Solvated electrons at the water-air interface: surface versus bulk signal in low kinetic energy photoelectron spectroscopy

Time-resolved photoelectron spectroscopy at low kinetic energies (?5 eV) is applied to dilute iodide solutions with different surface and bulk contributions. The results indicate a pronounced surface sensitivity. Signals assigned to solvated electrons near the liquid surface decay rapidly on a sub-ps timescale. In contrast to the literature, a long-lived surface solvated electron at 1.6 eV binding energy is not observed.     

A study of the adsorption of oxygen on silver at high pressure by electron spectroscopy

In situ X-ray photoelectron spectroscopic observation of a silver foil at 473 K exposed to oxygen at pressures up to 0.5 torr has identified three species, characterised by oxygen 1s binding energies of 528.3, 530.3 and 532.5 eV. The first two are identified as due to atomic oxygen, respectively chemisorbed at the surface and incorporated into the bulk. The third species is tentatively assigned to molecular oxygen, probably as a surface peroxidic group; both the UV photoelectron spectra and the O(1s) intensities determined at two different electron take-off angles are consistent with this interpretation.     

丙烯酸酯紫外光固化材料表面的XPS研究

分别在不同的固化氛围(空气和氮气)下对配方相同的双酚A环氧丙烯酸树脂进行紫外光固化,得到了具有不同表面能的固化膜.利用X射线光电子能谱(XPS),在不同的掠射角(60°和90°)下表征了固化膜样品的元素组成、电子结合能、相对含量以及表面官能团类型.结果表明:不同的固化环境会导致材料表面能的差异.与空气下固化的材料相比,氮气下固化的材料表面C_1s和O_1s原子的结合能均向低结合能方向位移,且低结合能的C—C/C—H含量大于前者的含量,而相对高结合能的CO和C—O含量均小于前者.在空气下固化时,其表面高结合能的单键氧含量大于氮气下固化的材料表面.材料表面与基材所形成的界面间的原子结合能以及原子各种结合状态的相对含量也不同.如在氮气下固化时,材料表面C_1s原子的结合能为286.62eV,而玻璃界面的结合能为287.48eV,且两面中C_1s和O_1s原子5种结合状态的相对含量都有差异.材料自身组分在固化过程中的迁移,使固化膜中C_1s和O_1s的原子浓度由表及里存在一定的梯度分布.CO和C—O等基团含量从表面到本体逐渐增多,而C—C/C—H含量从表面到本体逐渐减少.     

The Kohn-Sham density of states and band gap of water: from small clusters to liquid water

Electronic properties of water clusters (H2O)(n), with n=2, 4, 8, 10, 15, 20, and 30 molecules were investigated by sequential Monte Carlo/density-functional theory (DFT) calculations. DFT calculations were carried out over uncorrelated configurations generated by Monte Carlo simulations of liquid water with a reparametrized exchange-correlation functional that reproduces the experimental information on the electronic properties (first ionization energy and highest occupied molecular orbital-lowest unoccupied molecular orbital gap) of the water dimer. The dependence of electronic properties on the cluster size (n) shows that the density of states (DOS) of small water clusters (n>10) exhibits the same basic features that are typical of larger aggregates, such as the mixing of the 3a1 and 1b1 valence bands. When long-ranged polarization effects are taken into account by the introduction of embedding charges, the DOS associated with 3a1 orbitals is significantly enhanced. In agreement with valence-band photoelectron spectra of liquid water, the 1b1, 3a1, and 1b2 electron binding energies in water aggregates are redshifted by approximately 1 eV relative to the isolated molecule. By extrapolating the results for larger clusters the threshold energy for photoelectron emission is 9.6+/-0.15 eV (free clusters) and 10.58+/-0.10 eV (embedded clusters). Our results for the electron affinity (V0=-0.17+/-0.05 eV) and adiabatic band gap (E(G,Ad)=6.83+/-0.05 eV) of liquid water are in excellent agreement with recent information from theoretical and experimental works.     

High resolution and low-temperature photoelectron spectroscopy of an oxygen-linked fullerene dimer dianion: C(120)O(2-)

C(120)O comprises two C(60) cages linked by a furan ring and is formed by reactions of C(60)O and C(60). We have produced doubly charged anions of this fullerene dimer (C(120)O(2-)) and studied its electronic structure and stability using photoelectron spectroscopy and theoretical calculations. High resolution and vibrationally resolved photoelectron spectra were obtained at 70 K and at several photon energies. The second electron affinity of C(120)O was measured to be 1.02+/-0.03 eV and the intramolecular Coulomb repulsion was estimated to be about 0.8 eV in C(120)O(2-) on the basis of the observed repulsive Coulomb barrier. A low-lying excited state ((2)B(1)) was also observed for C(120)O(-) at 0.09 eV above the ground state ((2)A(1)). The C(120)O(2-) dianion can be viewed as a single electron on each C(60) ball very weakly coupled. Theoretical calculations showed that the singlet and triplet states of C(120)O(2-) are nearly degenerate and can both be present in the experiment. The computed electron binding energies and excitation energies, as well as Franck-Condon factors, are used to help interpret the photoelectron spectra. A C-C bond-cleaved isomer, C(60)-O-C(60) (2-), was also observed with a higher electron binding energy of 1.54 eV.     

Correlation between hydroxyl fraction and O/Al atomic ratio as determined from XPS spectra of aluminium oxide layers

A set of five different aluminium oxide layers has been investigated using XPS. The oxide layers were made by oxidizing aluminium in a vacuum, with an alkaline and acidic pretreatment and in boiling water. Hydroxyl fractions of the aluminium oxide layers ranging from 0.0 to 0.5 were determined by constrained curve-fitting of the O 1s peak. The O/Al atomic ratios of the aluminium oxide layers, ranging from 1.5 to 2.0, were determined from the O 1s and Al 2p photoelectron intensities. A method is presented to account for the attenuation of the photoelectron intensities by the contamination overlayer. For the studied oxide layers, a linear relation is observed between the hydroxyl fraction and the O/Al atomic ratio of the aluminium oxide layers. It is concluded that the results obtained by the curve-fitting procedure are reliable. Furthermore, a linear relation is observed between the hydroxyl fraction and the O 1s peak width. The O 1s binding energies of the O²⁻ and OH⁻ components of the oxide layers correspond to 531.0 ± 0.1 eV and 532.4 ± 0.1 eV, respectively. Only pseudoboehmite showed 0.5 eV lower binding energies for these components. Angle-resolved XPS analysis showed that most of the studied oxides are enriched in hydroxyl groups at their outermost surface. Copyright © 2004 John Wiley & Sons, Ltd.     

Electron binding energies of aqueous alkali and halide ions: EUV photoelectron spectroscopy of liquid solutions and combined ab initio and molecular dynamics calculations

Photoelectron spectroscopy combined with the liquid microjet technique enables the direct probing of the electronic structure of aqueous solutions. We report measured and calculated lowest vertical electron binding energies of aqueous alkali cations and halide anions. In some cases, ejection from deeper electronic levels of the solute could be observed. Electron binding energies of a given aqueous ion are found to be independent of the counterion and the salt concentration. The experimental results are complemented by ab initio calculations, at the MP2 and CCSD(T) level, of the ionization energies of these prototype ions in the aqueous phase. The solvent effect was accounted for in the electronic structure calculations in two ways. An explicit inclusion of discrete water molecules using a set of snapshots from an equilibrium classical molecular dynamics simulations and a fractional charge representation of solvent molecules give good results for halide ions. The electron binding energies of alkali cations computed with this approach tend to be overestimated. On the other hand, the polarizable continuum model, which strictly provides adiabatic binding energies, performs well for the alkali cations but fails for the halides. Photon energies in the experiment were in the EUV region (typically 100 eV) for which the technique is probing the top layers of the liquid sample. Hence, the reported energies of aqueous ions are closely connected with both structures and chemical reactivity at the liquid interface, for example, in atmospheric aerosol particles, as well as fundamental bulk solvation properties.     

Highly efficient electron stimulated desorption of O+ from gadolinia-doped ceria surfaces

Highly efficient electron stimulated desorption of O+ from gadolinia-doped ceria (GDC) surfaces annealed at 850 K in ultrahigh vacuum is observed and investigated. O+ desorption has a major threshold of approximately 40 eV and an intrinsic kinetic energy of approximately 5.6 eV. Since the threshold energy is close to Ce 5s and Gd 5s core levels, Auger decay of core holes is likely associated with O+ desorption from sites related to oxygen vacancies. The interactions of water and molecular oxygen with GDC surfaces result in a decrease in O+ desorption, suggesting that water and oxygen molecules adsorb mainly to oxygen vacancies. The dependence of O+ kinetic energies on the incident electron energy and temperature reveals surface charging as a result of electron trapping, hole trapping, and electron-hole recombination. The activation energy for surface charge dissipation is found to be 0.43 eV, close to the activation energy for ionic conduction (0.47 to 0.6 eV) in the same material.     

Probing the electronic and structural properties of chromium oxide clusters (CrO3)n(-) and (CrO3)n (n = 1-5): photoelectron spectroscopy and density functional calculations

Photoelectron spectroscopy has been conducted for a series of (CrO3)n(-) (n = 1-5) clusters and compared with density functional calculations. Well-resolved photoelectron spectra were obtained for (CrO3)n(-) (n = 1-5) at 193 nm (6.424 eV) and 157 nm (7.866 eV) photon energies, allowing for accurate measurements of the electron binding energies, low-lying electronic excitations for n = 1 and 2, and the energy gaps. Density functional and molecular orbital theory (CCSD(T)) calculations were performed to locate the ground and low-lying excited states for the neutral clusters and to calculate the electron binding energies of the anionic species. The experimental and computational studies firmly establish the unique low-spin, nonplanar, cyclic ring structures for (CrO3)n and (CrO3)n(-) for n > or = 3. The structural parameters of (CrO3)n are shown to converge rapidly to those of the bulk CrO3 crystal. The extra electron in (CrO3)n(-) (n > or = 2) is shown to be largely delocalized over all Cr centers, in accord with the relatively sharp ground-state photoelectron bands. The measured energy gaps of (CrO3)n exhibit a sharp increase from n = 1 to n = 3 and approach to the bulk value of 2.25 eV at n = 4 and 5, consistent with the convergence of the structural parameters.     

Excitation-energy dependence of the mechanism for two-photon ionization of liquid H(2)O and D(2)O from 8.3 to 12.4 eV

Transient absorption measurements monitor the geminate recombination kinetics of solvated electrons following two-photon ionization of liquid water at several excitation energies in the range from 8.3 to 12.4 eV. Modeling the kinetics of the electron reveals its average ejection length from the hydronium ion and hydroxyl radical counterparts and thus provides insight into the ionization mechanism. The electron ejection length increases monotonically from roughly 0.9 nm at 8.3 eV to nearly 4 nm at 12.4 eV, with the increase taking place most rapidly above 9.5 eV. We connect our results with recent advances in the understanding of the electronic structure of liquid water and discuss the nature of the ionization mechanism as a function of excitation energy. The isotope dependence of the electron ejection length provides additional information about the ionization mechanism. The electron ejection length has a similar energy dependence for two-photon ionization of liquid D(2)O, but is consistently shorter than in H(2)O by about 0.3 nm across the wide range of excitation energies studied.     

Flexible H2O2 in water: electronic structure from photoelectron spectroscopy and ab initio calculations

The effect of hydration on the electronic structure of H(2)O(2) is investigated by liquid-jet photoelectron spectroscopy measurements and ab initio calculations. Experimental valence electron binding energies of the H(2)O(2) orbitals in water are, on average, 1.9 eV red-shifted with respect to the gas-phase molecule. A smaller width of the first peak was observed in the photoelectron spectrum from the solution. Our experiment is complemented by simulated photoelectron spectra, calculated at the ab initio level of theory (with EOM-IP-CCSD and DFT methods), and using path-integral sampling of the ground-state density. The observed shift in ionization energy upon solvation is attributed to a combination of nonspecific electrostatic effects (long-range polarization) and of the specific interactions between H(2)O(2) and H(2)O molecules in the first solvation shell. Changes in peak widths are found to result from merging of the two lowest ionized states of H(2)O(2) in water due to conformational changes upon solvation. Hydration effects on H(2)O(2) are stronger than on the H(2)O molecule. In addition to valence spectra, we report oxygen 1s core-level photoelectron spectra from H(2)O(2)(aq), and observed energies and spectral intensities are discussed qualitatively.     

XPS analysis of aluminum nitride films deposited by plasma source molecular beam epitaxy

Ten samples of crystalline aluminum nitride (AlN) film were deposited on sapphire and silicon substrates by a plasma source molecular beam method. The samples were analyzed using X‐ray photoelectron spectroscopy (XPS) depth profiling and high‐resolution X‐ray diffraction. Oxygen levels were observed to decrease exponentially from the surface into the bulk film. Aluminum, nitrogen and oxygen peaks were fitted with subpeaks in a consistent manner and the subpeaks were assigned to chemical states. AlN subpeaks were observed at 73.5 eV for Al2p and 396.4 eV for N1s. An N1s subpeak at 395.0 eV was assigned to N? N defects. No direct N? O bonds are assigned; rather it is proposed that an N? Al? O bond sequence is the source of higher binding energy N1s subpeaks. The observations in this study support a model in which oxygen is bound only to aluminum in the form of Al? O octahedral complexes dispersed or clustered throughout the main AlN matrix or as Al? O bonds on the crystal grain boundaries. The data also suggest that the AlN lattice parameters are related to oxygen content, since the c‐axis is observed to increase with increasing oxygen content. Copyright © 2008 John Wiley & Sons, Ltd.     

Accurate vertical ionization energy and work function determinations of liquid water and aqueous solutions

The absolute-scale electronic energetics of liquid water and aqueous solutions, both in the bulk and at associated interfaces, are the central determiners of water-based chemistry. However, such information is generally experimentally inaccessible. Here we demonstrate that a refined implementation of the liquid microjet photoelectron spectroscopy (PES) technique can be adopted to address this. Implementing concepts from condensed matter physics, we establish novel all-liquid-phase vacuum and equilibrated solution–metal-electrode Fermi level referencing procedures. This enables the precise and accurate determination of previously elusive water solvent and solute vertical ionization energies, VIEs. Notably, this includes quantification of solute-induced perturbations of water''s electronic energetics and VIE definition on an absolute and universal chemical potential scale. Defining and applying these procedures over a broad range of ionization energies, we accurately and respectively determine the VIE and oxidative stability of liquid water as 11.33 ± 0.03 eV and 6.60 ± 0.08 eV with respect to its liquid-vacuum-interface potential and Fermi level. Combining our referencing schemes, we accurately determine the work function of liquid water as 4.73 ± 0.09 eV. Further, applying our novel approach to a pair of exemplary aqueous solutions, we extract absolute VIEs of aqueous iodide anions, reaffirm the robustness of liquid water''s electronic structure to high bulk salt concentrations (2 M sodium iodide), and quantify reference-level dependent reductions of water''s VIE and a 0.48 ± 0.13 eV contraction of the solution''s work function upon partial hydration of a known surfactant (25 mM tetrabutylammonium iodide). Our combined experimental accomplishments mark a major advance in our ability to quantify electronic–structure interactions and chemical reactivity in liquid water, which now explicitly extends to the measurement of absolute-scale bulk and interfacial solution energetics, including those of relevance to aqueous electrochemical processes.

A generalised liquid-phase photoelectron spectroscopy approach is reported, allowing accurate, absolute energy scale ionisation energies of liquid water and aqueous solutions, as well as liquid water''s work function to be reported.     

Electron photoemission from charged films: absolute cross section for trapping 0-5 eV electrons in condensed CO2

The electron trapping or attachment cross section of carbon dioxide (CO2) condensed as thin films on a spacer of Ar is obtained using a simple model for electron trapping in a molecular film and then charge releasing from the same film by photon absorption. The measurements are presented for different electron exposures and impact energies, film thicknesses, and probing photon energies. The cross section for trapping an electron of incident energy between 0 and 5 eV reveals three different attachment processes characterized by a maximum at about 0.75 eV, a structured feature around 2.25 eV, and a shoulder around 3.75 eV. From the measurement of their dependence with the probing photon energy, the two lowest processes produce traps having a vertical electron binding energy of approximately 3.5 eV, whereas the highest one yields a slightly higher value of approximately 3.7 eV. The 0.75 eV maximum corresponds to the formation of vibrational Feshbach resonances in (CO2)n- anion clusters. The 2.25 eV feature is attributed to the formation of a vibrationally excited 2Piu anion in (CO2)n- clusters, followed by fast decay into its vibrational ground state without undergoing autodetachment. Finally, 3.75 eV shoulder is assigned to the well-known dissociative electron attachment process from 2Piu anion state producing the O- anion in the gas phase and the (CO2)nO- anions in clusters.     

Photoelectron spectroscopy of the [glycine x (H2O)(1,2)]- clusters: sequential hydration shifts and observation of isomers

The electron binding energies of the small hydrated amino acid anions, [glycine x (H2O)(1,2)]-, are determined using photoelectron spectroscopy. The vertical electron detachment energies (VDEs) are found to increase by approximately 0.12 eV with each additional water molecule such that the higher electron binding isomer of the dihydrate is rather robust, with a VDE value of 0.33 eV. A weak binding isomer of the dihydrate is also recovered, however, with a VDE value (0.14 eV) lower than that of the monohydrate. Unlike the situation in the smaller (n < or = 13) water cluster anions, the [Gly x (H2O)(n > or = 6)]- clusters are observed to photodissociate via water monomer evaporation upon photoexcitation in the O-H stretching region. We discuss this observation in the context of the mechanism responsible for the previously observed [S. Xu, M. Nilles, and K. H. Bowen, Jr., J. Chem. Phys. 119, 10696 (2003)] sudden onset in the cluster formation at [Gly x (H2O)5]-.     

High-resolution soft X-ray photoelectron spectroscopy of liquid water

High-resolution soft X-ray photoelectron spectra of liquid water (H(2)O and D(2)O) were measured using a liquid beam photoelectron spectrometer. The 1a(1) (O1s) band and the lowest valence 1b(1) band had single peaks, which is not consistent with the split 1b(1)→ 1a(1) of the X-ray emission band of liquid water if the splitting is assumed to originate from level shifts in two different hydrogen bonding structures. The second valence 3a(1) band of liquid water exhibited a flat top implying that two bands exist underneath a broad feature, which is similar to the case of the 3a(1) band of amorphous ice. The energy splitting between the two 3a(1) bands is estimated to be 1.38 eV (H(2)O) and 1.39 eV (D(2)O). Ab initio calculations suggest that the large splitting of the 3a(1) band is characteristic of water molecules that function as both proton donor and acceptor. The overall result is consistent with the conventional model of a tetrahedral hydrogen-bonding network in liquid water.     

杂多阴离子的XPS研究 II:Keggin结构钨钼杂多酸热稳定性差异的研究

加入优级纯CsCl作内标,用其中Cs3d5/2校正荷电效应,准确地测定了HPWnMo12-n(n=0,3,6,9,12)及HXM12(X=P,As,Si,Ge;M=W,Mo)中Ols及HPM12中P2p的结合能,发现钨系中Ols的结合能比钼系中的高约0.5eV;P2p的结合能则相应地高0.8eV;同种金属的磷系杂多酸中Ols的结合能比其他杂原子中的高0.3eV。通过差热分析发现同类型杂多酸的热稳定性与阴离子中的Ols结合能呈近似平行的关系。结果表明,跟钼系杂多酸相比,钨系杂多酸中氧原子有较低的电子云密度,钨氧之间有更多的轨道重叠;含磷杂多酸跟其他杂多酸相比,金属、氧之间有更强的相互作用。     

XPS study of the major minerals in bauxite: gibbsite, bayerite and (pseudo-)boehmite

Synthetic corundum (Al2O3), gibbsite (Al(OH)3), bayerite (Al(OH)3), boehmite (AlO(OH)) and pseudoboehmite (AlO(OH)) have been studied by high resolution XPS. The chemical compositions based on the XPS survey scans were in good agreement with the expected composition. High resolution Al2p scans showed no significant changes in binding energy, with all values between 73.9 and 74.4 eV. Only bayerite showed two transitions, associated with the presence of amorphous material in the sample. More information about the chemical and crystallographic environment was obtained from the O1s high resolution spectra. Here a clear distinction could be made between oxygen in the crystal structure, hydroxyl groups and adsorbed water. Oxygen in the crystal structure was characterised by a binding energy of about 530.6 eV in all minerals. Hydroxyl groups, present either in the crystal structure or on the surface, exhibited binding energies around 531.9 eV, while water on the surface showed binding energies around 533.0 eV. A distinction could be made between boehmite and pseudoboehmite based on the slightly lower ratio of oxygen to hydroxyl groups and water in pseudoboehmite.     

Monitoring super- and subsurface oxygen on Ag(210) by high energy resolution X-ray photoelectron spectroscopy: subsurface diffusion and segregation

We report on a high energy resolution X-ray photoelectron spectroscopy plus supersonic molecular beam investigation of O/Ag(210). Two components are detected in the O1s spectra upon O2 adsorption, at binding energies EB=527.7 and 529.6 eV. The former peak persists up to 470 K, while the latter one decreases abruptly above 280 K. Comparison with a previous vibrational spectroscopy investigation on the same system (L. Vattuone, et al. Phys. Rev. Lett. 2003, 90, 228302) allows to assign both features to atomic oxygen. The low-energy peak is identified with adatoms, while the other is correlated to O atoms in subsurface sites. A minor contribution at the same binding energy, due to carbonates, is quantified by inspection of the C1s region and shows a different temperature behavior with respect to oxygen. Oxygen segregation into the subsurface region is observed when heating the crystal in the presence of supersurface oxygen.     

Solvation of O(2)(-) and O(4)(-) by p-difluorobenzene and p-xylene studied by photoelectron spectroscopy

Anion photoelectron spectra of the O(2)(-) . arene and O(4)(-) . arene complexes with p-xylene and p-difluorobenzene are presented and analyzed with the aid of calculations on the anions and corresponding neutrals. Relative to the adiabatic electron affinity of O(2), the O(2)(-) . arene spectra are blueshifted by 0.75-1 eV. Solvation energy alone does not account for this shift, and it is proposed that a repulsive portion of the neutral potential energy surface is accessed in the detachment, resulting in dissociative photodetachment. O(2)(-) is found to interact more strongly with the p-difluorobenzene than the p-xylene. The binding motif involves the O(2)(-) in plane with the arene, interacting via electron donation along nearby C-H bonds. A peak found at 4.36(2) eV in the photoelectron spectrum of O(2)(-) . p-difluorobenzene (p-DFB) is tentatively attributed to the charge transfer state, O(2)(-) . p-DFB(+). Spectra of O(4)(-) . arene complexes show less blueshift in electron binding energy relative to the spectrum of bare O(4)(-), which itself undergoes dissociative photodetachment. The striking similarity between the profiles of the O(4)(-) . arene complexes with the O(4)(-) spectrum suggests that the O(4)(-) molecule remains intact upon complex formation, and delocalization of the charge across the O(4)(-) molecule results in similar structures for the anion and neutral complexes.