Submonolayer Coverage Regime

3 Structural and Electronic Properties of Rare Earth Metal Systems 

LEED pattern as well as its interpretation is presented in the Fig. 3.18c and d, respectively. The atomic distances as determined from the diffiraction pattern are 9.48 A in the [001] direction and 4.47 A in the [110] direction of the substrate. 

A part of an atomically resolved STM image is inserted in the structure model at the same scale. Reprinted with permission from [38]. Copyright (1999) by the American Physical Society 

Comparing the structure model with the atomic scale STM images it becomes clear that one does only see one atomic species of the alloy, the Gd atoms. The fact that the Gd atoms are much larger than the Fe atoms may play a major role but a definite statement concerning this point can only be given if LDOS calculations for this structure will become available. 

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Fig. 3.16 Comparison of the growth of a Fe and b Gd on W(110) in the submonolayer coverage regime. For both metals the coverage is about 0.25 MF. The scan range for both images is 70 nm X 70 nm. Below the STM images structure models are shown to highlight the difference in the growth mode. Reprinted with permission from [38]. Copyright (1999) by the American Physical Society...

As the existence of this review demonstrates, the various experimental problems, described above, have not discouraged everybody from attempting to perform structural determinations of adsorbates on metal oxide surfaces. The following sections of this paper detail such work. It is structured such that there is a separate section on each metal oxide for which quantitative structural determinations of adsorbates have been carried out. We have restricted ourselves to only those studies involving monolayer/submonolayer adsorbate coverages, but have included atoms, molecules and metal adsorption in this coverage regime. 

Adsorption of Li, which is known to form bulk alloys with Al, also leads to the formation of surface alloys which are, perhaps surprisingly, similar to those formed by the heavier alkalis, at least at submonolayer coverages. However, Li adsorption in general continues beyond the submonolayer regime. For this system, the studies of surface alloys shed new light on the structure of the metastable AlsLi bulk alloy. 

The x-ray data [242] indicate that the orientational transition within the compressed monolayer at a coverage of about 1.17 + 10% occurs near 35 K and is completed at about 48 K (see Fig. 37) that is, it occurs at a higher temperature than in the submonolayer commensurate regime. This transition... 

Although the final symmetry cannot be deduced only from the spectroscopic data, it is important to stress that the spectroscopic data supply information on the strength of the chemical bonding [48] and on the effect of the applied electric field [55, 56]. Moreover, in situ STM data have been reported only for a full monolayer, but not for submonolayer coverage. The in situ FTIR is sensitive also in the submonolayer regime. By appropriate calibration (through the full monolayer as determined by in situ STM), it is possible to determine the adsorption isotherm, even for metals for which it is very difficult to obtain capacitance data. 

The data recorded ranges from the submonolayer to the multilayer coverage regime and is compared the gasphase PE spectra of TCE [5] (see Appendix Sect. A.3.5). For interpretation of the data the previously mentioned data treatment procedures (Sects. 2.2.3 and A.1.5) are applied with successful extraction of previously superimposed features in the spectra. 

Most microscopic theories of adsorption and desorption are based on the lattice gas model. One assumes that the surface of a sohd can be divided into two-dimensional cells, labelled i, for which one introduces microscopic variables Hi = 1 or 0, depending on whether cell i is occupied by an adsorbed gas particle or not. (The connection with magnetic systems is made by a transformation to spin variables cr, = 2n, — 1.) In its simplest form a lattice gas model is restricted to the submonolayer regime and to gas-solid systems in which the surface structure and the adsorption sites do not change as a function of coverage. To introduce the dynamics of the system one writes down a model Hamiltonian which, for the simplest system of a one-component adsorbate with one adsorption site per unit cell, is... 

Figure 3.19 shows a series of UPS spectra obtained from a Ru(001) substrate on which small amounts of Ag or Au have been evaporated. The spectrum of the clean Ru(001) substrate, being a d-metal, exhibits a high density of states at the Fermi level. The Ru 4d band is seen to extend to about 5 eV below the Fermi level. Silver and gold, on the other hand, are s-metals, with a low density of states at the Fermi level, as illustrated by the spectrum of the thick layers (top spectra of Fig. 3.19). The d-band falls between 4 and 8 eV below the Fermi level. The other spectra show what happens if the Ag or Au coverages increase submonolayers to a thick film. Photoemission from the Ru substrate is rapidly attenuated, while the d-band of Ag develops from narrow states in the submonolayer regime, via progressively broader bands at coverages between one to four ML, until the energy...

The heat capacity peaks obtained by adiabatic calorimetry [156] using Grafoil GTA show maxima at slightly lower temperatures in the submonolayer regime at about 27.5 0.1 K from 0.169 up to 1.000 monolayers. A vdue of 28.7 K was reported beyond the complete monolayer at a coverage... 

Before discussing compressed monolayers with their even richer behavior than the submonolayer regime we note that the experimental determination of the coverage gets more difficult due to possible second-layer adsorption [90, 93]. Thus, the coverages marking a phase boundary may differ between various experimental techniques and groups (see the discussion in Section I.C). 

Orientational ordering transitions [394] clearly occur in the commensurate phase (possibly in coexistence with the fluid) up to 1.101 monolayers at 28.1 K, as well as in the uniaxial compressed phase from 1.142 to 1.178 monolayers at 29-30 K based on adiabatic calorimetry [156] a heat capacity peak at 28.7 K was reported at a coverage of 1.109 which was interpreted as the transition to the uniaxially compressed incommensurate phase, whereas this peak is located near 27.5 K in the submonolayer regime. The ac heat capacity scans [394] in the triangular incommensurate phase at 1.200 and 1.302 monolayers show only very weak and broad features in the relevant temperature range from 22 K to 32 K. Thus, it is concluded [394] that their orientational order is only gradually lost as the temperature is increased... 

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