W. Sesselmann

The interaction of cesium with oxygen

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.

Low temperature formation of benzene from acetylene on a Pd(111) surface

Abstract Experiments with UPS, metastable noble gas deexcitation spectroscopy (MDS) and thermal desorption demonstrated that C2H2 adsorbed on Pd(111) at 140 K undergoes cyclotrimerisation to C6H6 after higher (≳ 100 L) exposures. If the surface is intermediately warmed up to 300 K, the low temperature state of adsorbed acetylene transforms irreversibly into another species which is unreactive. The surface species formed by reaction was identified by comparison with the electron spectroscopic data of C6H6 adsorbed from the gas phase as well as with those of free C6H6. The molecules are only weaky held on the surface and start to desorb already at about 150 K.

Electron spectroscopy of surfaces by impact of metastable He atoms: CO on Pd(110)

Abstract Deexcitation of metastable He∗ 2 1 S (excitation energy E ∗ = 20.6 eV) or 2 3 S ( E * =19.8 eV) atoms at a clean Pd(110) surface proceeds through a two-stage process (resonance ionization + Auger neutralization, RI + AN). The measured electron energy distribution reflects the self-convolution of the local density of states of the outmost atomic layer. A CO adlayer suppresses the RI step and the spectra are caused by Auger deexcitation (Penning ionization). Comparison with corresponding UPS data allows identification of the valence orbitals of the adsorbate. Emission up to the Fermi level is ascribed to contributions from the 5σ level. The effectively available excitation energy in front of the adlayer is lowered by 0.5 eV. Extensive data on the variation of the intensities from the adsorbate valence levels with angle of incidence as well as of emission are presented and are analyzed in terms of an empirical model.

Scattering of metastable rare gas atoms from solid surfaces

This work reports on measurements of the survival probabilities (SP) and angular distributions of metastable He atoms backscattered from clean and adsorbate covered metal surfaces, as well as on data for He∗, Ne∗ and Ar∗ reflected from LiF and NaCl surfaces. The SP for He∗ are always below ∼ 10−3, they increase for the clean metal surfaces in the order Cu<Pd<W, as well as upon adsorption of CO or oxidation. Equally small values (∼10−4) were found for He∗ and Ar∗ reflected from the ionic crystals, while a considerably higher value (∼ 10−2) was determined for Ne∗ in this case. The angular distributions vary considerably from those of the respective ground state atoms which has to be attributed to the marked change of the interaction potentials.

Reflection of metastable He atoms from solid surfaces

Abstract Survival probabilities of metastable He atoms scattered from clean and adsorbate-covered Pd, Cu, and W surfaces were found to be only of the order between 10 −6 and 10 −3 . A novel theoretical formalism is presented which on the basis of a “dimensionality effect” is able to qualitatively explain the experimental observations.

Deexcitation of Metastable He-Atoms Interacting with Clean and Adsorbate Covered Metal Surfaces

Metastable, electronically excited He-atoms impinging at clean or adsorbate covered metal surfaces are deexcited with a high probability causing electron emission. The deexcitation either proceeds via a combined resonance-ionization + Auger neutralization or a Auger-deexcitation (Penning-ionization) mechanism. It is shown that the latter mechanism dominates at geometrically shielded (adsorbate covered) or electronically shielded (low work function) surfaces.