond was proposed by Langmuir 2 and assumes that the alkali-metal atom transfers its valence s electron completely to the metal substrate. With increasing coverage the resulting surface dipoles depolarize each other mutually, thus causing the characteristic variation of the work function. The more realistic, quantum mechanical model of Gurney 3 assumes that the discrete s level of the free alkali-metal atom broadens and shifts down in energy when the atom approaches the surface. At the equilibrium separation the density of states of this resonance reaches partly down below the Fermi level E F so that ionization is incomplete. This picture underlies a series of more recent theoretical treatments of this problem 4 which predict partial occupancy of this 5-level resonance even for a single adatom, i.e., in the limit of zero coverage. Any attempts to verify experimentally the existence of this level close to the Fermi level by ultraviolet photoelectron spectroscopy (UPS) have been so far rather unsuccessful because of overlap with the emission from valence states of the substrate and of the low excitation cross section of s -electronic states by photons. 5 The only indication so far was reported for Cs adsorbed on Cu(lll) above half a monolayer where the UPS data showed the emergence of a peak below E F . 6 Since, however, at higher coverages an alkali-metal overlayer is expected to become more metalliclike, 4 this observation does not answer the question of whether isolated adsorbed alkali-metal atoms are completely ionized or not.Electron spectroscopy by deexcitation of metastable noble-gas atoms (MDS), on the other hand, is an extremely surface-sensitive technique and probes only the valence electronic structure of the outermost atomic layer of a solid. 7 Since, in addition, He* atoms exhibit a high cross section for excitation of s levels, this technique provides unique possibilities for studying the outlined question. Recent studies with K/NKlll) 8 and K/Cu(110) 9 already provided evidence for the existence of alkali-metal-derived states below E F . The present work reports on a systematic study with the system K/Cu(110) over the whole range of coverages where, for the first time, the existence of a partially occupied alkali-metal s resonance even at very low coverages could be demonstrated.The experimental system has been described previously. 10 MDS was performed by l S He* atoms with thermal kinetic energies. Since these undergo very ef...

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