Penning deexcitation spectra of alkali atoms adsorbed on metals show an intense peak at the Fermi level for coverages around the work-function minimum. This strong electron emission is assigned to the decay of a core-excited negative He ~ ion created by resonance transition of a substrate electron into the affinity level of the metastable rare-gas atom. The formation of this species also easily provides an explanation for the observed He* singlet-triplet conversion.

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Abstract. Electron spectra of various metastable rare gas atoms systematically measured on a P t ( l l l ) surface with Rb coverages ranging from submonolayers (3%) to multilayers are presented. The different decay channels of the excited particles are discussed in terms of resonant electron exchange processes between the substrate and the projectile in relation to the work function. It is shown that below a certain value of the work function a highly excited negative rare gas atom is formed which can undergo different deexcitation processes. A careful discussion of the branching ratios into the decay channels offers a natural explanation of the variations in the electron spectra induced by alkali metal adsorption. Additionally, an attempt is made to extract information about the alkali metal chemisorption state from the observed electron spectra. 79.20.Ne, 79.80.÷w The practicability of Penning spectroscopy as a technique for surface investigations has been demonstrated in the past by several groups [1][2][3][4][5][6][7]. Penning spectroscopy has been used in studies of adsorbed molecules on transition metals [1][2][3], as well as of molecular crystals [4,5] and alkali halides [6,7] as a method which is complementary to UV photoelectron spectroscopy but has the advantage of strongly enhanced surface sensitivity. The majority of investigations, however, has dealt with studies of alkali metal adsorption, a fact reflecting the essential drawback of the method, namely the existence of a competing process. This resonance ionization transition (RI) occurs predominantly at clean metal surfaces and leads to the formation of He* ions. Only for metallic substrates with a low enough work function to inhibit RI does the Penning transition dominate and produce intense, clear structures in the electron spectrum, comparable to UPS results. PACS:The electron spectra produced by impact of metastable rare gas atoms on clean transition metals (work function typically between 4 and 6 eV) are due to an ion neutralization process (IN) following RI and the resulting electron distributions have to be deconvoluted numerically to extract information about the surface density of states [8]. If the work function is lowered by alkali metal adsorption, sharp structures appear in the spectrum at the Fermi level which are clearly induced by Penning transitions [9-1 t]. The intensity of these peaks has previously been correlated with the alkali induced density of states at E F although the peak intensity strongly increases at intermediate coverages. A further difficulty in the interpretation of Penning spectra is connected to the fact that two He* species (1S and 3S), with an excitation energy difference of 0.8 eV, exist and that the conversion of 1S ~ 3S is very effective [12,13]. As a consequence, the spectra display double peak structures with the respective intensities not being representative of the composition of the incoming metastable beam.In a previous paper, we presented a new interpretation of the sudden appearance of the narrow p...

The HeI and Neo uv spectra and He Penning ionization electron spectra of coadsorbed K and CN annealed to above 800 K sho~extra features, not present in NeI and Ne spectra, that are observed at constant kinetic electron energies. Using the observed valence levels of KCNã nd the excitation energies, the extra features are showa to result from interatomic Auger transitions involving a K(3p) hole and mixed K-CN states. This strong interadsorbste interaction is attributed to the formation of molecular orbitals of a more covalently bonded KCN molecule.The K-CO (Refs. 1-5) and K-OH (Refs. 6 and 7) coadsorption systems on various transition metals have been studied very intensively. CO and CN are isoelectronic diatomics and neutral OH and CN are both radicals, motivating the measurement of CN-K coadsorption for comparison.The binding energy of the partially filled orbitals and the role of the symmetry of the lowest unoccupied molecular orbitals (LUMO's) may be investigated by comparison of CN to CO and OH. In the case of CO and OH the electronic valence structure remains essentially intact after K coadsorption. ' The alkali coadsorbate strengthens the bond to the substrate and reduces the strength of the intramolecular bond. s This is interpreted as an alkali-induced lowering of the energy of the LUMO [CO(2tr ) or OH (4a )] below the Fermi level EF, followed by partial filling of these orbitals by charge transfer from the substrate. For pure CN, on the other hand, the chemisorption bond is predicted to be mostly ionic without a dative bond involving the 2tr orbital, ' since the 2tr* level lies quite high above the Fermi level.It is the nature of an ionic bond that the ionic constituents (K+ and CN ) share no common molecular orbitals. The interatomic Auger transitions described in this paper, however, indicate that new mixed orbitals are formed when the KCN is in direct contact with the metal surface. Recent discussions of the K-CO adsorption system indirectly suggested that interatomic Auger transitions are responsible for broadening of the K(3p) level in photoemission spectra. We are able to observe such interatomic Auger transitions directly in photoemission and metastable He or Ne scattering spectra of adsorbed KCN. Penning ionization electron spectroscopy (PIES) is extremely surface sensitive, because the metastable probe atoms do not penetrate the surface and are deexcited through a tunneling process that takes place outside the sample surface. As a consequence of this sensitivity, emission from the substrate d band is suppressed, allowing the detection of adsorbate-induced emission at a low binding energy near Es. We are, therefore, able to observe all of the KCN valence levels participating in the Auger transitions.In previous studies, we have shown that adsorbed CN can be formed from the dissociative adsorption of CqN2 (cyanogen) at 300 K on Pd(100),(111)and Cu(111). "'We have reported the PIES spectra of cyanogen and CN on Pd(100), as well as ultraviolet photoemission spectroscopy (UPS) and thermal desorption...