Conventional Photoelectron Spectroscopy Research Articles

Spectroscopy of Radicals, Clusters, and Transition States Using Slow Electron Velocity-Map Imaging of Cryogenically Cooled Anions

Slow electron velocity-map imaging of cryogenically cooled anions (cryo-SEVI) is a high-resolution variant of anion photoelectron spectroscopy that has been applied with considerable success over the years to the study of radicals, size-selected clusters, and transition states for unimolecular and bimolecular reactions. Cryo-SEVI retains the versatility of conventional anion photoelectron spectroscopy while offering sub-meV resolution, thereby enabling the resolution of vibrational structure in the photoelectron spectra of complex anions. This Feature Article describes recent experiments in our laboratory using cryo-SEVI, including a new research direction in which anions are vibrationally pre-excited with an infrared laser pulse prior to photodetachment.

Dipole-Bound State, Photodetachment Spectroscopy, and Resonant Photoelectron Imaging of Cryogenically-Cooled 2-Cyanopyrrolide.

Valence-bound anions with polar neutral cores can have diffuse dipole-bound excited states just below the electron detachment threshold. Because of the similarity in geometry and vibrational frequencies between the dipole-bound states (DBSs) and the corresponding neutrals, DBSs have been exploited as intermediate states to conduct resonant photoelectron spectroscopy (PES), resulting in highly non-Franck-Condon photoelectron spectra via vibrational autodetachment and providing much richer vibrational information than conventional PES. Here, we report a photodetachment and high-resolution photoelectron imaging study of the 2-cyanopyrrolide anion, cooled in a cryogenic ion trap. The electron affinity of the 2-cyanopyrrolyl radical is measured to be 3.0981 ± 0.0006 eV (24 988 ± 5 cm-1). A DBS is observed for 2-cyanopyrrolide at 240 cm-1 below its detachment threshold using photodetachment spectroscopy. Twenty-three above-threshold vibrational resonances (Feshbach resonances) of the DBS are observed. Resonant PES is conducted at each Feshbach resonance, yielding a wealth of vibrational information about the 2-cyanopyrrolyl radical. Resonant two-photon PES confirms the s-like dipole-bound orbital and reveals a relatively long lifetime of the bound zero-point level of the DBS. Fundamental frequencies for 19 vibrational modes (out of a total of 24) are obtained for the cyanopyrrolyl radical, including six out-of-plane modes. The current work provides important spectroscopic information about 2-cyanopyrrolyl, which should be valuable for the study of this radical in combustion or astronomical environments.

Electron affinities of small-molecule organic semiconductors: Comparison among cyclic voltammetry, conventional inverse photoelectron spectroscopy, and low-energy inverse photoelectron spectroscopy

The electron affinity ( A ) of an organic semiconductor is an energy parameter representing the electron transport level. The precise value of A in a solid is indispensable for examining and designing organic semiconductor devices. In principle, A in a solid can be determined by inverse photoelectron spectroscopy (IPES). However, because IPES is normally available only for specialists of IPES, A is often estimated from the reduction potential E red measured in solution using a more easily available method, cyclic voltammetry (CV). Thus, the conversion relation from E red into A is practically important in the research of organic semiconductors. On the other hand, previous IPES data may have errors of about 0.3–0.5 eV owing to sample damage and low energy resolution. In 2012, one of the authors developed low-energy inverse photoelectron spectroscopy (LEIPS), which enables the determination of A with the precision of 0.1 eV. In this work, we establish the correlation between CV data E red and solid-state A based on LEIPS data and suggest the relation A = (1.24 ± 0.07) × e E red + (5.06 ± 0.15) eV. We also discuss the reliability of the A values determined by the previous IPES and the present LEIPS in terms of the LUMO spectral onset and the vacuum level. • Electron affinities of 30 small-molecule organic semiconductors. • Correlation between electron affinity and reduction potential. • Electron affinities determined by low-energy inverse photoelectron spectroscopy. • Comparison between conventional and low-energy inverse photoelectron spectroscopies.

Surface, Interface, and Bulk Electronic and Chemical Properties of Complete Perovskite Solar Cells: Tapered Cross-Section Photoelectron Spectroscopy, a Novel Solution.

The surface, interface, and bulk properties are a few of the most critical factors that influence the performance of perovskite solar cells. The photoelectron spectroscopy (PES) is used as a technique to analyze these properties. However, the information depth of PES is limited to 10-20 nm, which makes it not suitable to study the complete devices, which have a thickness of ∼1 μm. Here, we introduce a novel and simple technique of PES on a tapered cross section (TCS-PES). It provides both lateral and vertical resolutions compared to the conventional PES so that it is suitable to study a complete perovskite solar cell. It offers many benefits over conventional PES methods such as the chemical composition in the micrometer scale from the surface to the bulk and the electronic properties at the multiple interfaces. The chemical natures of different layers of the perovskite-based solar cells [(FAPbI3)0.85(MAPbBr3)0.15] can be identified precisely for the first time using the TCS-PES method. We found that the perovskite layer has higher iodine concentration at the Spiro/perovskite interface and higher bromine concentration at the TiO2/perovskite interface. UPS measurements on the tapered cross section revealed that the perovskite is n-type, and the solar cell studied here is a p-n-n structure type device. The unique possibilities to analyze the complete solar cell by XPS and UPS allow us to estimate the band bending in a working solar cell. Moreover, this technique can further be used to study the device under operating conditions, and it can be applied in other solid-state devices like solid electrolyte Li-ion batteries, LEDs, or photoelectrodes.

One-dimensionality of the spin-polarized surface conduction and valence bands of quasi-one-dimensional Bi chains on GaSb(110)-( 2×1 )

Surface electronic structure and its one-dimensionality above and below the Fermi level ($E_{\rm F}$) were surveyed on the Bi/GaSb(110)-(2$\times$1) surface hosting quasi-one-dimensional (Q1D) Bi chains, using conventional (one-photon) and two-photon angle-resolved photoelectron spectroscopy (ARPES) and theoretical calculations. ARPES results reveal that the Q1D electronic states are within the projected bulk bandgap. Circular dichroism of two-photon ARPES and density-functional-theory calculation indicate clear spin and orbital polarization of the surface states consistent with the giant sizes of Rashba-type SOI, derived from the strong contribution of heavy Bi atoms. The surface conduction band above $E_{\rm F}$ forms a nearly straight constant-energy contour, suggesting its suitability for application in further studies of one-dimensional electronic systems with strong SOI. A tight-binding model calculation based on the obtained surface electronic structure successfully reproduces the surface band dispersions and predicts possible one- to two-dimensional crossover in the temperature range of 60--100~K.

Coexisting Charge States in a Unary Organic Monolayer Film on a Metal.

The electronic and geometric structures of tetracene films on Ag(110) and Cu(110) have been studied with photoemission tomography and compared to that of pentacene. Despite similar energy level alignment of the two oligoacenes on these surfaces revealed by conventional ultraviolet photoelectron spectroscopy, the momentum-space resolved photoemission tomography reveals a significant difference in both structural and electronic properties of tetracene and pentacene films. Particularly, the saturated monolayer of tetracene on Ag(110) is found to consist of two molecular species that, despite having the same orientation, are electronically very different-while one molecule remains neutral, another is charged because of electron donation from the substrate.

Probing a battery electrolyte drop with ambient pressure photoelectron spectroscopy

Operando ambient pressure photoelectron spectroscopy in realistic battery environments is a key development towards probing the functionality of the electrode/electrolyte interface in lithium-ion batteries that is not possible with conventional photoelectron spectroscopy. Here, we present the ambient pressure photoelectron spectroscopy characterization of a model electrolyte based on 1M bis(trifluoromethane)sulfonimide lithium salt in propylene carbonate. For the first time, we show ambient pressure photoelectron spectroscopy data of propylene carbonate in the liquid phase by using solvent vapor as the stabilizing environment. This enables us to separate effects from salt and solvent, and to characterize changes in electrolyte composition as a function of probing depth. While the bulk electrolyte meets the expected composition, clear accumulation of ionic species is found at the electrolyte surface. Our results show that it is possible to measure directly complex liquids such as battery electrolytes, which is an important accomplishment towards true operando studies.

Anomalous photoelectric emission from Ag on zinc-phthalocyanine film

Photoelectric emission from organic and metal thin films is generally observed with irradiation of photon energy larger than 4 eV. In this paper, however, we report photoelectric emission from Ag on a zinc-phthalocyanine (ZnPc) layer at a photon energy of 3.4 eV. The threshold energy for this photoelectric emission is much smaller than the work function of Ag estimated by conventional photoelectron spectroscopy. The photoelectric emission by low-energy photons is significant for Ag thicknesses of less than 1 nm. Photoelectron spectroscopy and morphological study of the Ag/ZnPc suggest that the anomalous photoelectric emission from the Ag surface is caused by a vacuum level shift at the Ag/ZnPc interface and by surface plasmons of the Ag nanoparticles.

Photoelectron spectroscopy of wet and gaseous samples through graphene membranes.

Photoelectron spectroscopy (PES) and microscopy are highly important for exploring morphologically and chemically complex liquid-gas, solid-liquid and solid-gas interfaces under realistic conditions, but the very small electron mean free path inside dense media imposes serious experimental challenges. Currently, near ambient pressure PES is conducted using dexterously designed electron energy analyzers coupled with differentially pumped electron lenses which make it possible to conduct PES measurements at a few hPa. This report proposes an alternative ambient pressure approach that can be applied to a broad class of samples and be implemented in conventional PES instruments. It uses ultrathin electron transparent but molecular impermeable membranes to isolate the high pressure sample environment from the high vacuum PES detection system. We demonstrate that the separating graphene membrane windows are both mechanically robust and sufficiently transparent for electrons in a wide energy range to allow soft X-ray PES of liquid and gaseous water. The performed proof-of-principle experiments confirm the possibility to probe vacuum-incompatible toxic or reactive samples placed inside such hermetic, gas flow or fluidic environmental cells.

Surface Cu depletion of Cu(In,Ga)Se 2 films: An investigation by hard X-ray photoelectron spectroscopy

The origin of surface Cu depletion of polycrystalline chalcopyrite thin films and its consequences for the physics of related solar cells have been discussed for the past 15 years. In order to shed light on the composition and thickness of this Cu-depleted surface layer, depth-dependent compositional analysis by hard X-ray photoelectron spectroscopy was performed. The data from Cu-poor grown Cu(In,Ga)Se 2 samples point to a surface layer in the sub-nanometer regime, which is completely depleted of Cu. This result supports the surface reconstruction model proposed by first-principles calculations by other authors. Analysis of the surface morphology of the investigated samples confirms the conjunction of Cu depletion and faceting of the surface. Theoretical considerations show that the apparent surface concentration ratio of [Cu]/([In] + [Ga]) = 1/3 found by conventional photoelectron spectroscopy studies can be explained by the surface reconstruction model.

Potential and restriction of high resolution, high energy photoemission in 400–8,000 eV for studying strongly correlated electron systems

High energy photoelectron spectroscopy (PES) is an essential tool to overcome the high surface sensitivity of the conventional photoelectron spectroscopy for the study of bulk electronic structures of strongly correlated electron systems, which often have many different electronic structures in the surface and the bulk. The potential of the soft X-ray ARPES (SX-ARPES) in the several hundred eV region for bulk Fermiology of such systems is first demonstrated. Higher bulk sensitivity is achieved by hard X-ray PES (HAXPES) beyond several keV. Deconvolution of the surface, subsurface and bulk is feasible by combining SX-PES and HAXPES spectra measured at several very different h ν. Recoil effects of photoelectrons are observed in HAXPES not only in core spectra but also in valence band spectra of some materials with light elements. The present status and future possibility of high energy PES are discussed.

Threshold photoelectron spectroscopy of ozone

Threshold photoelectron spectrum of ozone is presented for the first time at a resolution of 21-38 meV using synchrotron radiation in the energy region of 12-21 eV. The ionization energies of the first ionized states were determined and an interpretation of the O3 spectrum with respect to its first three ionic states, 1 2A1, 1 2B2, and 1 2A2, is presented. Above 16 eV the enhancement of the intensities of the 2 2B1, 3 2A1, and 4 2B2 band systems due to the contribution of indirect processes was observed, not accessible by conventional photoelectron spectroscopy. It was also resolved and assigned the extensive vibrational structures of ozone. Between 15.5 and 18.5 eV the main band contours are similar to those found in conventional photoelectron spectroscopy, except that our threshold photoelectron spectrum reveals extensive additional vibrational structures. The band 2 2B1 was found to present an irregular vibrational spacing DeltaE, with a minimum value of 80 meV at approximately 16.47 eV.

ION PAIR DISSOCIATION: Spectroscopy and Dynamics

Ion pair dissociation processes may be studied using coherent vacuum ultraviolet laser sources in a manner entirely analogous to photoelectron spectroscopy, albeit with the anion playing the role of a heavy electron. If the excitation energy is above the dissociation energy and the kinetic energy of the fragment is measured using ion imaging, this approach is termed ion pair imaging spectroscopy (IPIS) and is related to conventional photoelectron spectroscopy. If the excitation energy is just below the dissociation energy and pulsed-field dissociation is employed, this approach is analogous to mass analyzed threshold ionization (MATI) spectroscopy and is termed threshold ion pair production spectroscopy (TIPPS). These approaches provide a novel means of investigating ion thermochemistry and spectroscopy and superexcited state decay dynamics at high resolution.

X-Ray Magnetic Circular Dichroism and Resonant Photomission ofV(TCNE)xHybrid Magnets

Thin films of V(TCNE)x were deposited in ultrahigh vacuum using a film growth technique based on in situ chemical vapor deposition of tetracyanoethylene, TCNE, and bis-benzene vanadium, V(C6H6)2. The in situ preparation method enabled, for the first time, experimental analysis of oxygen-free films. X-ray magnetic circular dichroism measurements recorded at the V L(2,3) edge confirmed room temperature magnetic ordering. A combination of conventional photoelectron spectroscopy (PES) and resonant photoemission (RPE) measured at the V L3 edge shows that the highest occupied electronic state is V(3d) derived. The rearrangements of the TCNE- related valence electronic states observed in PES and the evidence of V(3d) and TCNE- pi(pi*) orbital overlap contained in RPE spectra, indicate that strong, covalent type bonding occurs between the vanadium and the TCNE molecules.

Chemical reactions at Cu∕ZnS(001) and In∕ZnS(001) heterojunctions: A comparison of photoelectron and SL2,3 x-ray emission spectroscopy

Occurrence and extent of chemical reactions at Cu∕ZnS(001) and In∕ZnS(001) heterojunctions have been investigated by SL2,3 x-ray emission spectroscopy as well as photoelectron spectroscopy. With the formation of metal-sulfur bonds, spectral features originating from shallow metal d core levels (Zn3d, In4d) or valence states (Cu3d)) may appear in the SL2,3 emission spectra. Thus the x-ray emission spectroscopy was employed to detect chemical reactions at the heterojunctions, together with conventional photoelectron spectroscopy. Considerable reactions at the Cu∕ZnS(001) interface are more clearly indicated in the SL emission spectrum than in the Cu2p3∕2 or S2p core level spectra, whereas relatively confined reactions at the In∕ZnS(001) interface can only be probed in the In3d5∕2 core level spectra. The partial densities of states calculated for a reference CuInS2 on the basis of density functional theory agree well with features occurring in its SL2,3 emission spectrum.

Studies on the Core-level-excitation-induced Ion Desorption from Solid Surfaces.

The core-level-excitation-induced ion desorption from surface is investigated. Two studies using electron-ion coincidence spectroscopy are shown. On Si(100)/H2O surface, it is shown that ion desorption is mainly induced by the shake-up/off excitation accompanying the Auger decay when the photon energy is near the O1s threshold. At a photon energy higher than the shake-up threshold, most of ions desorb resulting from the shake-up excitation accompanying the core-excitation. In both cases, the desorption is induced by the multi-hole final state. On ice surface, the kinetic energy of O1s photoelectrons gives the highest coincidence yield of H+ desorption is shifted by about −0.7 eV compared to the O1s peak observed in the conventional core-level photoelectron spectroscopy. It is ascribed to a core-level shift in the O1s level from which hydrogen ions desorb.

Electron-Ion Coincidence Spectroscopy as a New Tool for Surface Analysis –an Application to the Ice Surface

Electron-ion coincidence (EICO) spectroscopy [K. Mase, M. Nagasono, S. Tanaka, M. Kamada, T.Urisu and Y. Murata: Rev. Sci. Inst. 68 (1997) 1703] has recently been developed to investigate the process of ion desorption induced by the core level excitation. In the present study, we apply EICO spectroscopy to determine the O1s level of condensed H2O (ice) at 100 K. The kinetic energy of O1s photoelectrons which gives the highest coincidence yield of H+ desorption is shifted by about -0.7 eV compared to the O1s peak observed in the conventional core-level photoelectron spectroscopy. It is ascribed to a core-level shift in the O1s level from which hydrogen ions desorb. The results indicate the advantages and the possibilities of the EICO spectroscopy for surface analysis.

Sb desorption from Sb/GaAs(001) and GaSb(001) analyzed by core-level photoelectron spectroscopy

Abstract The process of Sb desorption from GaSb(001) and Sb-terminated GaAs(001) was investigated by using a real-time analysis system that uses time-resolved core-level photoelectron spectroscopy. From the photoelectron intensity analysis, two competing processes with fast and slow time constants were found on the Sb desorption from both substrates and the activation energies were determined to be 0.71 eV and 0.90 eV for GaSb(001) and 1.1 eV and 2.5 eV for Sb-terminated GaAs(001). We investigated the relationship between the activation energies and surface bonding structure using spectrum differences based on the assignment of conventional core-level photoelectron spectroscopy of the surfaces whose Sb coverage is well controlled. This indicated that the activation energy of the desorption related to the transition from 2×8 to 2×4 of an Sb-terminated GaAs(001) is larger than the other desorption processes and it can be explained that the large potential barrier for bonding structure rearrangement between 2×8 superstructure and 2×4 superstructure exist in addition to Sb atoms desorption.

Temperature Controled Photoelectron Spectroscopic Investigation of Volatile Species on PbS(100) Surface

A special sample precooling technique has been utilised to preserve thin layers of volatile chemical species on the sample surface in ultra high vacuum conditions for investigation with conventional and synchrotron excited photoelectron spectroscopy. The precooling system has been constructed for the Scienta 300 electron spectrometer at the beam line 51 at MAX I synchrotron laboratory in Lund (Sweden) and it will be transferred to beam line 411 at the new MAX II storage ring. The spectrometer also includes a possibility for continuous temperature control of the sample during the measurement at temperature range 70 K – 450 K. In this work, temperature controlled measuring technique has been applied in a study of electrochemically treated PbS (100) surface. The measured sulfur 2p core level spectra clearly reveal a layer structure of elemental sulfur and metal polysulfides formed during anodic oxidation of PbS in acidic aqueous solution. Similar measuring technique can also be applied, e.g., in studies of adsorption layers on various substrates.

Photoelectron Microscopy and Its Applications to Semiconductor Science

Photoelectron spectromicroscopy, the combination of conventional photoelectron spectroscopy and high lateral resolution, is a novel technique that exploits the high brightness of the most recent synchrotron sources. We briefly review its main implementation modes and present some examples of applications, with particular emphasis on semiconductor interface research.