Main Valence Band Research Articles

Surface carrier dynamics and diffusion in inorganic metal-halide perovskite films

Carrier dynamics in metal-halide perovskites, in particular diffusion, has typically been investigated on micrometer-length scales and above. However, surfaces and interfaces modify the carrier dynamics, e.g., by interface band bending and recombination. To investigate the carrier dynamics and diffusion at the surface, we use time-resolved photoelectron spectroscopy which is sensitive to the photoexcited carriers in the topmost few nm of the material. We extract electron mobilities in well-ordered ${\mathrm{CsPbBr}}_{3}$(001) and ${\mathrm{CsSnBr}}_{3}$(001) films at room temperature of $31\ifmmode\pm\else\textpm\fi{}6$ and $13\ifmmode\pm\else\textpm\fi{}1\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$, respectively. The mobility in ${\mathrm{CsPbBr}}_{3}$(001) increases to $200\ifmmode\pm\else\textpm\fi{}8\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ at 90 K, indicating strong electron-phonon coupling for electrons at the conduction band minimum. Temperature-dependent photoemission experiments find different phonon coupling mechanisms for high-energetic electrons and holes at the valence band maximum and in the main valence bands.

Physical properties and superconductivity in the cubic A15-type Ta[formula omitted]Ge compound: A first principles study

In this theoretical study, we have targeted to present the characteristic features of the superconductivity in the cubic A15-type compound Ta3Ge by investigating its structural, electronic, elastic, mechanical, phonon, and electron–phonon interaction properties in detail. A critical examination of elastic moduli of Ta3Ge reveals that it is soft (ductile) due to significant metallic bonding. A critical surveying of electronic density of states reveals that Ge electronic states make negligible additive to the main valence band region of Ta3Ge while its transition metal d states dominate this region. An examination of our phonon results indicate that the low-frequency optical phonon branches and three acoustic phonon branches of Ta3Ge are mainly dominated by the vibrations of Ta atoms. Thereby, their d states and their vibrations become the main actors of the transition from the normal state to the superconducting state for Ta3Ge. From the integration of the Eliashberg spectral function of Ta3Ge, the values of average electron–phonon coupling parameter and logarithmically averaged phonon frequency are found to be 1.08 and 123.05 K, respectively, producing its superconducting temperature Tc = 8.3 K, being in good accordance with the experimental value of 8.0 K.

Multiple mobile excitons manifested as sidebands in quasi-one-dimensional metallic TaSe3.

Charge neutrality and their expected itinerant nature makes excitons potential transmitters of information. However, exciton mobility remains inaccessible to traditional optical experiments that only create and detect excitons with negligible momentum. Here, using angle-resolved photoemission spectroscopy, we detect dispersing excitons in the quasi-one-dimensional metallic trichalcogenide, TaSe3. The low density of conduction electrons and the low dimensionality in TaSe3 combined with a polaronic renormalization of the conduction band and the poorly screened interaction between these polarons and photo-induced valence holes leads to various excitonic bound states that we interpret as intrachain and interchain excitons, and possibly trions. The thresholds for the formation of a photo-hole together with an exciton appear as side valence bands with dispersions nearly parallel to the main valence band, but shifted to lower excitation energies. The energy separation between side and main valence bands can be controlled by surface doping, enabling the tuning of certain exciton properties.

Electronic Structure of Lanthanide-Doped Bismuth Vanadates: A Systematic Study by X-ray Photoelectron and Optical Spectroscopies

Monoclinic BiVO 4 has emerged in recent years as one of the most promising materials for photocatalytic evolution of oxygen under solar irradiation. However, it is in itself unable to phototcatalyze reduction of water to hydrogen due to the placement of the conduction band edge below the potential required for H 2 O/H 2 reduction. As a consequence, BiVO 4 only finds application in a hybrid system. Very recently, tetragonal lanthanide-doped BiVO 4 powders have been shown to be able to both reduce and to oxidize water under solar irradiation, but to date there has been no comprehensive study of the electronic properties of lanthanide-doped bismuth vanadates aimed at establishing the systematic trends in the electronic structure in traversing the lanthanide series. Here, the accessible family of lanthanide-doped BiVO 4 quaternary oxides of stoichiometry Bi 0.5 Ln 0.5 VO 4 (Ln = La to Lu, excluding Pm) has been studied by X-ray powder diffraction, X-ray photoemission spectroscopy, and diffuse reflectance optical spectroscopy. The compounds all adopt the tetragonal zircon structure, and lattice parameters decrease monotonically in traversing the lanthanide series. At the same time, there is an increased peak broadening in the diffraction patterns as the mismatch in ionic radius between Bi 3+ and the Ln 3+ ions increases across the series. Valence band X-ray photoemission spectra show that the final state 4f n-1 structure associated with ionization of lanthanide 4f n states is superimposed on the valence band structure of BiVO 4 in the quaternary materials: in the case of the Ce-, Pr- and Tb-doped BiVO 4 , 4f-related states appear above the top of the main valence band of BiVO 4 and account for the small bandgap in the Ce compound. In all cases, the 4f structure is characteristic of the lanthanide element in the Ln(III) oxidation state. Vanadium 2p and lanthanide 3d or 4d core level photoelectron spectra of those compounds where the lanthanide may in principle adopt a higher (Ln = Ce, Pr, Tb) or lower (Ln = Eu, Yb) oxidation state further confirm the prevalence of the Ln(III) valence state throughout. The visible region optical properties of all samples were studied by diffuse reflectance spectroscopy, with a particular focus on the optical bandgap and the details of transitions associated with localized 4f states. Taken together, the results demonstrate the remarkable tunability of optical and electronic properties for these quaternary materials.

Multi-zone single-shot femtosecond laser ablation of silica glass at variable multi-photon ionization paths

Single-shot femtosecond laser ablation of silica glass was studied at moderate peak intensities, providing laser energy deposition in the material via multi-photon absorption prior formation of dense opaque plasma, in order to evaluate the corresponding nonlinear absorption coefficients. For this purpose, tightly focused 515-nm, 220-fs laser pulses produce on a silica glass surface at variable pulse energies single-shot micro-craters, characterized by 3D-scanning confocal laser microscopy. Our analysis of crater diameter dependences on incident laser energy in terms of focusing parameters (Liu analysis) reveals in certain intensity ranges different multi-photon—one-, two-, and four-photon—absorption mechanisms with the increasing integer number of photons. Their depth profiles were considered as sets of distances, representing nonlinear transmission of the incident focused Gaussian beams until the fixed ablation threshold iso-fluence level, and analyzed in terms of the multi-photon absorption mechanisms. These inter-band transitions involve the electronic density-of-states tails in the bandgap (one- and two-photon transitions) and between the main valence and conduction bands (four-photon transitions). In these intensity ranges the depth of the single-shot craters can be fitted, accounting for the derived multi-photon absorption mechanisms, to evaluate the corresponding multi-photon absorption coefficients. The intensity-dependent variation of multi-photon absorption mechanisms provides the three-zone ablation both across the surface and into the bulk of the silica glass.

Communication: Evidence of halogen bonds in clathrate cages.

We present a theoretical characterization of the interaction of Cl2 and Br2 in the 512 and 51262 clathrate cages, respectively, based on energy partitioning analysis and a study of the electronic shifts associated with transitions to the main valence bands. Our analysis clearly shows that while Br2@51262 does not show halogen bonding interactions in its equilibrium geometry, Cl2@512 presents all the characteristics expected for halogen bonding. This is accomplished by the interaction of the usual sigma-hole with the lone pair of the closest oxygen atom involved in hydrogen bonding within the cage framework, though breaking of the hydrogen bond is not required. This possibility, which had not been considered in previous analyses, opens up a new way of looking at the interactions of dihalogens with the nearest water molecules in the cage.

Origin of band-gap bowing in wurtzite AlN1−xPx alloys

The structural and electronic properties of AlN1−xPx semiconductors have been investigated from first principles within the density functional theory (DFT). An anomalously large bowing of calculated band-gaps was revealed in these materials. For low P contents x, in homogeneous atomic configurations of the alloy, this effect originates from the hybridised P 3p and N 2p bands located above the valence band maximum of the host AlN system. Such a band-gap bowing is strongly enhanced in clustered configurations of atoms in studied alloys as a result of broadening and incorporation of these states into the main valence band region.

Valence Band Splitting on Multilayer MoS2: Mixing of Spin-Orbit Coupling and Interlayer Coupling.

Understanding the origin of valence band splitting is important because it governs the unique spin and valley physics in few-layer MoS2. We explore the effects of spin-orbit coupling and interlayer coupling on few-layer MoS2 using first-principles methods. We find spin-orbit coupling has a major contribution to the valence band splitting at K in multilayer MoS2. In double-layer MoS2, the interlayer coupling leads to the widening of the gap between the already spin-orbit split states. This is also the case for the bands of the K-point in bulk MoS2. In triple-layer MoS2, the strength of interlayer coupling of the spin-up channel becomes different from that of spin-down at K. This combined with spin-orbit coupling results in the band splitting in two main valence bands at K. With the increase of pressure, this phenomenon becomes more obvious with a decrease of main energy gap in the splitting valence bands at the K valley.

Effect of Sn surface states on the photocatalytic activity of anatase TiO2

The influence of surface Sn-doping on the photocatalytic properties of anatase TiO2 has been investigated in samples prepared by a grafting route using Sn(IV) tert-butoxide as Sn precursor. The grafting procedure leads to the formation of isolated Sn(IV) sites on the surface of anatase TiO2 powders as gauged by structural characterisation based on XRD, Raman spectroscopy and XAS. Studies of the surface reduction based on TPR experiments and XPS provide the conditions for a selective reduction of surface Sn(IV) to the divalent oxidation state. Electronic structure characterisation based on valence band XPS and DRS shows that there is a slight widening of the band gap upon Sn(IV)-grafting, but Sn(II) related states emerge at the top of the main valence band upon reduction at temperatures up to 350°C, and this induces visible light absorption. Grafting of TiO2 with Sn(IV) increases the formation rate of OH radicals on the surface of the material. Reduction of the Sn(IV)-grafted TiO2 to form surface Sn(II) brings about substantial increase of the photocatalytic efficiency for the methylene blue degradation under irradiation with λ≥320nm compared with Sn(IV)-grafted and pure anatase TiO2. This observation is explained based on a surface hole trapping by the Sn(II)-related surface states which lie above the top of the main valence band and can therefore act as trapping sites for holes produced under photoexcitation.

Features of the energy spectrum and hole-scattering mechanisms in PbSb2Te4

The kinetic Hall Rikl, electrical-conduction σkk, thermopower αkk coefficients, and their anisotropy are investigated in the temperature range of 77–450 K for a series of copper-doped PbSb2Te4 crystals with various hole concentrations of (1.6–3.2) × 1020 cm−3. The thermopower anisotropy observed in all investigated crystals is indicative of a mixed hole-scattering mechanism. The main scattering mechanisms are scattering at acoustic phonons and at the Coulomb potential of impurities and defects. The experimental effective scattering parameters and partial mobilities are determined in the cleavage plane and along the trigonal axis. The theoretical estimates of mobility are in satisfactory agreement with the experiment. It is established that the character of the temperature dependence for the ratio αkk/T = f(T) depends on the hole concentration. It is shown that the physical properties can be described within the framework of the one-band model for a crystal with pmin = 1.6 × 1020 cm−3; the value of the effective mass density of the hole state in the main-valence band extremum is estimated as md ≈ 0.5m0. The energy gap between the valence-band extrema is estimated as ΔEv ≈ 0.23 eV.

Spectroscopic characterization of a multiband complex oxide: Insulating and conducting cement 12CaO·7Al2O3

Natural 12CaO$\cdot$7Al$_2$O$_3$ (C12A7) is a wide bandgap insulator, but conductivity can be realized by introducing oxygen deficiency. Currently, there are two competing models explaining conductivity in oxygen-deficient C12A7, one involving the electron transfer via a "cage conduction band" inside the nominal band gap, the other involving electron hopping along framework lattice sites. To help resolve this debate, we probe insulating and conducting C12A7 with X-ray emission, X-ray absorption, and X-ray photoemission spectroscopy, which provide a full picture of both the valence and conduction band edges in these materials. These measurements suggest the existence of a narrow conduction band between the main conduction and valence bands common in both conducting and insulating C12A7 and support the theory that free electrons in oxygen-deficient C12A7 occupy the low-energy states of this narrow band. Our measurements are corroborated with density functional theory calculations.

Electronic basis of visible region activity in high area Sn-doped rutile TiO2 photocatalysts

The influence of Sn doping on the anatase-to-rutile phase transition has been investigated in high area powders prepared by a sol-gel route involving alkoxide precursors. Sn doping facilitates conversion of anatase to rutile at lower temperatures than observed for undoped material. At the same time Sn-doping inhibits sintering as gauged by line widths in X-ray diffraction and gas-adsorption surface area measurements. These observations are linked to the finding of pronounced segregation of Sn to the surface of rutile TiO(2) observed in X-ray photoemission spectra. Sn-doped TiO(2) is found to exhibit enhanced visible region photocatalytic activity as compared with undoped material in dye degradation experiments. This is attributed to narrowing of the bulk bandgap at low doping levels coupled with the introduction of surface states associated with segregated Sn ions in the divalent state. The Sn(II) surface states lie above the top of the main valence band and can therefore act as trapping sites for holes produced under photoexcitation.

The electronic states of 1,2,5-oxadiazole studied by VUV absorption spectroscopy and CI, CCSD(T) and DFT methods

The 1,2,5-oxadiazole VUV absorption spectrum in the range 5–11.5eV, shows broad bands centred near 6.2, 7.1, 8.3, 8.8, 10.6 and 11.3eV. Rydberg states associated with three ionisation energies (IE) were identified in the complex fine structure above 8.7eV. Electronic vertical excitation energies for singlet and triplet valence, and Rydberg states were computed using ab initio multi-reference multi-root CI methods. There is generally a good correlation between the envelope of the theoretical intensities and the experimental spectrum. The nature of the more intense calculated Rydberg states, and positions of the main valence and Rydberg bands are discussed. The lowest triplet, singlet and Rydberg 3s excited states have equilibrium structures that are non-planar with CS symmetry, in a chair-like orientation where the O and H atoms lie out of the NCCN plane. This finding is consistent with the doubling of the low energy UV spectral lines [B.J. Forrest, A.W. Richardson, Can. J. Chem., 50 (1972) 2088].The nearly degenerate IE of the UV-photoelectron spectrum (UV–PES, Palmer et al. 1977) makes analysis of the VUV spectrum difficult, leading to the necessity for reinvestigation. Vertical studies (IEV) using CI, Tamm–Dancoff (TDA) and Green’s Function (GF) methods all gave similar results, with near degeneracy of the first 3IEV confirming the earlier study. Studies of the adiabatic IE (IEA) using CCSD(T) and B3LYP methods, showed the energy sequence 2A2<2B1<2B2, but these states are all saddle points, in contrast to the 4th state (2A1) which is a minimum. In contrast, MP2 study of the 2B2 state showed a minimum, with only two saddle points.Complete minima were found after minor twisting of the structures. The lowest energy cationic state is 2A″ (CS), which closely resembles the 2B2 state. The O–N–C–C skeleton is twisted by 8°. The corresponding 2A′ state (CS) is effectively identical to the 2B1 state. Attempts to find minima for other symmetry states were unsuccessful.

The electronic states of 1,2,5-thiadiazole studied by VUV absorption spectroscopy and ab initio configuration interaction methods

The 1,2,5-thiadiazole VUV absorption spectrum over the range 5–12 eV has been obtained for the first time. It shows broad bands centred near 5.0, 7.2, 7.7, 8.7, 9.6 and 10.6 eV; several of these show well resolved vibrational structure. A number of valence states and Rydberg states relating to the first and second ionisation energies have been identified. The doubling of some bands in the 4.7 eV region (previously reported) is confirmed; a 3s Rydberg state shows a similar phenomenon, which we also attribute to a non-planar upper state; a study of the (planar) equilibrium geometry of the corresponding triplet Rydberg state shows it is a saddle point. Electronic excitation energies for valence (singlet and triplet) and Rydberg-type states have been computed using ab initio multi-reference multi-root CI methods. These studies used a triple-zeta + double polarisation basis set, augmented by diffuse (Rydberg) orbitals. The theoretical study shows the nature of the more intense Rydberg state types, and positions of the main valence and Rydberg bands. There is generally a good correlation between the theoretical intensities and the experimental envelope. Reconsideration of the order of cationic states in the UV-photoelectron spectrum, crucial to the Rydberg state interpretation, has led to the sequence: 2B 1 < 2B 2 < 2A 2 < 2A 1. Study of the excitation energies to specific upper states, also supports this order. The value of IE 1 is refined to 10.111 eV. Equilibrium structures show the π- and σ-cations are planar, in contrast to the lowest triplet state.

Doping of a one-dimensional Mott insulator: Photoemission and optical studies ofSr2CuO3+δ

Angle-resolved photoemission and optical spectroscopy are used to probe the electronic structure of the one-dimensional Mott insulator ${\mathrm{Sr}}_{2}\mathrm{Cu}{\mathrm{O}}_{3+\ensuremath{\delta}}$, both at half-filling and with small concentrations of excess oxygen doping. Spin-charge separation can be seen as evidenced by the existence of spinon and holon branches in the photoemission spectra. Optical studies reveal no significant doping dependence, while photoemission studies show a large energy shift in the spinon and holon states with respect to the main valence band states which remain nearly unaffected. The results suggest the excess dopant carriers are incorporated solely within the one-dimensional Hubbard band which is electronically isolated from the rest of the states in the system.

The electronic states of isothiazole studied by VUV absorption spectroscopy and ab initio configuration interaction methods

The isothiazole VUV absorption spectrum over the range 5–12 eV shows (broad) intense bands centred near 5.17, 6.11, 7.37, 7.75, 9.18 and 10.43 eV. The lowest Rydberg states relating to the first ionisation energy are difficult to identify, but higher members are particularly numerous on the region from 8.4 to 9.6 eV. Electronic excitation energies for valence (singlet and triplet) and Rydberg-type states have been computed using ab initio multi-reference multi-root CI methods. These studies used a triple zeta + double polarisation basis set, augmented by diffuse (Rydberg) orbitals. The theoretical study shows the nature of the more intense Rydberg state types, and positions of the main valence and Rydberg bands. By study of the excitation energies to specific upper states, the vertical ionisation energies (IE) are confirmed as π 4 - 1 < π 3 - 1 < σ 18 - 1 ( LP N ) < σ 17 - 1 ( LP S ) . Structures for the π- and σ-cations, and the (neutral) ππ ∗-triplet states have been obtained. Calculated energies for low-lying Rydberg states are close to those observed, and there is generally a good correlation between the theoretical intensities and the experimental envelope. The ground state atomic and molecular properties are in good agreement with experiment.

The electronic states of but-2-yne studied by VUV absorption, near-threshold electron energy-loss spectroscopy and ab initio configuration interaction methods

The photoabsorption spectrum of but-2-yne in the range 5.5–11 eV (225–110 nm) has been recorded using a synchrotron radiation source. The spectrum is dominated by three d-type Rydberg series, converging to the first ionisation energy (IE) (π −1, 9.562 eV). Origins of the π3d members are 7.841, 7.977 and 8.018 eV, respectively. Transitions of low intensity, arising from excitation of the π3s state (origin, ∼6.35 eV) and two π3p Rydberg states (7.38 and 7.51 eV, respectively) have also been identified in the spectrum. Near-threshold electron energy-loss spectra reveal valence excited triplet states at about 5.2 and 5.8 eV, respectively. Electronic excitation energies for valence and Rydberg-type states have been computed using ab initio multi-reference multi-root CI methods. These studies used a triple zeta + polarisation basis set, augmented by diffuse (Rydberg) orbitals, to generate the theoretical singlet and triplet energy manifolds. The correlation of theory and experiment shows the nature of the more intense Rydberg state types, and identification of the main valence and Rydberg bands. Calculated energies for Rydberg states are close to those expected, and there is generally a good correlation between the theoretical and experimental envelopes. It was possible to generate singlet Rydberg states which relate to the 5-lowest IEs of but-2-yne; furthermore, the separation of these sequences shows that the IE order (under D 3h symmetry) is: 2 e ′ < 5 a 1 ′ < 1 e ″ < 1 e ′ < 4 a 2 ″ , also supported by direct calculation of the IEs by CI. The lowest valence singlet states are ππ ∗, optically forbidden, and calculated to lie near 7.3 and 7.6 eV. The states which contribute strongly to the observed spectrum are πσ ∗ ( 1 E ′ + 1 A 2 ″ ) near 7.9 eV having 2 e ′ 6 a 1 ′ ∗ excitation, followed by several ππ ∗ and πσ ∗ states ( 1 E ′ + 1 A 2 ″ ) between 10.0 and 10.5 eV; an 1E′ antisymmetric combination(2e′2e″ − 2e′2e″) is by far the strongest in intensity. A further group of symmetry-allowed valence states are calculated to lie near 12.3 and 12.9 eV. The two lowest triplet states, both of E′ symmetry (ππ ∗), have vertical excitation energies of 5.7 and 6.2 eV, but are strongly bent with a trans-CCCC unit (C S and C 2h). The theoretical work confirms that, on intensity grounds, valence excited states do not contribute significantly to the spectrum. CI calculations of the ionic states give the ionisation energy sequence ( D 3 h ) : 2 E ′ < 2 A 1 ′ < 2 E ″ < 2 E ′ < 2 A 2 ″ . Adiabatic structures for the first cation, two triplets, and a singlet (C 2h) were obtained; these show shortening of C–C, and lengthening of C C, in a trans-CCCC, as is found with ethyne.

Photon energy dependence of final state screening in a dilute electron gas system: A synchrotron radiation photoemission study of β-PbO 2

Photoemission spectra of thick films of β-PbO 2 have been measured over a range of exciting photon energies between 40 eV and 150 eV. There is evidence for occupation of conduction band states above the top of the main valence band to give a degenerate electron gas and the spectra terminate in a sharp metallic Fermi edge. Structure associated with screening of shallow 5d core holes by the mobile conduction electrons observed in Al Kα excited X-ray photoemission spectra is suppressed in the lower energy spectra and the Pb 5d core level appears as a simple spin-orbit doublet.

The electronic states of oxazole studied by VUV absorption and electron energy-loss (EEL) spectroscopies, and ab initio configuration interaction methods

The oxazole VUV absorption spectrum over the range 5–12 eV shows intense bands centred near 6.3, 7.5, 8.3, 9.6 and 10.8 eV. The electron energy-loss (EEL) spectrum shows additional structure with a strong peak (∼1.4 eV) arising from resonant vibrational excitation of the molecule via a shape resonance, and a spin-forbidden 3ππ ∗ state at 4.6 eV. Electronic excitation energies for valence and Rydberg-type states have been computed using ab initio multi-reference multi-root CI methods. The CI studies used a triple zeta + polarisation basis set, augmented by diffuse (Rydberg) orbitals, to generate the theoretical singlet and triplet energy manifolds. The correlation of theory and experiment shows the nature of the more intense Rydberg state types, and identification of the main valence and Rydberg bands. Calculated energies for low-lying Rydberg states are relatively close (SD 0.38) to those expected, and there is generally a good correlation between the theoretical and experimental envelopes. Two of the three lowest electronic states arise from ππ ∗ excitation of the outer (3a″ and 2a″) π-orbitals, with one state (LP Nπ ∗) originating from the lone pair on nitrogen (15a′) between them.

Experimental and theoretical study of the electronic structures of α-PbO and β-PbO2

The electronic structures of α-PbO and β-PbO2 have been investigated by X-ray photoemission, X-ray absorption and X-ray emission spectroscopies, supported by bandstructure calculations performed within the framework of density functional theory. The relative intensity of a peak found at the bottom of the valence band for both oxides changes dramatically between Al Kα X-ray photoemission and O K shell X-ray emission spectra, demonstrating that the states associated with this peak possess dominant Pb 6s character. This finding is in accord with partial densities of states derived from bandstructure calculations but is at variance with the conventional view that the Pb 6s states in PbO are close to the Fermi energy and hybridise with empty 6p states to give a metal based directional 6s–6p lone pair. The photoemission onset of β-PbO2 contains a well-defined metallic Fermi edge. The position of the onset structure suggests that the metallic nature of PbO2 arises from occupation of conduction band states above the main valence band, probably arising from oxygen vacancy defects. The conduction electrons of β-PbO2 are strongly perturbed by ionisation of Pb core levels, giving rise to distinctive satellites in core XPS whose energies correspond to those of the conduction electron plasmon.