Absolute Surface Coverage Research Articles

Barium on (110) tungsten

The W(110)-Ba system studies were undertaken to determine the essence of structure transformation stages in monoatomic layers. The first structure observed to form at the initial condensation stage ( n < 1 × 10 14 atoms/cm 2) is an incoherent hexagonal film, while the following structures at higher coverages are coherent. A detailed analysis of these data allows one to conclude that the variations in potential of the adatom (potential relief) are a function of the absolute surface coverage. The work function, thermal desorption energy and surface plasma losses are also presented and discussed. Linear segments were found on the work function versus coverage curve to be correlated with the structure transformations. The work function minimum is believed to be closely related to the appearance of metallic bonding.

Electron spectroscopic study of nitrogen species adsorbed on copper

The interaction of a number of nitrogen containing molecules (NO, NH3 and N2H4) with copper has been investigated by electron spectroscopy. Nitric oxide is shown to be molecularly adsorbed in a linearly bonded configuration at 85 K; this adlayer is unstable above 120 K, resulting in an exclusively oxygen adlayer at 295 K, the nitrogen being desorbed. At 85 K N(1s) peaks observed at 401 and 406 eV are associated with NOδ–(ads) and NOδ+(ads) species, the former dissociating and the latter desorbing on thermal activation of the adlayer.Interaction of nitric oxide with Cu at 295 K results in dissociative chemisorption with both oxygen and nitrogen retained at the surface. The surface concentration suggests that each nitrogen and oxygen adatom is bridge-bonded to two surface copper atoms. Estimates of absolute surface coverages are made by comparing the intensities of the N(1s) and O(1s) peaks with the Cu(2p) substrate intensity, while valence level spectroscopy (He I and II radiation) supplements core-level spectroscopy for discriminating between molecular and dissociated surface species.Ammonia dissociates at 295 K on copper while with oxidized copper the surface oxide is replaced by an imide-type surface. With hydrazine, adsorption is molecular over the temperature range 85 to 295 K, in contrast to our observations with iron.

Simultaneous determination of structure, absolute surface coverage, and nucleation kinetics: Cd on Ge(111)

The feasibility of performing simultaneous RHEED–mass-spectrometric-molecular-beam experiments is demonstrated with a study of the nucleation of Cd on a contaminated Ge(111) surface. The crossed-beam experimental geometry and the scheme for determining absolute adatom density is described. The nucleation mechanism is found to have the appearance of a two-dimensional gas-solid phase transformation. The critical adatom density was found to be dependent on the level of contamination. The onset of clustering was found to correlate well with the MSMB spectra and clusters approaching critical dimensions were observed.

Adsorption of CO on the (110) plane of tungsten; temperature dependence of the sticking coefficient and absolute surface coverages

Absolute coverages and sticking coefficients as functions of both gas and surface temperatures are presented for CO adsorption on the (110) plane of tungsten. For a low temperature layer the CO/W ratio at saturation is 1.1, while that for alpha or beta adsorption is 0.5, indicating some crowding in the low temperature phase at high coverage. Initial sticking coefficients drop with increasing gas or surface temperature but eventually level off at ~ 0.5. Qualitative reasons for this behavior are given.

Correlation of fractional-monolayer oxygen determinations obtained by proton-excited x-ray and Auger electron spectroscopy analysis of Fe surfaces

Oxygen Kα x rays excited by the impact of 120-keV protons have been used to determine absolute oxygen surface coverages ranging from 4.8×1014 to 5.2×1015 atoms/cm2 on polycrystalline iron samples mounted in an Auger spectroscopy system. Amplitudes of the electron-beam-excited oxygen KL2L2 Auger electron spectra obtained from the samples corresponded linearly with the coverages determined by the proton-excited x rays (PEX). Estimates of the oxygen adatom coverages using the Auger results and the theoretical approach of Bishop and Riviere were found to agree within a factor of 2.15 (± 0.22) with the PEX-determined coverages.

Preferential Chemisorption on Crystalline Surfaces—Cu/O System

ALTHOUGH it has long been appreciated that chemisorption is an anisotropic property of the crystalline substrate, few systematic studies have been reported. This communication reports a recent study in which absolute surface coverages of oxygen on copper have been measured as a function of crystallographic orientation and related to a model of surface structure.

Structure analysis of alkali metal adsorption on single crystal nickel surfaces

Studies of alkali metal adsorption on single crystal Ni surfaces were undertaken to determine surface crystallography, electronic structure and atomic binding in these adsorption systems as a function of absolute surface coverage. A detailed analysis of surface crystallography using low energy electron diffraction (LEED) is presented and discussed in this paper. The work function and thermal desorption energy studies will be presented elsewhere. Significant information concerning the nature of surface forces was deduced from the surface crystallography. It is shown that alkali metal adatoms on Ni repel one another at all coverages forming a single layer. Furthermore, the forces between alkali metal adatoms on Ni (110) are highly anisotropic. These non-classical anisotropic forces are probably related to the indirect inter-adsorbate interactions via the metal's conduction electrons treated by Grimley 6). The strong repulsive forces between adsorbed alkali metal atoms on Ni (111) and Ni (100) resulted in the adatoms being uniformly spaced. By uniformly spaced, it is meant that the standard deviation of the average distance between nearest neighbor adatoms is small. Incoherent hexagonal close-packed surface structures are demonstrated to form with Na on Ni (111) and Ni (110) at high coverages in order to maximize the packing of adatoms. The distance between nearest neighbor adatoms in these structures decreases continuously with coverage. The existence of these incoherent structures shows that the repulsive forces among alkali metal adatoms on Ni (111) and Ni (110) are more important in determining their positions at high coverages than are the variations in potential of the adatom due to substrate surface geometry. The distance between nearest neighbor alkali metal atoms adsorbed on Ni at one physical monolayer coverage is shown to be less than the diameter of bulk alkali metal atoms. The occurrence of a reduced alkali adatom diameter coupled with the existence of incoherent, close-packed surface structures raises significant uncertainty about the validity of conventional calculations of surface roughness factors from the density of adsorbed alkali metal atoms.