M. Kiskinova

Oxygen adsorption on a Pd(111) surface

The interaction of oxygen with Pd(111) has been studied by means of AES, ELS, thermal desorption (TD), electron stimulated desorption (ESD) and work function measurements. It was found that a very small part ( ∼ 2–3%) of the available adsorption sites are contributing to the O+ electron stimulated yield, the population of the latter being accompanied by enormously large work function changes (up to ∼ 0.9 eV). A mechanism of adsorption and depopulation of these sites involving oxygen bulk and surface diffusion has been proposed.

Interaction of oxygen with an alkali modified Ru(001) surface

Abstract The interaction of oxygen with potassium predosed Ru(001) has been studied by means of thermal desorption (TD), Auger electron spectroscopy (AES), electron energy loss spectroscopy (EELS) and work function (WF) measurements. These results were compared with those reported earlier on Na and Cs precovered Ru(001). It was found that the initial sticking coefficient, S 0 , increased linearly with alkali coverages θ am , in the sequence Na-K-Cs. The TD, WF and EELS data indicated predominance of the strong oxygen-ruthenium interactions over the alkali-oxygen interactions at low potassium and oxygen coverages (θ K and θ O , respectively). Above certain θ K and θ O (θ K > 0.12 and θ O > 0.6), the coincident O 2 , K and K 2 O TD peaks at 890 K, the EL feature at ~ 28 eV, and the appearance of a shoulder in the WF I–V curves were interpreted as the formation of K-O-Ru complexes gathered in patches on the Ru(001) surface. This behaviour was common for the three alkali metals when coadsorbed with oxygen.

Interaction of oxygen with a Cs-covered Si(111)7 × 7 surface

Abstract The influence of submonolayer Cs coverages, θCs, on Si(111)7 × 7 at varying surface temperatures, Ts, on the initial oxygen uptake and Si oxide growth has been studied by means of thermal desorption, work function and Auger electron spectroscopy. The experimental data have shown that the presence of Cs always causes an increase of the oxygen initial uptake whereas promotion of the Si oxide growth is observed only at θCs above half a monolayer. It was found that in the presence of Cs the oxygen initial sticking coefficient becomes independent of Ts (in the range 300–700 K) and the oxygen uptake rate remains constant up to a certain oxygen coverage. The value of this oxygen coverage increases with increasing θCsand Ts. Experimental evidence is given from the thermal desorption data that at θCs > 0.5, Cs participates in the formation of an intermediate with oxygen which is thermally stable at temperatures below 500 K. The mechanism of the Cs-promoted Si oxide growth and the conditions when Cs can act as a catalyst for Si oxidation are discussed.

Adsorption of CO on Ru(1010) doped with different amounts of K and Cs

Abstract The interaction of CO with K and Cs doped Ru(10 1 0) has been studied by means of Auger electron spectroscopy (AES), thermal desorption (TD), electron energy loss spectroscopy (EELS), LEED and work function (WF) measurements. It was found that both alkali metals (AM) induce similar changes in the CO adsorption kinetics and energetics, with preferential filling of the promoted sites. The initial heat of CO adsorption, Δ H CO 0 , associated with promoted CO ∗ , increases linearly from 145 up to 224 kJ/mol with increasing alkali coverage, θ AM , the maximum Δ H CO 0 value being reached when coincident CO and AM desorption is detected. LEED and WF data for coadsorbate systems indicate formation of patches with different WF and compositions on the surface. EELS results show that the presence of AM causes an energy shift of the CO 2 π b → 2 π a transitions from 6.3 to ∼7.0 eV, removal of the AM ns valence transitions and plasmon excitations and appearance of new loss structures. Comparing the present data with the results on CO adsorption on Na doped Ru(10 1 0) the type and contribution of the possible direct COue5f8AM interactions are discussed.

Interaction of oxygen with Si(111) 7 × 7 and Si(100) 2 × 1 surfaces modified with small amounts of Cs

Abstract The effect of small amounts of Cs (∼ 0.2 of the saturation Cs coverage at 300 K) on the oxidation of Si(111)7 × 7 and Si(100)1 × 2 at reaction temperatures of 300–700 K has been studied by means of Auger and electron energy loss (EEL) spectroscopies, thermal desorption and work function (WF) measurements. It has been found that at these low Cs coverages only the first stage of the oxidation process — the initial oxygen uptake — is affected. The result is: (i) an increase and temperature independence of the initial sticking coefficient, S0; (ii) coverage independence of S upto certain (reaction temperature determined) oxygen coverages. EEL and WF data have shown that at 300 K oxygen occupies preferentially Cs promoted sites. The liberation of oxygen from this Cs influenced state in order to form usual Si-O bonding configurations takes place at temperatures exceeding 400 K as evidenced by the rise of sharp SiO related ∼ 5 and 7 eV loss peaks and reduction of the work function. Thermal desorption data showed that the oxidation of Si leads to an appearance of a higher temperature desorption state of Cs. Because of this oxygen induced stabilization of the Cs layer the removal of Cs requires annealing the mixed layer at 950 K. At oxygen coverages less than 1 ML this annealing procedure led also to complete SiO desorption.

Electron-stimulated and thermal desorption studies on palladium: II. Carbon monoxide

Abstract Chemisorption of CO on a Pd ribbon has been studied using a combination of electron-stimulated and thermal desorption methods. From the thermal desorption spectra the desorption energy was evaluated as a function of coverage. Electron bombardment of the Pd surface covered with CO produced an O + ion current whose intensity depended on the CO coverage. The observed changes in the adsorption energy and in the ion current with increase in coverage were interpreted on the basis of a lateral interaction and the Blyholder model for a chemisorption bond between CO and a transition metal.

Co adsorption on Ru(101̄0) modified with different amounts of Cu

Abstract The adsorption of CO at 300 K on Ru(1010) modified with different amounts of Cu has been studied using Auger electron spectroscopy (AES), thermal desorption (TD), electron energy loss spectroscopy (EELS), LEED and work function (WF) measurements. It was found that the effect of the coadsorbed Cu on the CO adsorption kinetics and energetics depends strongly on the actual Cu surface site occupation and coverage, θ Cu . The presence of Cu up to θ Cu = 1 ML was found to cause no changes in the CO initial sticking coefficient, S 0 , and initial dipole moment, but reduces the CO saturation coverage. At θ Cu > 1 ML, both the CO initial sticking coefficient and the initial dipole moment are reduced, accompanied with a complete removal of the CO adsorption states at 300 K, starting with the most tightly bound one. The observed behavior of CO on Cu/Ru(1010) was discussed in the framework of the precursor state model for CO adsorption and ensemble and ligand effects on modified surfaces.

Electron-stimulated and thermal desorption studies on palladium: I. Oxygen

Abstract Chemisorption of oxygen on a Pd ribbon has been studied by means of thermal and electron-stimulated desorption mass spectrometry. The results of combined thermal desorption (TD) and electron-stimulated desorption (ESD) experiments indicated the existence of two adsorption states, the more tightly bound one being responsible for the ESD signal. After resolving the desorption curves into two single desorption traces, the desorption energies of the two states were derived. An attempt was made to explain the observed dependence of oxygen coverage and ESD current intensity on adsorption temperature.

Temperature effects on the oxidation kinetics of clean and cesiated Si(111) and Si(100) surfaces

Abstract We present new experimental results from a comparative study of the oxidation kinetics of the clean, and modified with 0.2 ML Cs, Si(111) and Si(100) surfaces at substrate temperatures ranging from 300 to 900 K. The observed difference in the temperature dependence of the oxygen sticking coefficient and the oxygen saturation coverage for clean and cesiated Si surfaces illustrates that the presence of small amounts of Cs affects in a diverse manner the main stages of the oxidation process — the initial sticking coefficient characterizing the relatively fast initial uptake is enhanced and becomes temperature independent whereas the following slow oxide growth characterized by the achieved saturation oxygen coverage remains unaffected and becomes less temperature dependent.

Auger electron spectroscopy and mass spectroscopy studies on hydrogenation of graphite in the presence of nickel and tungsten

Hydrogenation of polycrystalline graphite in the presence of nickel and tungsten was studied by means of Auger electron Spectroscopy and mass spectroscopy at temperatures up to 850K and a hydrogen pressure ranging from 1.10−8 to 5.10−6 Torr. The changes in the carbon Auger lineshape with increasing metal surface concentration revealed a tendency to formation of a carbide phase, the latter being much stabler in the case of tungsten. The mass spectrometric studies of the interaction of hydrogen with the metal graphite systems showed the formation of CH4 at temperatures higher than 750K. The electronic and adsorption properties of the metal-graphite systems and the formation of active surface “carbide” carbon were considered in explaining the different catalytic activities of nickel and tungsten.