Abstract
Xenon adsorption
Xenon adsorbs on Ru(10-10) at 30 K in 3 different ordered phases within the monolayer range. First a (3x1)-phase with ΘXe/Ru= 0.33 grows, followed by a (2x5)-phase with ΘXe/Ru=0.5 and after that an uniaxially commensurate phase develops until Xe/Ru ΘXe/Ru=0.65 (= 1 ML). As on Ni(110) the Xe atoms thereby take the shortest next neighbour distance of 4.16 Å, which was found with surface adsorption by xenon so far. In contrast to previously published multilayer data of other Xe systems showing a fcc(111) face of the Xe crystal, multilayer adsorption of Xe on Ru(10-10) is characterized by a fcc(100) face at least up to the 5th layer.
The change in the work function of 1.3 eV for the monolayer is strong, although the density compared with Xe monolayers on other surfaces is not particularly high. However, no direct referring to chemical contributions in the interaction with the substrate could be found. A separation of the entire interaction into a vertical portion between substrate and adsorbate and into a lateral portion between the adparticels is not possible.
The electronic bandstructure of the monolayer of Xe/Ru(10-10) shows two-dimensional character. This statement could not be met alone due to the LEED phases and the deduced geometrical structure. Although the individual Xe atoms are strongly compressed within the Xe chains, the Xe chains are further separated in the monolayer as with unreconstructed surfaces with smaller lattice constant c earlier examined. Hence, the bandwidth of the laterally interacting Xe5p levels is comparatively small.
As an outlook for resuming investigations concerning the dimensionality of Xe adsorption the strongly anisotropic rhenium (10-10)-surface could serve, which has a still larger lattice constant c of 4.46 Å compared to Ru(10-10) so that the overlap of the 5p-levels will be still smaller. This system could supply a further intermediate point between the one-dimensional bandstructure of Xe/Pt(110) and the two-dimensional bandstructure of Xe/Ru(10-10) and other systems. On the other hand, the (3x1)-phase documented in this work is an already ideal 1D-system, which certainly gives an accurately fixed Xe-Xe distance of 4.282 Å in [0001]-direction and 8.118 Å in [1-210]-direction that can not be tuned by the crystal temperature or the coverage.
Oxygen adsorption
With oxygen adsorption on Ru(10-10) at low temperatures followed by annealing to 600 K a previously unknown c(2x8)-phase came to light, which is either due to the influence of oxygen - dissociated during adsorption - in the subsurface region or to the onset of oxide formation. However, because of the technical difficulty to desorb atomic oxygen at high temperatures with high rates on the one hand and to measure low temperatures with the given equipment and sufficient accuracy on the other hand an experimental proof could not be furnished so far. Still higher oxygen concentrations than within the c(2x8)-phase lead to striped (2xn)-structures, which are known from other transition metal surfaces as oxidic phases.
Oxygen exposure at temperatures above 850 K led to a (2x1)pg-phase, which like the (2x1)pg2O-phase prepared at 300 K exhibits a glide mirror plane and contrary to this phase is stable up to 1000 K. Upon cooling below 250 K this phase reversibly converts into an already well-known p(2x1)-phase.
Molecularly physisorbed oxygen could be observed on the Ru(10-10)-surface as on many other transition metal surfaces only after passivation of the surface with atomic oxygen. No molecularly chemisorbed oxygen species were found. While the IPE data at the well-known oxygen phases prepared at 300 K do not exhibit significant changes with respect to the clean surface, the situation changes at 30 K. With IPE the first, vacant ?-orbital of the molecularly physisorbed oxygen was proven. Polarization-dependent measurements at the synchrotron showed that the molecule axis is oriented in [1-210]-direction of the substrate.
Using LEED, an ordered phase of molecular oxygen was observed, which shows an accurately hexagonal arrangement and exhibits an O2-O2 distance of 2.8 Å. This phase is destroyed by electron-induced dissociation and electron-stimulated desorption within a few seconds and leads to a disorder of the oxygen molecules in the monolayer range. Also longer UV irradiation within a few minutes led to the reduction of the signal intensities in the photoemission spectra.
Since with the use of UV radiation or with electron bombardment a non destructive investigation of the molecularly physisorbed and the condensed oxygen layers is not possible, a next step in the investigation of the O2/O/Ru(10-10)-System could be a time-resolved EELS spectrometry of the decay of the O2 molecule and the development of atomic oxygen states. |
