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 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).     

TDS and LEED studies of H2O adsorption on GaAs(110)

The UHV cleaved (110) face has been exposed to water in the range from 10 L to 2 × 10⁴ L. The main TDS peak in H₂O desorption appears at 350 K, independent of coverage. The low desorption energy of 0.7 eV (16 kcal/mol) is reasonable for oxygen atoms bound via the lone pair orbital to As as was earlier derived from UPS measurements. A broad spur between 450 and 600 K may be related to O-Ga bonds. The sticking probability shows values below 10^-4; only near 4.8 × 10³ L (6 × 10¹⁵ cm^-2 s^-1 H₂O molecules for 300 s) corresponding to a coverage of about 0.4 monolayes a steep maximum appears. At about one monolayer saturation is observed. Exposures to more than 10⁴ L of water quench the intensity of the (10) LEED spot considerably stronger than the intensity of the (11) spot. A comparison of the I(E) curves with existing model calculations suggests that the observed behaviour of the LEED spots is caused by a change in surface structure towards the unrelaxed configuration. The higher sticking coefficient observed near 0.4 monolayers may be connected with this rearrangement of surface atoms.     

Evidence for bicarbonate formation on vacuum annealed TiO2(110) resulting from a precursor-mediated interaction between CO2 and H2O

The reaction of CO₂ and H₂O to form bicarbonate (HCO⁻₃) was examined on the nearly perfect and vacuum annealed surfaces of TiO₂(110) with temperature programmed desorption (TPD), static secondary ion mass spectrometry (SSIMS) and high resolution electron energy loss spectrometry (HREELS). The vacuum annealed TiO₂(110) surface possesses oxygen vacancy sites that are manifested in electronic EELS by a loss feature at 0.75 V. These oxygen vacancy sites bind CO₂ only slightly more strongly (TPD peak at 166 K) than do the five-coordinated Ti⁴⁺ sites (TPD peak at 137 K) typical of the nearly perfect TiO₂(110) surface. Vibrational HREELS indicates that CO₂ is linearly bound at the latter sites with a ν_a(OCO) frequency similar to the gas phase value. In contrast, oxygen vacancies dissociate H₂O to bridging OH groups which recombine to liberate H₂O in TPD at 490 K. No evidence for a reaction between CO₂ and H₂O is detected on the nearly perfect surface. In sequentially dosed experiments on the vacuum annealed surface at 110 K, CO₂ adsorption is blocked by the presence of preadsorbed H₂O, adsorbed CO₂ is displaced by postdosed H₂O, and there is little or no evidence for bicarbonate formation in either case. However, when CO₂ and H₂O are simultaneously dosed, a new CO₂ TPD state is observed at 213 K, and the 166 K state associated with CO₂ at the vacancies is absent. SSIMS was used to tentatively assign the 213 K CO₂ TPD state to a bicarbonate species. The 213 K CO₂ TPD state is not formed if the vacancy sites are filled with OH groups prior to simultaneous CO₂+H₂O exposure. Sticking coefficient measurements suggest that CO₂ adsorption at 110 K is precursor-mediated, as is known to be the case for H₂O adsorption on TiO₂(110). A model explaining the circumstances under which the proposed bicarbonate species is formed involves the surface catalyzed conversion of a precursor-bound H₂O–CO₂ van der Waals complex to carbonic acid, which then reacts at unoccupied oxygen vacancies to generate bicarbonate, but falls apart to CO₂ and H₂O in the absence of these sites. This model is consistent with the conditions under which bicarbonate is formed on powdered TiO₂, and is similar to the mechanism by which water catalyzes carbonic acid formation in aqueous solution.     

The surface chemistry of H2O on clean and oxygen-covered Cu(110)

Adsorption of water at 100 K. on clean and oxygen-covered Cu(110) has been studied using UPS, TDS, Δφ and LEED measurements. The results indicate that two-dimensional hydrogenbonded islands are formed on the clean surface. The long-range order in these islands is in registry with the substrate lattice and gives rise to a c(2×2) LEED pattern. Upon the formation of multilayer ice, the ordering disappears. The presence of oxygen on the surface disrupts the hydrogen bonding, and composite oxygen-water layers are formed. A model of the arrangement of oxygen atoms and water molecules is presented, based upon the LEED observations for these layers and an estimate of the relative oxygen and water coverages. The intensity variation of a thermal desorption peak at 290 K, attributed to adsorbed OH species, with oxygen coverage is in accordance with this model. For low oxygen coverages, the TDS and Δφ results indicate that small oxygen-water clusters with an enhanced ratio of water molecules per adsorbed oxygen atom are present.     

INS investigation on disaccharide/H2O mixtures

New Inelastic Neutron Scattering findings on homologues disaccharides (C₁₂H₂₂O₁₁)/H₂O mixtures are presented. The comparison among the spectra of trehalose, maltose and sucrose/H₂O mixtures, besides evidencing a different destructuring effectiveness on the H₂O hydrogen bond network, and hence different cryoprotectant properties, shows a higher ‘crystallinity’ degree for the trehalose/H₂O system which accounts for its higher ‘rigidity’. This result justifies the better cryptobiotic action of trehalose in respect to maltose and sucrose.     

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.     

The adsorption of CO,H2, H2, CO2 and H2O on carburized and graphitized Ni(110)

A study of the adsorption/desorption behavior of CO, H₂O, CO₂ and H₂ on Ni(110)(4 × 5)-C and Ni(110)-graphite was made in order to assess the importance of desorption as a rate-limiting step for the decomposition of formic acid and to identify available reaction channels for the decomposition. The carbide surface adsorbed CO and H₂O in amounts comparable to the clean surface, whereas this surface, unlike clean Ni(110), did not appreciably adsorb H₂. The binding energy of CO on the carbide was coverage sensitive, decreasing from 21 to 12 $\text{kcal}\text{mol}$ as the CO coverage approached 1.1 × 10¹⁵ molecules cm^?2 at 200K. The initial sticking probability and maximum coverage of CO on the carbide surface were close to that observed for clean Ni(110). The amount of H₂, CO, CO₂ and H₂O adsorbed on the graphitized surface was insignificant relative to the clean surface. The kinetics of adsorption/desorption of the states observed are discussed.     

Dynamics of interaction of H2 and D2 with Ni(110) and Ni(110) surfaces

Elastic and direct-inelastic scattering as well as dissociative adsorption and associative desorption of H₂ and D₂ on Ni(110) and Ni(111) surfaces were studied by molecular beam techniques. Inelastic scattering at the molecular potential is dominated by phonon interactions. With Ni(110), dissociative adsorption occurs with nearly unity sticking probability s₀, irrespective of surface temperature T_s and mean kinetic energy normal to the surface 〈 E_⊥ 〉. The desorbing molecules exhibit a cos θ_e angular distribution indicating full thermal accommodation of their translation energy. With Ni(111), on the other hand, s₀ is only about 0.05 if both the gas and the surface are at room temperature. s₀ is again independent of T_s, but increases continuously with 〈 E⊥ 〉 up to a value of ～0.4 for 〈 E⊥ 〉 = 0.12 eV. The cos⁵θ_e angular distribution of desorbing molecules indicates that in this case they carry off excess translational energy. The results are qualitatively rationalized in terms of a two-dimensional potential diagram with an activation barrier in the entrance channel. While the height of this barrier seems to be negligible for Ni(110), it is about 0.1 eV for Ni(111) and can be overcome through high enough translational energy by direct collision. The results show no evidence for intermediate trapping in a molecular “precursor” state on the clean surfaces, but this effect may play a role at finite coverages.     

The reaction of H2S with surface oxygen on Ni(110)

The reactions of H₂S with predosed surface oxygen on Ni(110) surfaces were studied for a variety of coverage conditions. The primary reaction product is H₂O, but the details of the water formation and desorption depends on the coverage of both O and H₂S.

For high coverages of oxygen (p(2 × 1)−O; 0.5 ML), the reaction to form water is quantitative. The loss of oxygen from the surface (as measured by AES) is equal to the increase in sulfur coverage. XPS and HREELS measurements indicate the presence of chemisorbed H₂O immediately following large exposures of H₂S on the oxygen predosed surface at 110 K. Deuterium incorporation results suggest that the primary mechanism for these coverage conditions involves direct transfer of hydrogen from SH or H₂S moieties to the oxygen.

A second mechanism involving reaction of surface hydroxyl groups with surface hydrogen was also identified. This mechanism is particularly important for high coverages of oxygen (0.5 ML) and low coverages of H₂S (0.15 ML), where water desorption was observed at 235 K, but was not observed spectroscopically at 110 K. The sequential addition of two surface hydrogen atoms to surface oxygen is not an important mechanism in this system.

These reactions were modeled using a bond-order conservation method, and the model successfully reproduced the important mechanistic conclusions.     

The adsorption of H2O on TiO2 and SnO2(110) studied by first-principles calculations

First-principles calculations based on density functional theory and the pseudopotential method have been used to investigate the energetics of H₂O adsorption on the (110) surface of TiO₂ and SnO₂. Full relaxation of all atomic positions is performed on slab systems with periodic boundary conditions, and cases of full and half coverage are studied. Both molecular and dissociative (H₂O→OH⁻+H⁻) adsorption are treated, and allowance is made for relaxation of the adsorbed species to unsymmetrica configurations. It is found that for both TiO₂ and SnO₂ an unsymmetrical dissociated configuration is the most stable. The symmetrical molecularly adsorbed configuration is unstable with respect to lowering of symmetry, and is separated from the fully dissociated configuration by at most a very small energy barrier. The calculated dissociative adsorption energies for TiO₂ and SnO₂ are in reasonable agreement with the results of thermal desorption experiments. Calculated total and local electronic densities of states for dissociatively and molecularly adsorbed configurations are presented, and their relation with experimental UPS spectra is discussed.     

Spectroscopically determined potential energy surfaces of the H2O, H2O, and H2O isotopologues of water

Adiabatic potential energy surfaces (PESs) for three major isotopologues of water, H₂¹⁶O, H₂¹⁷O, and H₂¹⁸O, are constructed by fitting to observed vibration-rotation energy levels of the system using the nuclear motion program DVR3D employing an exact kinetic energy operator. Extensive tests show that the mass-dependent ab initio surfaces due to Polyansky et al. [O.L. Polyansky, A.G. Császár, S.V. Shirin, N.F. Zobov, P. Barletta, J. Tennyson, D.W. Schwenke, P.J. Knowles, Science 299 (2003) 539-542.] provide an excellent starting point for the fits. The refinements are performed using a mass-independent morphing function, which smoothly distorts the original adiabatic ab initio PESs. The best overall fit is based on 1788 experimental energy levels with the rotational quantum number J = 0, 2, and 5. It reproduces these levels with a standard deviation of 0.079 cm⁻¹ and gives, when explicit allowance is made for nonadiabatic rotational effects, excellent predictions for levels up to J = 40. Theoretical linelists for all three isotopologues of water involved in the PES construction were calculated up to 26 000 cm⁻¹ with energy levels up to J = 10. These linelists should make an excellent starting point for spectroscopic modelling and analysis.     

Adsorption of H2S,H2O and O2 on Si(111) surfaces

Electron energy-loss spectroscopy has been applied to the study of Si(111) surfaces covered with H₂S, H₂O and O₂ at room temperature and the surfaces annealed at ～ 600°C. The experimental results strongly suggest that H₂S and H₂O adsorb in the molecular states at room temperature. It is proposed that O₂ is first adsorbed in a molecular state, then adsorbs as atoms, and finally oxidizes forming SiO₂.     

Electron-stimulated desorption of D (H) from condensed D2O (H2O) films

The electron-stimulated desorption (ESD) of D⁻ and H⁻ ions from condensed D₂O and H₂O films is investigated. Three low-energy peaks are observed in the ESD anion yield, which are identified as arising from excitation of ²B₁, ²A₁ and ²B₂ dissociative electron attachment (DEA) resonances. Additional structure is observed between 18 and 32 eV, which may be due to ion pair formation or to DEA resonances involving the 2a₁ orbital. The ion yield resulting from excitation of the ²B₁ resonance increases as the film is heated. We attribute the increase in the ion yield to thermally induced hydrogen bond breaking near the surface, which enhances the lifetimes of the excited states that lead to desorption.     

Site-specific interaction of H2O with ZnO single-crystal surfaces studied by thermal desorption and UV photoelectron spectroscopy

The adsorption and condensation of H₂O(D₂O) on ZnO(101&#x0304;0), (0001)Zn and (0001&#x0304;)O surfaces was investigated by means of thermal desorption (TDS) and UV photoelectron spectroscopy (UPS). The clean ZnO single-crystal surfaces were prepared by Ar-ion sputtering and annealing and characterised by Auger electron spectroscopy, LEED, UPS and work-function measurements. On all three surfaces six different adsorption states were found. In the monolayer regime there is a stronger bonding to Zn sites (desorption temperature 340 K) than to O sites (190 K), The bonding to the Zn sites seems to be accompanied by some clustering. Before the chemisorption layer is completed a first ice state is found whose desorption temperature shifts from 162 to 168 K with increasing exposures. At higher exposures the multilayer ice state is found at 152 K. On the (0001&#x0304;)O face defect-induced features were identified. The water lone-pair orbital 1b₁, whose energy falls between the O p and the Zn 3d emission of the substrate and which is known to show bonding shifts, was analysed using angle-resolved UPS. In the monolayer, the main chemisorption states are found at E_B^V(1b₁) = ?9.6 eV for the (0001)Zn face and at ? 10.6 eV for the (0001&#x0304;)O face and are compared with the multilayer ice emission at 1&#x0304;1.1 eV. The difference in binding energies shows the same trend as the TDS data. For the (101&#x0304;0) face the 1b₁ emission is very broad, indicating some overlap between different states.     

A Raman spectroscopic study of the polyimide/Ag(110) interface

We have investigated the bonding of the monomers pyromellitic dianhydride (PMDA) and oxydianiline (ODA) and their subsequent polymerization to form polyimide on Ag(110) in ultrahigh vacuum. Unenhanced surface Raman spectroscopy was used to identify adsorbed species and to follow the polymerization reaction. ODA was unreactive on the cold (140 K) surface; spectral features were identical to those in the condensed multilayer indicating that the monolayer is physisorbed. PMDA, on the other hand, adsorbed dissociatively, the evidence being consistent with a bidentate surface carboxylate mode of bonding. Finally, heating a 30 Å film of codosed monomers to 473 K resulted in the formation of a polyimide film as shown by characteristic features in the carbonyl stretching region of the Raman spectrum.     

A study of H2S adsorption kinetics on the clean and oxygen covered (110) surface of copper

The very low pressure adsorption kinetics of H₂S on the clean and oxygen covered Cu(110) face have been examined by Auger Electron Spectroscopy (AES) and Mirror Electron Microscopy (MEM, used for continuous surface potential variations of the copper surface). The AES experimental curves on the clean copper face have been interpreted using a model of island growth by surface diffusion. The presence of an adsorbed oxygen layer on the copper surface changes notably the induction times observed on both AES and MEM measurements.     

Core-level X-ray photoelectron spectra and X-ray photoelectron diffraction of RuO2(110) grown by molecular beam epitaxy on TiO2(110)

We have measured Ru 3d, 4s, 4p and O 1s high-resolution core-level X-ray photoelectron spectra, along with Ru 3d and O 1s scanned-angle X-ray photoelectron diffraction angular distributions, for RuO₂(110). The surfaces were prepared by oxygen-plasma-assisted molecular beam epitaxial growth of RuO₂ on TiO₂(110). XPS spectral interpretation and the nature of the XPD scans strongly suggest that the complex line shapes are due to final-state screening effects, rather than the presence of Ru in oxidation states other than +4.     

The decomposition of H2S on Ni(110)

Adsorbed H₂S decomposes on Ni(110) to form primarily surface S and H for coverages of less than 0.5 ML. The hydrogen evolves in two separate TPD peaks, characteristic of hydrogen recombination and desorption from the clean surface and from regions perturbed by chemisorbed sulfur. XPS and HREELS indicate the presence of SH and possibly H₂S groups on the surface at 110 K. The XPS data indicates that for coverages less than about 0.5 ML, the concentration of molecular H₂S is small, but it is difficult to asess the coverage of SH groups. However, all of the molecular species decompose prior to hydrogen desorption (for high coverage, 180 K). Physisorbed H₂S is observed on the surface for coverages greater than about 0.5 ML.

The sulfur Auger lineshape was observed to be a function of both coverage and temperature. The changes in the lineshape were attributed to perturbations in local bonding interactions between the S and the Ni surface, perhaps involving some change in either bonding sites or distances but not involving SH bond scission.

The decomposition reaction was modeled using a bond order conservation method which successfully reproduced the experimental results.