The electronic structure of the valence bands of solid NH3 and H2O studied by ultraviolet photoelectron spectroscopy

The results of a photoelectron study using ultraviolet 40.81 eV photons (UPS) of the outermost bands of the molecular solids NH₃ and H₂O are reported. The binding energies, the energy separation, the band widths and the branching ratio of the two outermost bands of solid NH₃ are found not to be significantly different from the 3a_l and 1e molecular orbital states of the gaseous NH₃ UPS spectrum. This implies that hydrogen bonding has not produced any significant change in the electronic structure of the valence bands of solid NH₃. Because of a much smaller intermolecular hydrogen bond length in solid H₂O compared to that in solid NH₃, the hydrogen bond does, however, produce a significant change in the valence bands of H₂O on solidification, and because of the orbital geometry it predominantly affects the 3a_l molecular orbital state.     

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.     

The adsorption of SO2 on iron surfaces studied by x-ray photoelectron spectroscopy

The interaction of SO₂ with evaporated iron surfaces in the temperature range 80–450 K was investigated by using X-ray photoelectron spectroscopy. At 300 K, SO₂ decomposed at the initial stage of the interaction and gave adsorbed S with the S2p peak at 161.9 eV and adsorbed O with the O1s at 530.0 eV. Further exposure of SO₂ gave adsorbed SO₄ with S2p at 166.8 eV O1s at 531.3 eV, being different in binding energies from ionic SO₄^2?. This indicates the two stage reaction Of SO₂ with iron surface; SO₂(gas) → S(ads) + 20(ads), SO₂(gas) + 2O(ads) → SO₄(ads). The first reaction did not occur at low temperature or in the presence of adsorbed O. The adsorbed SO₄ formed at 80 K showed a quantitative decomposition reaction into S(ads) and O(ads) in the temperature range 200–350 K.     

Bonds and Fermi surfaces studied by photoelectron spectroscopy

Photoelectron spectroscopy with synchrotron radiation employing high energy and angular resolutions is a very efficient tool for experimental investigations of the electronic structure of solids and their surfaces. In addition to standard band-mapping applications, photoemission intensity and line-shape analyses provide valuable information about wave functions, bonds and interactions of a many-electron system. In this report we choose covalent semiconductor surfaces as well as metallic clean and nanostructured surfaces of layered materials to serve as model systems for assessing the spatial origin of photoelectrons and the three-dimensional shape of Fermi surfaces. Received: 11 July 2001 / Accepted: 23 July 2001 / Published online: 3 April 2002     

Oxidation of Zircaloy-4 by H2O followed by molecular desorption

The interaction of H₂O with Zircaloy-4 (Zry-4) is investigated using Auger electron spectroscopy (AES) and temperature programmed desorption (TPD) methods. Following adsorption of H₂O at 150 K the Zr(MNV) and Zr(MNN) Auger features shift by ∼6.5 and 4.5 eV, respectively, indicating surface oxidation. Heating H₂O/Zry-4 results in molecular desorption of water at both low and high temperatures. The low-temperature desorption is attributed to ice multilayers, whereas, three overlapping high-temperature features are presumably due to recombinative desorption. This high-temperature desorption begins before the surface oxide is dissolved, continues upon its removal, and is atypical for water/metal systems. Unexpectedly, no significant desorption of hydrogen is observed near 400 K, as is typically observed following O₂ adsorption on Zr-based materials. However, we do observe that H₂O adsorption on Zry-4 surfaces roughened by argon ion sputtering results in H₂ desorption.     

Penning ionization electron spectroscopy of H2O,D2O,H2S and SO2

Electron energy peak shifts and peak shapes were determined in the ionization of H₂O, D₂O, H₂S and SO₂ by Ne(³P₂) and He(2¹S, 2³S) metastable atoms. The shifts are large, especially in ionization of H₂O and D₂O into the ionic ground state and are probably mostly due to chemical interaction during the collision.In a previous paper the electron energy distribution curves for ionization of CO, HCl, HBr, N₂O, NO₂, CO₂, COS and CS₂ by helium, neon and argon metastables and the characteristics of this ionization were described¹. In this paper the series of triatomic molecules was extended to the molecules H₂O, D₂O, H₂S and SO₂. Because all these molecules have considerable dipole moments it could be expected that the peak shifts might be enhanced as compared with other triatomic molecules.     

Oxidation of nickel studied by UV and X-ray photoelectron spectroscopy

The nature of argon-ion bombarded nickel surfaces (polycrystalline, and (111), (110) and (100) single crystals) and their subsequent interaction with oxygen at ordinary temperatures have been studied using X-ray and UV photoelectron spectroscopy, including angular variation measurements and the determination of work function changes, in combination in the same apparatus. Variations between the HeI spectra of the four clean substrates were taken to confirm the presence of substantial order within the depth sampled by UPS. The four surfaces exhibited similar but not identical behaviour during oxidation, resembling that reported by other workers from studies of both annealed single crystals and evaporated polycrystalline films. The initial process was deduced to be essentially dissociative chemisorption: no evidence supporting a previous suggestion of associative adsorption at low coverages was found. Oxygen commenced to penetrate below the surface of all samples before oxygen equivalent to a monolayer had been taken up (~10 L exposure) and further substantial uptake followed resulting in the formation of a stable film (~18 Å) of nickel oxide by ~100 L exposure. This oxide layer was not stoichiometric nickel(II) oxide: it was characterized by the presence of two distinct O 1s signals, the relative intensities of which depended on the crystallographic nature of the surface. It is tentatively suggested that the oxygen signal with the higher BE be associated with Ni^III. Comparison of the X-ray and UV spectra suggests that the oxide film is very non-uniform in thickness, some Ni metal remaining very close to the surface.     

A photoelectron spectroscopic investigation of the interaction between H2O and oxygen on Ag(110)

The adsorption and reaction of H₂O with adsorbed oxygen atoms on Ag(110) was examined by UPS. In agreement with previous EELS results, H₂O formed multilayers of ice upon adsorption at 140 K. The ice layers could be easily distinguished from monolayer coverages of chemisorbed H₂O (present above 160 K) by UPS. The ice layers produced (1) strong attenuation of the emission from the Ag d-bands, (2) a nearly 2 eV shift of H₂O valence levels to higher binding energy and (3) strong attenuation of emission from the H₂O 3a₁ orbital. H₂O was observed to react stoichiometrically with O(a) above 250 K to produce a pure layer of adsorbed hydroxyl species. The UPS spectra for these species exhibited features at ?5.8 and ?8.7 eV, as well as strong features above the d-bands. These spectra were compared with those for OH(a) on other surfaces, and the difficulties of identifying OH by UPS due to contamination by excess H₂O are discussed.     

Oxidation of Al single crystal surfaces by exposure to O2 and H2O

The initial stages of oxidation of Al single crystals are studied by soft X-ray photoemission spectroscopy at photon energies hv = 30 eV and 111.13 eV using synchroton radiation. Both the valence band region and the substrate Al 2p core levels are measured with high resolution to clarify the differences between (a) the geometrical effects at different surfaces, (100) and (110), and (b) between the oxidation by pure O₂ and H₂O. There is a well established but not very dramatic differences in the O 2p induced band between the two crystal surfaces when oxidizing with O₂. The Al 2p spectra reveal an initial state of oxidation with less O atoms per Al atom than in Al₂O₃ate disappears at higher exposures with O₂ while it is absent when oxidizing with H₂O. Only about 1/4 of the exposure with H₂O is needed to obtain the same coverage as with O₂.     

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.     

DFT study on H2O activation by stepped and planar Rh surfaces

In this research the effect of steps (lower coordinated surface atoms) and the presence of pre-adsorbed oxygen on the activation energy of water are studied with DFT. Without oxygen water activation is found to be structure insensitive. When oxygen is adsorbed on the surface and acts as the acceptor for the hydrogen at the step edge, the barrier will decrease significantly.     

Collisional broadening and shifting of the 211-202 transition of H2O, H2O, H2O by atmosphere gases

Broadening and shifting of the 2₁₁-2₀₂ transition of H₂¹⁶O, H₂¹⁷O, H₂¹⁸O by pressure of water, nitrogen and oxygen were precisely measured at room temperature using spectrometer with radio-acoustic detection of absorption. Shift parameters for all studied lines as well as broadening parameters of H₂¹⁷O, H₂¹⁸O lines were measured for the first time. Comparison of obtained results with previously known experimental and theoretical data is presented.     

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.     

H2O surface interaction of oxygen-free polyphenylacetylene films investigated by XPS

The interaction of water with the surface of polyphenylacetylene (PPA) films is dependent on the preparation and casting procedures. Films with an oxygen-free surface were obtained. Molecular adsorption of H₂O is indicated to be the major phenomenon by XPS. On annealing up to ≈ 250°C partial loss of adsorbed water occurs. The thickness of the reacted surface layer is estimated to be ≈ 6.0Å when the PPA film is exposed to water vapour at pressure p ≈ 760 Torr.     

A study of LaCoO3 by UV photoelectron spectroscopy

He(II) photoelectron spectra of LaCoO₃ and La_0.7Sr_0.3CoO₃ have been recorded. Spectra of LaCoO₃ were recorded at various temperatures in the range 77 K ? T ? 873 K and changes in the spectra were observed corresponding to the variation of the Co spin state in the lattice.     

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.     

H2 and CO chemisorption on polycrystalline titanium studied by flash desorption mass spectrometry

We have used flash desorption mass spectroscopy to study the adsorption and desorption of H₂ and CO from clean titanium at room temperature. CO flash desorption occurs predominantly from a low temperature state whose binding energy is 20.3 kcal/mole. H₂ flash desorption is complex. Only one peak is observed; it is broader than flash desorption spectra normally corresponding to first or second order kinetics. The shift in the peak temperature to lower values with increasing coverage has been analysed using the isothermal desorption rate technique. The apparent order of H₂ desorption is 1.5 and is independent of temperature from 888 to 1077 K. The activation energy is 21 kcal/mole. These results will be discussed in terms of absorption of H₂ into titanium and thermal decomposition of a titanium hydride compound.     

Antisymmetric exchange interaction and spin-spinrelaxation in CuCl2·2H2O

Spin-spin relaxation measurements support the introduction in CuCl₂·2H₂O of an antisymmetric exhange as proposed by Dzialoshinsky and Moriya.