Angular resolved Auger emission from clean and sulfur covered Ni(110) surface

The angular dependence of the nickel M₂₃VV and of the sulfur L₂₃VV Auger transitions are studied in detail, on clean and sulfur covered Ni(110) surfaces. New experimental data are presented for the anisotropy of both transitions as a function of polar and azimuthai angles of emission. Our model, which incorporates at the same time the multiple scattering effects in the final state wave function and the intrinsic anisotropy of the Auger emitter, is found to give a satisfactory account of the observed auger anisotropy. We find a large sensitivity to the position of the sulfur adsorbed atoms. The best agreement is obtained for the hollow site. slightly less than 0.9 Å above the top nickel layer. This conclusion is consistent with previous LEED and MEIS studies, but does not agree with the long bridge site obtained from quantum chemistry calculations. Moreover the sulfur emitter on this particular Ni(110) face appears to have an intrinsic anisotropy.     

Electron loss spectroscopy of clean and oxygen covered GaAs(110) surfaces

Electron energy loss spectra of clean and oxygen covered GaAs(110) surfaces have been measured with a four grid retarding field analyser. Loss spectra of clean cleaved p- and n-type surfaces are slightly different and different states of adsorption for the oxygen on the two surfaces are found. The loss peaks which are common in the spectra obtained from clean surfaces of both types of material have been interpreted in terms of bulk and surface excitations. The data associated with the bulk excitations are in good agreement with previous optical and electron transmission data while loss peaks at 11.5 and 18.5 eV are interpreted as the surface plasma loss and a surface state transition respectively. For n-type material extra loss peaks were observed. In the case of oxygen adsorption on these surfaces new loss peaks were found at 13.5, 17.2 and 28.1 eV in both spectra and are assumed to be characteristic of the oxygen. Further, for n-type material an extra peak occurs at 8.2 eV.     

The adsorption of water on clean and oxygen covered Cu(110)

The water adsorption on clean and oxygen precovered Cu(110) surfaces is studied by means of UPS, LEED, work function measurements and ELS. At 90 K on the clean surface molecular water adsorption is indicated by UPS. The H₂O molecules are bonded at the oxygen end and the H-O-H angle is increased as compared with the free molecule. In the temperature range between 90 and 300 K distorted H₂O molecules and adsorbed hydroxyl species (OH) are detected, which are desorbed at room temperature. On an oxygen covered surface hydroxyl groups are formed by dissociation of adsorbed water molecules at a lower temperature than on the clean surface. Multilayers of condensed water are found below 140 K in both cases.     

Adsorption-desorption kinetics of CO from clean and sulfur covered Ru(001)

The total uptake of CO, its adsorption kinetics and its desorption kinetics from clean and partially sulfur covered surfaces of the basal plane of ruthenium have been investigated. The method of desorption rate isotherms applied to the CO flash desorption spectra from these surfaces was used to evaluate the coverage dependence of the binding energy of CO as well as the effect of various levels of sulfur on this binding energy. Below a total surface concentration of 1 adsorbate atom per 3 surface Ru atoms, the binding energy and sticking probability of CO on the clean and sulfur covered surfaces are the same. Above this concentration of total adsorbates, the adsorption kinetics is the same on all surfaces studied, the binding energy decreases linearly with CO coverage while the magnitude of the decrease increases with sulfur coverage. The total uptake of CO depends on the amount of preadsorbed sulfur. At low coverages of sulfur, total CO uptake is effected by the excluded volume of sulfur. At higher coverages of sulfur (approaching $\text{1}\text{2}$ the maximum sulfur concentration on the clean surface) the site requirements of sulfur limits the amount of CO that can adsorb on the remaining surface, to the quantity of 1 adsorbate atom per 2 Ru atoms.     

Adsorption of CO on clean and oxygen covered Ni(111) surfaces

Adsorption of CO on Ni(111) surfaces was studied by means of LEED, UPS and thermal desorption spectroscopy. On an initially clean surface adsorbed CO forms a √3 × $\text{√3}\text{R30°}$ structure at θ = 0.33 whose unit cell is continuously compressed with increasing coverage leading to a c4 × 2-structure at θ = 0.5. Beyond this coverage a more weakly bound phase characterized by a $\text{√7}\text{2}$ × $\text{√7}\text{2R19°}$ LEED pattern is formed which is interpreted with a hexagonal close-packed arrangement (θ = 0.57) where all CO molecules are either in “bridge” or in single-site positions with a mutual distance of 3.3 Å. If CO is adsorbed on a surface precovered by oxygen (exhibiting an O 2 × 2 structure) a partially disordered coadsorbate 2 × 2 structure with θ_o = θ_co = 0.25 is formed where the CO adsorption energy is lowered by about 4 kcal/mole due to repulsive interactions. In this case the photoemission spectrum exhibits not a simple superposition of the features arising from the single-component adsorbates (i.e. maxima at 5.5 eV below the Fermi level with O_ad, and at 7.8 (5σ + 1π) and 10.6 eV (4σ) with CO_ad, respectively), but the peak derived from the CO 4σ level is shifted by about 0.3 eV towards higher ionization energies.     

LEED intensities from clean and hydrogen covered Ni(100) and Pd(111) surfaces

Hydrogen adsorbs on Ni(100) and Pd(111) surfaces without the formation of additional diffraction spots in the LEED patterns. Measurements of LEED intensities revealed that adsorbed hydrogen layers cause considerable changes even in such cases where displacements of surface atoms (“reconstructive adsorption”) may be excluded. After hydrogen adsorption on Ni(100) the intensities of Bragg beams are uniformly lowered whereas the background intensity increases which is attributed to the formation of a disordered adsorbed layer. With Pd(111) adsorbed hydrogen causes a slight decrease of the background intensity and characteristic modifications of the intensity/voltage curve of the (0,0) beam, suggesting the formation of an ordered 1 × 1 structure. In the latter case energy shifts of the primary Bragg maxima were observed and are interpreted as being caused by an expansion of the layer spacing in the surface region by about 2% owing the partial dissolution of the hydrogen.     

The adsorption and reaction of Acetonitrile on clean and oxygen covered Ag(110) surfaces

The adsorption and reaction of acetonitrile (CH₃CN) on clean and oxygen covered Ag(110) surfaces has been studied using temperature programmed reaction spectroscopy (TPRS), isotope exchange, chemical displacement reactions and high resolution electron energy loss spectroscopy (EELS). On the clean Ag(110) surface, CH₃CN was reversibly adsorbed, desorbing with an activation energy of 10 kcal mol^-1 at 166 K from a monolayer state and at 158 K from a multilayer state. Vibrational spectra of multilayer, monolayer and sub-monolayer CH₃CN were in excellent agreement with that of gas phase CH₃CN indicating that CH₃CN is only weakly bonded to the clean Ag(110) surface. On the partially oxidized surface CH₃CN reacts with atomic oxygen to form adsorbed CH₂CN, OH and H₂O in addition to forming another molecular adsorption state with a desorption peak at 240 K. This molecular state shows a CN stretching frequency of 1840 cm^-1, which is indicative of substantial rehybridization of the CN bond and is associated with side-on coordination via the π system. The CH₂CN species is stable up to 430 K, where C-H bond breaking and reformation begins, leading to the formation of CH₃CN at 480 K and HCN at 510 K and leaving only carbon on the surface. In the presence of excess oxygen atoms C-H bond breaking and reformation is more facile leading to additional desorption peaks for CH₃CN and H₂O at 420 K. This destabilizing effect of O_(a) on Ch₂CN_(a) is explained in terms of an anionic (CH₂CN^-1) species. Comparison of the vibrational spectra from CH₂CN_(a) and CD₂CN_(a) supports the following assignment for the modes of adsorbed CH₂CN: ν(Ag-C) 215: δ(CCN) 545; ϱ_t(CH₂) 695; ϱ_w(CH₂) 850; ν(C-C) 960; ϱ_r(CH₂) 1060; δ(CH₂) 1375; ν(CN) 2075; and ν(CH₂) 2940 cm^-1. These results serve to further indicate the wide applicability of the acid-base reaction concept for reactions between gas phase Brönsted acids and adsorbed oxygen atoms on solver surfaces.     

H2O interaction with clean and oxygen precovered Ni(110)

The interaction of water vapour with clean as well as with oxygen precovered Ni(110) surfaces was studied at 150 and 273 K, using UPS, ΔΦ, TDS, and ELS. The He(I) (He(II)) excited UPS indicate a molecular adsorption of H₂O on Ni(110) at 150 K, showing three water-induced peaks at 6.5, 9.5 and 12.2 eV below E_F (6.8, 9.4 and 12.7 eV below E_F). The dramatic decrease of the Ni d-band intensity at higher exposures, as well as the course of the work function change, demonstrates the formation of H₂O multilayers (ice). The observed energy shift of all water-induced UPS peaks relative to the Fermi level (ΔE_max = 1.5 eVat 200 L) with increasing coverage is related to extra-atomic relaxation effects. The activation energies of desorption were estimated as 14.9 and 17.3 kcal/mole. From the ELS measurements we conclude a great sensitivity of H₂O for electron beam induced dissociation. At 273 K water adsorbs on Ni(110) only in the presence of oxygen, with two peaks at 5.7 and 9.3 eV below E_F (He(II)), being interpreted as due to hydroxyl species (OH)^δ? on the surface. A kinetic model for the H₂O adsorption on oxygen precovered Ni(110) surfaces is proposed, and verified by a simple Monte Carlo calculation leading to the same dependence of the maximum amount of adsorbed H₂O on the oxygen precoverage as revealed by work function measurements. On heating, some of the (OH)^δ? recombines and desorbs as H₂O at ? 320 K, leaving behind an oxygen covered Ni surface.     

The kinetics and mechanism of the autocatalytic decomposition of HCOOH on clean Ni(110)

The flash decomposition of DCOOH was studied on a clean nickel (110) surface following adsorption at 37°C. The reaction proceeded by a two-dimensional autocatalytic mechanism to form D₂, CO₂ and CO products. The results indicated DCOOH adsorbed dissociatively at 37°C by splitting off H₂O and forming an adsorbed molecule composed of DCO and DCOO. Above ten percent of saturation coverage these molecules formed a condensed phase or island structure. The decomposition of the molecules was rate determining for the formation of CO₂ and D₂ products. Theoretical calculations for branched chain mechanisms and coadsorption experiments with CO and H₂ separately with DCOOH indicated the intermediate involved in the explosion was not associated with the observed product molecules. The intermediate in the explosive decomposition was shown by interrupted flashes to be stable at 37°C. The autocatalytic flash decomposition curves were explained by reaction occurring at bare metal sites within the islands, and as product molecules desorbed the number of sites increased, causing the rate to accelerate. The rate of decomposition was well described by the $\text{equation Rate = ?k(}\text{c}\text{c}{}_{\mathrm{I}}\text{)(c}{}_{\mathrm{I}}\text{?c + fc}{}_{\mathrm{I}}\text{)}$, where c is the surface concentration, c_I is the initial surface concentration, and f is the density of initiation sites. The activation energy of 26.6kcal/mol was determined from heating rate variation. The narrow flash curves were fit with a first order pre-exponential factor of 1.6 × 10¹⁵ sec^?1 with a density of initiation sites of 0.004.     

Surface science studies of cobalt overlayers on clean and sulfur covered Mo(100) single crystal surfaces

The interaction of cobalt with clean and sulfur covered Mo(100) surfaces was investigated with Auger electron spectroscopy (AES), low energy electron diffraction (LEED) and temperature programmed desorption (TPD). On the clean surface, the deposition and subsequent annealing of one monolayer of cobalt resulted in the formation of an ordered overlayer with (1 × 1) surface structure. When cobalt was deposited on sulfur covered Mo(100) surfaces, after annealing the sulfur overlayer migrated on top of the cobalt layer. This topmost sulfur overlayer did not significantly affect the thermal desorption of cobalt from the Mo(100) surface. Various ordered structures of sulfur, cobalt and coadsorbed sulfur and cobalt were observed by LEED. A new surface structure showing (3 × 1) symmetry was observed when at least one monolayer of cobalt was deposited and annealed at 870 K on an ordered monolayer of sulfur on the Mo(100) surface. This surface structure was stable in ultrahigh vacuum up to 940 K.     

The adsorption and reaction of H2O on clean and oxygen covered Ag(110)

The adsorption and reaction of water on clean and oxygen covered Ag(110) surfaces has been studied with high resolution electron energy loss (EELS), temperature programmed desorption (TPD), and X-ray photoelectron (XPS) spectroscopy. Non-dissociative adsorption of water was observed on both surfaces at 100 K. The vibrational spectra of these adsorbates at 100 K compared favorably to infrared absorption spectra of ice Ih. Both surfaces exhibited a desorption state at 170 K representative of multilayer H₂O desorption. Desorption states due to hydrogen-bonded and non-hydrogen-bonded water molecules at 200 and 240 K, respectively, were observed from the surface predosed with oxygen. EEL spectra of the 240 K state showed features at 550 and 840 cm^?1 which were assigned to restricted rotations of the adsorbed molecule. The reaction of adsorbed H₂O with pre-adsorbed oxygen to produce adsorbed hydroxyl groups was observed by EELS in the temperature range 205 to 255 K. The adsorbed hydroxyl groups recombined at 320 K to yield both a TPD water peak at 320 K and adsorbed atomic oxygen. XPS results indicated that water reacted completely with adsorbed oxygen to form OH with no residual atomic oxygen. Solvation between hydrogen-bonded H₂O molecules and hydroxyl groups is proposed to account for the results of this work and earlier work showing complete isotopic exchange between H₂¹⁶O_(a) and ¹⁸O_(a).     

The binding energy of CO on clean and sulfur covered platinum surfaces

The desorption of CO from clean Pt(111) and (100), and from the same surfaces with partial overlayers of sulfur, was studied by Thermal Desorption Spectroscopy. The method of desorption rate isotherms was employed for data analysis. The desorption of CO from the (111) surface and both surfaces with ordered sulfur overlayers can be described as a first order process with coverage dependent activation energies. The desorption of CO from the clean Pt(100) surface is complicated by the dynamic interaction of the molecule with a thermally activated change of platinum surface structure. On both platinum faces surface sulfur decreases the initial binding energy of CO. As the CO concentration increases, its binding energy decreases very rapidly. This is due to a repulsive interaction which exists between co-adsorbed species.     

Adsorption of ethylenediamine on clean and oxygen covered Fe/Ni(100) surfaces studied by XPS

The interaction of ethylenediamine with Fe/Ni(100) surfaces oxidized to various extents has been studied in the temperature range 260–450 K by means of X-ray photoelectron Spectroscopy. The use of ~ 1 monolayer of Fe enables us to characterize the oxidation states of the topmost layer atoms unambiguously, based on the XPS spectra using a conventional spectrometer. On clean and c(2 × 2)-O surfaces the ethylenediamine can dissociate the N-H bond at 260 K. On heating the adlayer to 340 K the dissociation was further developed. On the surfaces whose Fe atoms were oxidized to FeO/Ni(100) and further, only molecularly adsorbed species were present at 260 K and desorbed partly without dissociation of the N-H bond after heating to 340 K.     

Trapping probabilities and desorption energies of alkali atoms on a clean and an oxygen covered tungsten (110) surface

Alkali atoms were scattered with hyperthermal energies from a clean and an oxygen covered (θ ≈ 0.5 ML) W(110) surface. The trapping probability of K and Na atoms on oxygen covered W(110) has been measured as a function of incoming energy (0–30 eV) and incident angle. A considerable enhancement of trapping on the oxygen covered surface compared to a clean surface was observed. At energies above 25 eV there are still K and Na atoms being trapped by the oxygen covered surface. From the temperature dependence of the mean residence time τ of the initially trapped atoms the pre-exponential factor τ₀ and the desorption energy Q were derived using the relation: $\text{τ = τ}{}_0\text{exp}\text{(}\text{Q}\text{kT}{}_{\mathrm{<mtext>s</mtext>}}\text{)}$. On clean W(110) we obtained for Li: $\text{τ}{}_0\text{= (8 ±}\text{8}\text{4}\text{) × 10}{}^{\mathrm{?14}}\text{sec}$, Q = (2.78 ± 0.09) eV; for Na: τ₀ = (9 ± 3) × 10^?14 sec, Q = (2.55 ± 0.04) eV; and for K: τ₀ = (4 ± 1) × 10^?13 sec, Q = (2.05 ± 0.02) eV. Oxygen covered W(110) gave for Na: τ₀ = (7 ±3) × 10^?15 sec, Q = (2.88 ± 0.05) eV; and for K: $\text{τ}{}_0\text{= (1.3 ±}\text{0.9}\text{0.6}\text{) × 10}{}^{\mathrm{?14}}\text{sec}$, Q = (2.48 ±0.05) eV. The adsorption on clean W(110) has the features of a supermobile two-dimentional gas; on the oxygen covered W(110) adsorbed atoms have the partition function of a one-dimen-sional gas. The binding of the adatoms to the surface has a highly ionic character in the systems of the present experiment. An estimate is given for the screening length of the non-perfect conductor W(110):k_s^?1≈ 0.5 Å.     

Electron ejection by helium metastable atoms incident on the clean and chalcogen covered Ni(100) surface

An intense, essentially photon free, helium metastable beam has been used to cause electron ejection from the clean, oxygen and sulphur covered Ni(100) surface, in a system equiped with an AES facility to monitor surface cleanness. The ejected electron energy distribution obtained for the clean Ni(100) surface is narrow and peaked at ～2 eV, unlike the distribution obtained from INS studies, and consequently indicates different de-excitation mechanisms for incident ions and excited atoms. The ejected electron distribution from the adsorbate covered surface is also narrow, but peaked at ～1 eV with structure which is essentially independent of the nature of the adsorbate. The yield of ejected electrons is found to increase linearly with coverage of both oxygen and sulphur, in contrast to the results obtained from INS. These data indicate that Auger neutralization does not occur at the surface; the possibility of Auger de-excitation is considered.     

Comparison of Low-Energy Neutral Scattering (LENS) with Low-Energy Ion Scattering (LEIS) at clean and adsorbate covered Ni surfaces

Ne and He projectiles were scattered at clean and sulphur or oxygen-covered polycrystalline Ni surfaces. The analysis of backscattered neutrals results in an oxygen to nickel surface atom density of 0.7 for NiO and a sulphur to nickel density of 0.25 for a saturation coverage of sulphur on polycrystalline nickel, which is in good accord with expectation. Only differential scattering cross-sections, which can be calculated with sufficient reliability enter the data reduction. This is a definite advantage over ion scattering which is complicated by the widely unknown neutralization probabilities.