Mean Free Path Of Photoelectrons Research Articles

Interfacial Properties of a PEM Water Electrolysis Pt Electrode Probed by Operando Tender Ambient Pressure X-Ray Photoelectron Spectroscopy

The probe of the chemical, structural, and electronic properties at the electrode-electrolyte interfaces under operational conditions is critical for the mechanistic study and optimization of all the electrochemical systems and has therefore been at the center of numerous studies [1]. Soft X-ray Ambient pressure X-ray photoelectron spectroscopy (APXPS) has been known as a promising in-situ/operando technique, but its applications in the probe of a realistic solid/liquid interface under ambient conditions has been limited by the short inelastic mean free path (IMFP) of photoelectrons in the liquid medium. Here, inspired by prior work [2,3] and taking advantage of the synchrotron tender X-ray source (Beamline 9.3.1) at the Advanced Light Source of the Lawrence Berkeley National Laboratory [4], the IMFP can be significantly increased. A larger IMFP enables interfacial characterization under higher background pressures and through liquid overlayer greater than 30 nm in thickness. We have designed and built a graphene sealed operando electrolysis electrochemical cell that is compatible with an existing tender X-ray APXPS setup for the investigation of electrode-electrolyte interfaces under operational conditions. The present setup will be utilized to probe various systems involving solid/liquid interfaces, including but not limited to the interactions of ions, the solvation of salts, and electrocatalysis.In this work, the utilization of tender X-rays (2000 – 6000 eV) facilitates investigations of the electrode-electrolyte interface with a liquid-layer thickness of tens of nanometers, which can be preserved within the sandwiched region between the topmost graphene and the Pt nanoparticle loaded proton exchange membrane (PEM). In addition, the environmental water vapor pressure within the analysis chamber was kept at the water vapor pressure at 298 K (~15 Torr). The setup allows the study of a water splitting electrolyzer under real working conditions, where the electrocatalytic performance and the operando photoemission spectroscopy can be simultaneous investigated. As a proof-of-concept, Pt nanoparticle-catalyzed oxygen evolution reaction (OER) was investigated under operando conditions. The electrochemistry and detailed analysis of the photoemission spectra demonstrating the evolution of the chemical, structural, and electronic properties at the H2O/Pt interface under the influence of external electrical field will be presented.

(Invited) Application of a Laboratory-Based Scanning XPS/Haxpes Instrument for the Characterization of Buried Interfaces

X-ray Photoelectron Spectroscopy (XPS) is a widely used surface analysis technique with many well established industrial and research applications. The surface sensitivity (top 5-10 nm) of XPS and its ability to provide short-range chemical bonding information make the technique extremely popular in materials characterization and failure analysis laboratories. While its surface sensitivity is an important attribute, in some cases, the depth of analysis of XPS is not sufficient to analyze buried interfaces without first sputter etching the sample surface. However, sputter etching can often lead to alterations of the true surface chemistry. An alternative to sputter etching the sample is Hard X-ray Photoelectron Spectroscopy (HAXPES), available at some synchrotron facilities. HAXPES utilizes X-rays typically defined as having energies greater than 5 keV. By increasing the photon energy of the X-ray source, the mean free path of photoelectrons is increased, resulting in an increased information depth obtained from the sample. Depending on the energy used, these hard X-rays can provide depths of analysis three or more times than that of soft x-rays used on conventional XPS systems. HAXPES measurements are, therefore, more sensitive to the bulk, and contributions from the surface are minimized.1,2 This presentation will describe a laboratory-based instrument, the PHI Quantes, equipped with two scanning microprobe monochromated X-ray sources, Al Kα (1486.6 eV) and Cr Kα (5414.9 eV), thus enabling both traditional XPS and HAXPES experiments in the same instrument. Combining both soft and hard X-ray analyses, we can gain an even better understanding of composition with depth and information at buried interfaces. References Kobayashi, K. Hard X-ray photoemission spectroscopy, Nucl. Instr. Meth. Phys. Res. A 2009, 601, 32-47.Fadley, C.S. Hard X-ray Photoemission: An Overview and Future Perspective. In Hard X-ray Photoelectron Spectroscopy (HAXPS); Woicik, J. C., Ed; Springer: Switzerland 2016.

GaP/Si(0 0 1) interface study by XPS in combination with Ar gas cluster ion beam sputtering

The study of the chemical composition of buried interfaces by X-ray photoelectron spectroscopy (XPS) is limited by the inelastic mean free path of emitted photoelectrons (PE). Soft X-ray sources (AlKα) are generally suitable for careful probing of surfaces or very thin films. Here we applied gas cluster ion beam sputtering in combination with in-situ XPS (GCIB-XPS) to analyze buried GaP/Si(0 0 1) heterointerfaces. The GCIB method was used to dig a crater into the 20 nm thick GaP(0 0 1) film. We found optimal parameters for GCIB sputtering and achieved interface layers without severe damage. Destructive effects, i.e. broadening of core level peaks, could not be completely avoided, however, and the formation of metallic Ga on the GaP surface was observed. PE spectra of the sputtered heterostructures were compared with corresponding reference spectra of sputtered bulk crystals. Interface contributions to the intensity of phosphorus and sillicon core level peaks were revealed: interface components are shifted to high binding energies of P 2p Si 2p core levels. Similar results were obtained on 4 nm thick GaP/Si(0 0 1) by XPS. Finally, a top-to-bottom concept for buried semiconductor interfaces studies by GCIB-XPS is demonstrated.

2D Material Membranes for Operando Atmospheric Pressure Photoelectron Spectroscopy

Probing the chemistry that occurs at catalyst interfaces under realistic process conditions is key to the rational design of better materials for industrial catalytic reactions. Ambient pressure X-ray photoelectron spectroscopy, has until recently been limited to pressures two orders of magnitude below atmosphere. However, the development of photoelectron transparent membranes based on two-dimensional materials that can maintain large pressure differences and yet have thicknesses approaching or even falling below the inelastic mean free path of photoelectrons, now allows the atmospheric pressure regime and above to be accessed. We introduce here the fundamental principles underlying this membrane-based approach to atmospheric pressure photoelectron spectroscopy, and in this context highlight some of the key design concepts and challenges in performing experiments with this technique. We discuss a number of recent proof-of-concept studies, and highlight the potential of the membrane-based approach for operando characterisation of catalyst interfaces under reaction conditions, as well as some current challenges and limitations in this area.

X-ray Photoelectron Spectroscopy Studies of Nanoparticles Dispersed in Static Liquid.

For nanoparticles active for chemical and energy transformations in static liquid environment, chemistries of surface or near-surface regions of these catalyst nanoparticles in liquid are crucial for fundamentally understanding their catalytic performances at a molecular level. Compared to catalysis at a solid-gas interface, there is very limited information on the surface of these catalyst nanoparticles under a working condition or during catalysis in liquid. Photoelectron spectroscopy is a surface-sensitive technique; however, it is challenging to study the surfaces of catalyst nanoparticles dispersed in static liquid because of the short inelastic mean free path of photoelectrons traveling in liquid. Here, we report a method for tracking the surface of nanoparticles dispersed in static liquid by employing graphene layers as an electron-transparent membrane to separate the static liquid containing a solvent, catalyst nanoparticles, and reactants from the high-vacuum environment of photoelectron spectrometers. The surfaces of Ag nanoparticles dispersed in static liquid sealed in such a graphene membrane liquid cell were successfully characterized using a photoelectron spectrometer equipped with a high vacuum energy analyzer. With this method, the surface of catalyst nanoparticles dispersed in liquid during catalysis at a relatively high temperature up to 150 °C can be tracked with photoelectron spectroscopy.

Ambient Pressure Photoelectron Spectroscopy: Opportunities in Catalysis from Solids to Liquids and Introducing Time Resolution

AbstractPhotoelectron spectroscopy is an excellent technique to explore chemically complex systems in catalysis. However, due to a small mean free path of photoelectrons in gas, liquid, and solid media, the study of the gas–solid, liquid–gas, solid–liquid interfaces, as well as liquid homogeneous systems are a serious challenge. With differentially pumped analyzers this limitation ceases to exist and no longer restricts photoelectron spectroscopy only to ultra‐high vacuum conditions. Presently, photoemission studies at tens of mbar of pressure are possible. To reach atmospheric pressure and even higher, membrane‐covered closed cells have been developed. Graphene membranes are impermeable to molecules and almost transparent to photoelectrons. They are used as a pressure barrier between the enclosed cell at atmospheric pressure and the electron analyzer at vacuum while allowing transmission of photoelectrons. By accessing to atmospheric pressure range, with this kind of cell, photoemission studies will become a versatile in situ and operando tool for catalysis. Studies involving liquids in static conditions are an important aspect in this direction, which can be extended to homogeneous catalytic systems. Liquids are presently accessible using micro‐jets. Another aspect which is of paramount interest to investigate catalysts is time resolution. By improving the time resolution of photoemission measurements to the sub‐second regime it is possible to follow the kinetic changes that are crucial of a catalytic reaction, the changes that occur during catalyst pretreatment and activation, and notably to differentiate active species from spectator ones, which may be the dominating species in a classical experiment.

Correlation effect in Sr1−xLaxRuO3 studied by soft x-ray photoemission spectroscopy

To clarify how the electronic state of Sr1-xLaxRuO3 evolves with La doping, we conducted photoemission (PES) experiments using soft x-rays. The spectral shape of the Ru 4d derived peak near the Fermi level changes significantly with increasing x. This variation indicates that a spectral weight transfer from the coherent to incoherent component occurs due to an enhancement of the electron correlation effect. Resonant PES experiments at the La 3d_{5/2} edge have confirmed that there is no significant contribution of the La 5d state in the energy range where the spectral weight transfer is observed. Using the dependence of the photoelectron mean free path on the photon energy, we subtracted the surface components from the PES spectra and confirmed that the enhancement of the electron correlation effect with La doping is an intrinsic bulk phenomenon. On the other hand, a large portion of the coherent component remains at the Fermi level up to x = 0.5, reflecting that the Ru 4d state still has itinerant characteristics. Moreover, we found that the PES spectra hardly depend on the temperature and do not exhibit a discernible change with magnetic ordering, suggesting that the temperature variation of the exchange splitting does not follow the prediction of the Stoner theory. The presently obtained experimental results indicate that the electron correlation effect plays an important role in Sr1-xLaxRuO3 and that the Ru 4d electrons possess both local and itinerant characteristics.

(Invited) Hard X-Ray Photoelectron Spectroscopic Study on High-k Dielectrics Based Resistive Random Access Memor

Resistive random access memory (ReRAM) has been proposed as a new application for oxide materials and advanced to the commercial manufacturing stage. An oxide sandwiched between two metal electrodes shows reversible electric field–induced resistance switching behaviors. These are many resistivity changing mechanisms for the oxide based ReRAM structures such as the insulator–metal transition in perovskite oxides and conductive-bridging (CBRAM). In our research, we focus on the CBRAM with nano-electrolyte reaction, whose advantage is the low forming voltage. The CBRAM with nano-electrolyte reaction comprises the generation and rupture of a metal filament using a metal such as Ag and Cu acting as a fast mobile ion in oxides. Hafnium oxide (HfO2), which is used as a high-k gate insulator for advanced complementary metal-oxide-semiconductor (CMOS) technologies, has shown resistance switching phenomena and been increased interest in the use of HfO2 and related oxides as potential ReRAM materials.1) To put the oxide based ReRAM on practical applications, understanding on controls of metal/oxide interface is essentially important. Here, we employed hard x-ray photoelectron spectroscopy (HX-PES) under bias operation. HX-PES is a powerful tool for investigating the electronic structure and chemical state of the surface/interface of stacking structures for nanoelectronics devices without any degradation because it has a longer photoelectron mean free path than conventional x-ray photoelectron spectroscopy using Al Kα radiation (hν= 1486.6 eV). The detection depth of HX-PES with an energy of 6 keV is approximately three times deeper than that of conventional XPS, so the photoelectron from a metal/oxide interface, which works as an electrical device, can be detected by HX-PES. With this method, bias-induced compositional changes around the metal/oxide interface during device operation have been directly observed. HX-PES was performed at the SPring-8 BL15XU undulator beamline. The incident X-ray energy and the total energy resolution were 5.95 keV and 240 meV, respectively. We have demonstrated resistance switching using HfO2 film with a Cu top electrode for nonvolatile memory applications, and revealed the Cu diffusion into the HfO2 layer during the conductive filament formation process. Resistive switching was clearly observed in the Cu/HfO2/Pt structure by performing current-voltage measurements. The current step from a high resistive state to a low resistive state was of the order of 103-104, which provided a sufficient on/off ratio for use as a switching device. The filament formation process was investigated by employing HX-PES under bias operation. The application of a bias to the structure reduced the Cu2O state at the interface and the intensity ratio of Cu 2p3/2/Hf 3d5/2, providing evidence of Cu2O reduction and Cu diffusion into the HfO2 layer. These results also provide evidence that the resistance switching of the Cu/HfO2/Pt structure originates in a solid electrolyte reaction containing Cu ions. HX-PES also revealed the top or bottom electrode dependences of interface reduction or oxidization, and ion migration behaviors.2-5) In the presentation, we will show the details of the correlation between the switching mechanism and the interface reaction in the electrode/high-k dielectrics based ReRAM structure.6) We are grateful to HiSOR, Hiroshima Univ. and JAEA/SPring-8 for the development of HX-PES at BL15XU of SPring-8. The HX-PES measurements were performed under the approval of the NIMS Beamline Station (Proposal Nos. 2009A 4600, 2010B 4600, 2011A 4611, 2011B 4613, and 2012A 4613).

(Invited) Investigation of the Si/TiO2/Electrolyte Interface Using Operando Tender X-ray Photoelectron Spectroscopy

Photoelectrochemical (PEC) cells based on semiconductor-liquid interfaces provide a method of converting solar energy to electricity or fuels 1-3. Recently, we have demonstrated operational systems that involved stabilized semiconductor-liquid junctions 4,5. We have employed operando ambient-pressure X-Ray photoelectron spectroscopy (AP-XPS) on these highly stable semiconductor-liquids junctions to directly characterize the semiconductor-liquid junction at room temperature under real-time electrochemical control 6,7. The escape depth of photoelectrons that originate from tender X-ray synchrotron radiation — 2.3 to 5.2 keV — allows for sampling of the sample simultaneously near the sample surface but through an electrolyte layer as the inelastic mean free path of such photoelectrons is on the order of 5 to 10 nanometers. Accordingly, tender X-ray AP-XPS has enabled simultaneous monitoring of the semiconductor surface, the semiconductor-electrolyte interface, and the bulk electrolyte of a PEC cell as a function of the applied potential, E. Accumulation, depletion and Fermi level pinning were observed. The observed shifts in the core level emission binding energies with respect to the applied potential directly revealed ohmic and rectifying junction behavior on metallized and semiconducting samples, respectively. 1. H. J. Lewerenz et al., Energ. Environ. Sci., 3, 748–760 (2010). 2. H. J. Lewerenz, in Photoelectrochemical Materials and Energy Conversion Processes, R. C. Alkire, D. M. Kolb, J. Lipkowski, and P. N. Ross, Editors, vol. 12, p. 61–181, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany (2010). 3. Z. Huang, C. Xiang, H. J. Lewerenz, and N. S. Lewis, Energ. Environ. Sci., 7, 1207–1211 (2014). 4. S. Hu et al., Science, 344, 1005–1009 (2014). 5. M. F. Lichterman et al., Energy Environ. Sci., 7, 3334–3337 (2014). 6. S. Axnanda et al., Meet. Abstr., MA2013-02, 921–921 (2013). 7. S. Axnanda et al., (2014), submitted.

Investigation of group IVA elements combined with HAXPES and first-principles calculations

The core level and valence band spectra of group IVA elements were investigated with hard x-ray photoemission spectroscopy (HAXPES) photon energy of 7.939 keV by bulk sensitive manner. The survey and valance band spectra were presented, relative peaks intensity are discussed by thinking about inelastic mean free path (IMFP) and photoionization cross section of photoelectrons (PICS). In order to understand bulk band structures, valence bands are compared with the calculated ones by considering PICS, IMFP and total energy resolution. The calculated results by GGA, HSE06 and GW0 methods are simply discussed by comparing with experiment spectra.

(Invited) Photoelectron Spectroscopic Study on High-k Dielectrics Based Nanoionics-Type ReRAM Structure under Bias Operation

Resistive random access memory (ReRAM) has been proposed as a new application for oxide materials. An oxide sandwiched between two metal electrodes shows reversible electric field–induced resistance switching behaviors. Hafnium oxide (HfO2), which is used as a high-k gate insulator, has also shown resistance switching phenomena and been increased interest in the use of HfO2 and related oxides as potential ReRAM materials [1]. For the oxide based ReRAM with the conductive filament formation model, two mechanisms of resistance switching have been proposed. One is the nanoionics model, which comprises the generation and rupture of a metal filament using a metal such as Cu acting as a fast mobile ion in oxides. The other model is that of oxygen vacancy nucleation at the metal/oxide interface. To put the oxide based ReRAM on practical applications, understanding on controls of metal/oxide interface is essentially important.Here, we employed hard x-ray photoelectron spectroscopy (HX-PES) under bias operation to examine the electronic structure of Pt or Cu/HfO2 ReRAM structures in an operating device. HX-PES is a powerful tool for investigating the electronic structure and chemical state of the surface/interface of stacking structures for nanoelectronics devices without any degradation because it has a longer photoelectron mean free path than conventional x-ray photoelectron spectroscopy using Al Kα radiation (hν= 1486.6 eV). The detection depth of HX-PES with an energy of 6 keV is approximately three times deeper than that of conventional XPS, so the photoelectron from a metal/oxide interface, which works as an electrical device, can be detected by HX-PES. With this method, bias-induced compositional changes around the metal/oxide interface during device operation have been directly observed. The interface electronic states of 10-nm-thick Pt or Cu top electrodes/HfO2/Pt structures were measured with HX-PES in the SPring-8 BL15XU undulator beamline. The incident X-ray energy was 5.95 keV. In the case of the Pt/HfO2 interface, applying a forward bias increased the Pt–O bonding peak as shown in Fig 1, indicating evidence of Pt electrode oxidization and oxygen vacancy formation around the interface [2]. In contrast, the application of a bias to the Cu/HfO2 interface reduced the copper oxide bonding state, providing evidence of oxygen reduction and Cu diffusion into the HfO2 layer [3,4]. We achieved direct observation of oxygen migration at the metal/HfO2 interface under device operation, which is the key to controlling the electrical properties of oxide based ReRAM. Based on these results, we demonstrated the control of the switching voltage and the initial conductive filament formation process of the Ta−Nb binary oxide ((TaxNb1−x)2O5) ReRAM structure using a combinatorial method [5]. The relationship between the interface structure and the electrical properties also will be discussed in detail at the presentation.

Determination of electronic and atomic properties of surface, bulk and buried interfaces: Simultaneous combination of hard X-ray photoelectron spectroscopy and X-ray diffraction

Hard X-ray photoelectron spectroscopy (HAXPES) is a powerful novel emerging technique for bulk compositional, chemical and electronic properties determination in a non-destructive way. It benefits from the exceptionally large escape depth of high kinetic energy photoelectrons enabling the study of bulk and buried interfaces up to several tens of nanometres depth. Its advantage over conventional XPS is based on the long mean free path of high kinetic energetic photoelectrons. Using the advantage of tuneable X-ray radiation provided by synchrotron sources the photoelectron kinetic energy, i.e. the information depth can be changed and consequently electronic and compositional depth profiles can be obtained. The combination of HAXPES with an atomic structure sensitive technique, as X-ray diffraction, opens a new research field with great potential for many systems in which their electronic properties are intimately linked to their crystallographic structure. At SpLine, the Spanish CRG Beamline at the European Synchrotron Radiation Facility (ESRF) we have developed a novel and exceptional set-up that combine grazing incidence X-ray diffraction (GIXRD) and HAXPES. Both techniques can be operated simultaneously on the same sample and using the same excitation source. The set-up includes a heavy 2S+3D diffractometer and UHV chamber equipped with an electrostatic analyzer. The UHV chamber has also MBE evaporation sources, an ion gun, a LEED optic, a sample heating and cooling device, an electron gun, a UV discharge lamp, a low and medium energy X-ray tube, leak valves and a load-lock port. The photon energy ranges between 7 and 45keV. The HAXPES analyzer is an electrostatic cylinder-sector (FOCUS HV CSA), with a compact geometry and high transmission due to second order focusing. The analyzer is capable to handle kinetic energies both up to 15keV and down to a few eV with the same analyzer setup and power supply.

Three-Dimensional Electron Realm inVSe2by Soft-X-Ray Photoelectron Spectroscopy: Origin of Charge-Density Waves

The resolution of angle-resolved photoelectron spectroscopy (ARPES) in three-dimensional (3D) momentum k is fundamentally limited by ill defined surface-perpendicular wave vector k(perpendicular) associated with the finite photoelectron mean free path. Pushing ARPES into the soft-x-ray energy region sharpens the k(perpendicular) definition, allowing accurate electronic structure investigations in 3D materials. We apply soft-x-ray ARPES to explore the 3D electron realm in a paradigm transition metal dichalcogenide VSe2. Essential to break through the dramatic loss of the valence band photoexcitation cross section at soft-x-ray energies is the advanced photon flux performance of our synchrotron instrumentation. By virtue of the sharp 3D momentum definition, the soft-x-ray ARPES experimental band structure and Fermi surface of VSe2 show a textbook clarity. We identify pronounced 3D warping of the Fermi surface and show that its concomitant nesting acts as the precursor for the exotic 3D charge-density waves in VSe2. Our results demonstrate the immense potential of soft-x-ray ARPES to explore details of 3D electronic structure.

Spin Polarimetry and Magnetic Dichroism on a Buried Magnetic Layer Using Hard X-ray Photoelectron Spectroscopy

The spin-resolved electronic structure of buried magnetic layers is studied by hard X-ray photoelectron spectroscopy (HAXPES) using a spin polarimeter in combination with a high-energy hemispherical electron analyzer at the high-brilliance BL47XU beamline (SPring-8, Japan). Spin-resolved photoelectron spectra are analyzed in comparison with the results of magnetic linear and circular dichroism in photoelectron emission in the case of buried Co2FeAl0.5Si0.5 layers. The relatively large inelastic mean free path (up to 20 nm) of fast photoelectrons enables us to extend the HAXPES technique with electron-spin polarimetry and to develop spin analysis techniques for buried magnetic multilayers and interfaces.

Spin Polarimetry and Magnetic Dichroism on a Buried Magnetic Layer Using Hard X-ray Photoelectron Spectroscopy

The spin-resolved electronic structure of buried magnetic layers is studied by hard X-ray photoelectron spectroscopy (HAXPES) using a spin polarimeter in combination with a high-energy hemispherical electron analyzer at the high-brilliance BL47XU beamline (SPring-8, Japan). Spin-resolved photoelectron spectra are analyzed in comparison with the results of magnetic linear and circular dichroism in photoelectron emission in the case of buried Co2FeAl0.5Si0.5 layers. The relatively large inelastic mean free path (up to 20 nm) of fast photoelectrons enables us to extend the HAXPES technique with electron-spin polarimetry and to develop spin analysis techniques for buried magnetic multilayers and interfaces.

Effect of photoelectron mean free path on the photoemission cross-section of Cu(111) and Ag(111) Shockley states

The photoemission cross-section of Shockley states of Cu(111) and Ag(111) surfaces is studied over a wide range of photon energies. The constant initial-state spectra are very different for the two surfaces and show rich structure that does not follow the generally accepted nearly free electron model for the final state. Angle resolved photoemission data are interpreted within a one-step ab initio theory, revealing a multiple Bloch wave structure of photoemission final states. The inelastic scattering parameter-optical potential-is determined, and the energy dependence of the mean free path of the outgoing electron is calculated, which turns out to be the key for the understanding of the photoemission cross-section curve. These are essential steps for future exploration of wave function perturbations in the presence of surface nanostructures.

Attosecond spectroscopy of solids: Streaking phase shift due to lattice scattering

Theory of laser-assisted photoemission from solids is developed for a numerically exactly solvable model with full inclusion of band structure effects. The strong lattice scattering in the vicinity of band gaps leads to a distortion and a temporal shift of the streaking spectrogram of the order of 100 as. The effect is explained in terms of Bloch electron dynamics and is shown to remain large for an arbitrarily small photoelectron mean free path. The implications for the streaking experiment on W(110) [A. L. Cavalieri et al., Nature (London) {\bf 449}, 1029 (2007)]

Site-Specific Stereograph of SiC(0001) Surface by Inverse Matrix Method

The 2π-steradian (full hemisphere) Si 2p and C 1s photoelectron intensity angular distributions (PIADs) of the 6H-SiC(0001) surface 4° off towards the [1\bar100] direction were measured. In a bulk crystal, pairs of mirrored local atomic sites with respect to the 1\bar100 planes exist. Thus, a sixfold symmetry is expected for PIADs from the bulk. However, all the measured PIADs showed a threefold symmetry owing to the preferential appearance of terraces with one type of local atomic site caused by anisotropic step bunching along the [11\bar20] direction. Taking the finite inelastic mean free path of photoelectrons into account, PIADs for one kind of Si and C atomic sites were successfully derived by solving an inverse matrix. Three strong forward focusing peaks due to nearby Si and C atoms have been separated from those formed by farther atoms. They showed a circular dichroism of rotational shift around the incident-light axis, which corresponds to the parallax in stereo viewing.

Spatial Distribution of Nitrate and Nitrite Anions at the Liquid/Vapor Interface of Aqueous Solutions

Depth-resolved ion spatial distributions of nitrate and nitrite anions in aqueous solution have been quantitatively measured using X-ray photoemission spectroscopy on a 15 microm aqueous liquid jet containing 3 M NaNO(3), 3 M NaNO(2), or an equimolar mixture of the two. The surface region, which extends to photoelectron kinetic energies of 400-500 eV, is partially depleted in anions relative to the bulk 3 M concentration. The nitrate and nitrite solutions exhibit similar depth-dependent anion profiles. The results presented here are compared with recent molecular dynamics simulations of a NaNO(3) solution and are found to agree at high photoelectron kinetic energies. At shallower probe depths, the experiment measured a surface anion concentration less than that predicted by theory. Possible origins of the discrepancy are discussed in terms of the confined size of the simulation box and uncertainties that remain in regard to the inelastic mean free path of photoelectrons in aqueous media. The importance of our findings is discussed in relation to the observed increase in photochemical activity of nitrate-containing aerosols in the atmosphere.

Characterization of oxidized gallium droplets on silicon surface: An ellipsoidal droplet shape model for angle resolved X-ray photoelectron spectroscopy analysis

Deposition and oxidation of metallic gallium droplets on Si(111) were studied by angle resolved X-ray photoelectron spectroscopy. Two gallium peaks – Ga 3d and Ga 2p – were simultaneously measured in order to get an advantage of different inelastic mean free paths of photoelectrons from these two energy levels differing in binding energy by 1100 eV. Together with the angular dependent data it enhances the precision of the size characterization of Ga droplets and oxide thickness determination. A model for the calculation of theoretical intensities based on an ellipsoidal shape of droplets is presented and a simple procedure for estimation of droplet height and actual surface coverage based on measurement on a single emission angle is suggested.