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.     

Adsorption behavior of H2O on clean and oxygen precovered Ni(s)(111)

Photoelectron spectroscopy (UPS), thermal desorption spectroscopy (TDS), isotope exchange experiments, work function change (δφ) and LEED were used to study the adsorption and dissociation behavior of H₂O on a clean and oxygen precovered stepped Ni(s)[12(111) × (111)] surface. On the clean Ni(111) terraces fractional monolayers of H₂O are adsorbed weakly in a single adsorption state with a desorption peak temperature of 180 K, just above that of the ice multilayer desorption peak (T_m = 155 K). In the angular resolved UPS spectra three H₂O induced emission maxima at 6.2, 8.5 and 12.3 eV below E_F were found for θ ≈ 0.5. Angular and polarization dependent UPS measurements show that the C_2v symmetry of the H₂O gas-phase molecule is not conserved for H₂O(ad) on Ni(s)(111). Although the Δφ suggest a bonding of H₂O to Ni via the negative end of the H₂O dipole, the O atom, no hints for a preferred orientation of the H₂O molecular axes were found in the UPS, neither for the existence of water dimers nor for a long range ordered H₂O bilayer. These results give evidence that the molecular H₂O axis is more or less inclined with respect to the surface normal with an azimuthally random distribution. H₂O adsorption at step sites of the Ni(s)(111) surface leads in TDS to a desorption maximum at T_m = 225 K; the binding energy of H₂O to Ni is enhanced by about 30% compared to H₂O adsorbed on the terraces. Oxygen precoverage causes a significant increase of the H₂O desorption energy from the Ni(111) terraces by about 50%, suggesting a strong interaction between H₂O and O(ad). Work function measurements for H₂O+O demonstrate an increase of the effective H₂O dipole moment which suggests a reorientation of the H₂O dipole in the presence of O(ad), from inclined to a more perpendicular position. Although TDS and Δφ suggest a significant lateral interaction between H₂O+O(ad), no changes in the molecular binding energies in UPS and no “isotope exchange” between ¹⁸O(ad) and H₂¹⁶O(ad) could be observed. Also, dissociation of H₂O could neither be detected on the oxygen precovered Ni(s)(111) nor on the clean terraces.     

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.     

An adsorption-desorption study of Cu on Mo(110)

Thermal desorption spectroscopy (TDS), adsorption-desorption equilibrium (ADE), adsorption transient (AT) and desorption transient (DT) measurements are used to study the adsorption and condensation of Cu on Mo(110). Fitting the ADE results with various statistical-mechanical models gives good agreement with the desorption energies derived from the TDS, AT, and DT measurements. At coverages above two monolayers (ML) three-dimensional nucleation is seen in the kinetic measurements.     

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.     

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.     

Thermal behavior of the roughened Cu(110) surface

The roughened Cu(110) surface was prepared by annealing the clean surface at various temperatures ranging from 700 to ∼1000 K. A significant drop in intensity of reflection anisotropy spectroscopy (RAS) peak at 2.1 eV photon energy as a function of increasing sample temperature was found for annealing above the roughening transition at 900 K. The observed change of 2.1 eV peak in RAS spectra is because of the surface state Fermi level shift due to temperature change. The RAS result is in good agreement with an unoccupied surface state energy using inverse photoemission spectroscopy (IPES). New IPES results indicate that the unoccupied surface state intensity decreases with increasing annealing temperature. It was also found that the unoccupied surface state was shifted. IPES results provide that the contributions of the surface state to surface optical properties at 2.1 eV are relevant for the RAS technique.     

Phase transitions on clean Si(110) surfaces

The properties of the structure of clean Si(110) surfaces have been investigated by LEED. The phase transitions between surface structures Si(110)?(4 × 5), Si(110)?(2 × 1) and Si(110)(5 × 1) take place at about 600 and 750°C. The time of reconstruction from the high temperature phase to the low temperature phase may exceed the time of the sample cooling. That explains why the Si(110)?(2 × 1) and the Si(110)?(5 × 1) superstructures may be seen at room temperature. Surface defects favour the retaining of high temperature phases on the surface at room temperature. The transition from the Si(110)?(5 × 1) structure to the Si(110)?(2× 1) structure and conversely in the temperature range of 720–750°C apparently occurs through formation of the intermediate structures Si(110)?(7 × 1) and Si(110)?(9 × 1). The models are given of superstructures observed by LEED.     

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.     

In-situ observation of oxygen adlayer formation on Cu(110) electrode surfaces

In-situ atomic force microscopy (AFM) was used to image Cu(110) single crystals in aqueous solutions during the initial stages of oxidation. Images obtained in pH 2.5–2.7 HClO₄ and H₂SO₄ solutions revealed the growth of oxygen adlayers consisting primarily of [001] oriented chains. A majority of these chains (ca. 70%) were arranged in (2 × 1) and (3 × 1) structures. These chain structures were observed in the thermodynamically forbidden region of the pH-potential phase diagram, which indicates that stable oxygen adlayers develop prior to bulk oxide formation.     

Reflectometric study of surface states and oxygen adsorption on clean Si(100) and (110) surfaces

External differential reflection measurements were carried out on clean Si(100) and (110) surfaces in the photon energy range of 1.0 to 3.0 eV at 300 and 80 K. The results for Si(100) at 300 K showed two peaks in the joint density of states curve, which sharpened at 80 K. One peak at 3.0 ± 0.2 eV can be attributed to optical transitions from a filled surface states band near the top of the valence band to empty bulk conduction band levels. The other peak at 1.60 ± 0.05 eV may be attributed to transitions to an empty surface states band in the energy gap. This result favours the asymmetric dimer model for the Si(100) surface. For the (110) surface at 300 K only one peak was found at 3.0 ± 0.2 eV. At 80 K the peak height diminished by a factor of two. Oxygen adsorption in the submonolayer region on the clean Si(100) surface appeared to proceed in a similar way as on the Si(111) 7 × 7 surface. For the Si(110) surface the kinetics of the adsorption process at 80 K deviated clearly. The binding state of oxygen on this surface at 80 K appeared to be different from that on the same surface at 300 K.     

Interaction of H2O with a clean and oxygen precovered Ni(110) surface studied by XPS

The adsorption and reaction of H₂O on clean and oxygen precovered Ni(110) surfaces was studied by XPS from 100 to 520 K. At low temperature (T<150 K), a multilayer adsorption of H₂O on the clean surface with nearly constant sticking coefficient was observed. The O 1s binding energy shifted with coverage from 533.5 to 534.4 eV. H₂O adsorption on an oxygen precovered Ni(110) surface in the temperature range from 150 to 300 K leads to an O 1s double peak with maxima at 531.0 and 532.6 eV for T=150 K (530.8 and 532.8 eV at 300 K), proposed to be due to hydrogen bonded O_ads… HOH species on the surface. For T>350 K, only one sharp peak at 530.0 eV binding energy was detected, due to a dissociation of H₂O into O_ads and H₂. The s-shaped O 1s intensity-exposure curves are discussed on the basis of an autocatalytic process with a temperature dependent precursor state.     

Electron energy loss spectroscopy of ethylene on Ag(110) precovered with oxygen: energy dependence of the cross section

The intensities of the CH out-of-plane bending (v₇) and CH stretch (v₁) vibrations of ethylene adsorbed on Ag(l 10) precovered with oxygen have been measured in EELS as a function of beam energy from 2 to 20 eV. The energy dependence of the v₇ vibration is satisfactorily described by the dipole approximation. For the v₁ vibration, which is purely dipole forbidden, an entirely different energy dependence is found, with a maximum intensity at a beam energy of 2 eV. Comparison of the EEL intensities observed experimentally with those predicted from the dipole moments of the free molecule confirms the conclusion that the molecular plane of chemisorbed ethylene is parallel to the surface.     

Adsorption of bismuth on Si(110) surfaces

The adsorption of bismuth on Si(110) surfaces has been studied by means of quadrupole mass spectrometry (QMS), LEED and AES. The existence of three main adsorption states (the first, second and third phases) were observed. Two of them (the first and second phases) are two-dimensional phases and the other (the third phase) is a three-dimensional phase. After the completion of the first phase at a high substrate temperature, the LEED pattern showed a clear Si(110) 2 × 3 structure. A shift of the saturation coverage of the two-dimensional phase with the change of the deposition condition was also observed.     

Atomic adsorption of oxygen on Cu(111) and Cu(110)

We have studied angle-resolved inverse photoemission ( = 9.7 eV) after room temperature adsorption of oxygen on Cu(111) and Cu(110). On Cu(111) exposure to 500 L induces a band (3.0 eV aboveE _F at) which shows clear dispersion (1.0 eV) to higher energies for off normal incidence. Since no LEED superstructure is seen for that system, our results present strong evidence for the presence of short-range surface order. Two adsorbate bands are identified (2.8 eV and 6.3 eV at) on Cu(110)p(2×1)-O. Our results are in good agreement with a long-bridge adsorption site.     

Photoenhancement mechanism for oxygen chemisorption on GaAs(110) using visible light

Visible light from an argon ion laser (514.5 nm, 3 W/cm²) is seen to increase oxygen chemisorption on cleaved GaAs(110) surfaces up to a final coverage between one and two monolayers. Using photoemission spectroscopy to measure the oxygen coverage after simultaneous exposure of the surface to oxygen and light, we have determined that oxygen uptake for photoenhanced exposures is independent of sample temperature and doping type. In addition, significantly less enhancement is seen for weakly bound oxidizing molecules (N₂O) relative to the effects with molecular oxygen. These results are explained by a photoenhancement mechanism in which energy is released in a surface recombination event, possibly in the form of nonthermal phonons, causing physisorbed gas molecules to dissociate and thereby overcoming a major rate limiting step of the reaction in the dark. This reaction mechanism is supported by calculations of the surface recombination rates and free carrier densities at the surface which show that only the recombination rate is correlated with enhanced oxygen uptake. Other mechanisms and experimental data are also discussed.     

The interaction of hydrogen and carbon monoxide with oxygen on Cu(110)-Fe surfaces

The interaction of hydrogen and carbon monoxide with oxygen adsorbed on Cu(110)-Fe surfaces has been studied with ellipsometry, Auger electron spectroscopy and low energy electron diffraction. With carbon monoxide copper can be reduced completely and Fe_0.95O partially. With a model which is only an extension of the scheme for the reduction of pure Cu(110) by CO, the reduction of Cu(110)-Fe can be simulated. The lateral orientation of Fe_0.95O with respect to the copper matrix changes during repetitive oxidation-reduction cycles. At 725 K oxygen deficient iron oxide segregates to the surface. With hydrogen all oxygen can be removed.     

Adsorption of oxygen on clean gaas surfaces investigated by ultraviolet photoemission

The adsorption of oxygen at exposures of up to 10⁵ L on differently oriented, ion-bombarded and annealed GaAs surfaces was investigated by UPS. Coverages θ_As for the clean surfaces and oxygen coverages for the oxygen exposed surfaces were estimated by additional SXPS measurements. It was concluded that at small exposures molecular and atomic adsorption are comparable in quantity and that atomic adsorption (oxidation) becomes maximum at θ_As, ≈ 0.2 for (111)Ga and (001) surfaces. Bonding of oxygen molecules should involve Ga sites. Specific bonding of oxygen atoms (O-Ga or O-As) was not indicated by the two stable UPS peaks as they occurred for arsenic coverages from 0 to 0.5 and did not shift their energy positions. They simply indicate the two states of the adsorbate atoms, single and double bonds, to substrate atoms. For the surfaces prepared here, monolayer coverage by oxygen was obtained at about 10¹⁰L. Likewise adsorption of H₂O was investigated.