Electronic structure of the valence bands of H2-, Mg- and Pt-phthalocyanine derived from soft X-ray emission and photoelectron emission spectra

The soft X-ray emission and photoelectron emission spectra of H₂-, Mg- and Pt- phthalocyanine (PC) obtained using synchrotron radiation are reported and compared. In this way, an overall view of the pattern of valence bands is obtained and the electronic structure determined in terms of the component partial densities of states. In particular, from the valence p → 1s carbon and nitrogen K-emission spectra we determine for all three compounds the C and N 2p-like valence-band density of states with strong maxima located at binding energies of 8, 11 and 13.5 eV (carbon 2p) and 8 eV (nitrogen 2p) below the vacuum level. For PtPC the partial density of d-like valence states is determined from photoelectron emission difference-spectra and compared to previous XPS results. The sharp (1.2 eV FWHM) maximum of the Pt-derived partial density of states, observed at 6.9 eV binding energy, is assigned to the ⁴F term of a 5d⁸6s final-state configuration. A second, broader maximum at around 9.5 eV binding energy contains contributions from other terms of this 5d⁸ configuration, as well as from a 5d⁷ satellite (shake-up multiplet).     

Photoemission studies of single crystals of titanium carbide

The electron distribution in the valence band from single crystals of titanium carbide has been studied by photoelectron spectroscopy with photon energies h?ω = 16.8, 21.2, 40.8 and 1486.6 eV. The most conspicious feature of the electron distribution curves for TiC is a hybridization between the titanium 3d and carbon 2p states at ca. 3–4-eV binding energy, and a single carbon 2s band at ca. 10 eV. By taking into account the strong symmetry and energy dependence of the photoionization crosssections, as well as the surface sensitivity, we have identified strong emission from a carbon 2p band at ? 2.9-eV energy. Our results are compared with several recent energy band structure calculations and other experimental data. Results from pure titanium, which have been used for reference purposes, are also presented.The valence band from single crystals of titanium carbide have been studied by means of photoelectron spectroscopy, with photon energies ranging from 16.8 to 1486.6 eV.By taking into account effects such as the symmetry and energy dependence of the photoionization cross-sections and surface sensitivity, we have found the valence band of titanium carbide to consist of two peaks. The upper part of the valence band at 3–4 eV below the Fermi level consists of a hybridization between Ti 3d and C 2p states. The C 2p states observed in our spectra were mainly excited from a band about 2.9 eV below the Fermi level. The APW^5–9, MAPW¹⁰ and EPM¹¹ band structure calculations predict a flat band of p-character between the symmetry points X′₄ and K₃, most likely responsible for the majority of C 2p excitations observed. The C 2s states, on the other hand, form a single band centered around ?10.4 eV.The results obtained are consistent with several recent energy band structure calculations^{5–11, 13} that predict a combined bonding of covalent, ionic and metallic nature.     

Electronic structure of PbI2 from photoelectron spectra and the exciton problem

The valence band density of states for PbI₂ is determined from X-ray and u.v. induced photoelectron spectra. It is shown that the band derived from Pb 6s states is at 8 eV binding energy and not at the top of the valence bands as suggested by band structure and charge density calculations. A rigid shift in the predominantly iodine 5p derived bands to lower binding energy brings the band structure calculations into essential agreement with experiment. Pb 5d core level binding energies determined here are used to derive core level exciton energies of 0.7 eV from published reflectivity spectra.     

Photoemission spectra for gold for 15≤hv≤90 eV and the X-ray limit

Photoemission spectra for gold have been measured for photon energies 15≤hv≤90 eV using synchrotron radiation. We observe the transition from the ‘band structure’ regime (hv?30 eV) where spectra are strongly modulated by matrix elements to the high energy regim (hv?30 eV) where spectra reflect structure in the single-particle density of states.     

New method for photoelectron spectroscopy of solutions

A new method of rather general scope is reported for the determination of the photoionization (PI) spectra of solutions. Spectra thus obtained correspond to PI for ejection of quasi-free electrons into the liquid phase. The method is based upon photoelectron emission (PEE) into vacuum by the solution upon irradiation with adjustable photon energy. A new instrument is described which features irradiation (up to 10 eV) of a film of the solution on the rim of a rotating disk target (72 r.p.m.). Two types of data are obtained: PEE quantum yield per incident photon as a function of photon energy; energy distribution curve (EDC) of electrons emitted into vacuum at given photon energies. The PI cross-section is obtained as a function of photon energy by the previously developed method of EDC superposition. The method is applicable even when the contribution by the solvent to total PEE is dominant. Application is made to PI by 0.4 M indole in glycol for irradiation from 6.5 to 10 eV. Experimental results include: the effect of equilibrium vapor pressure (3.5 × 10^?3 ? 4.6 × 10^?2 torr) on EDC's; PEE spectral response of solvent and solution; EDC's from 6.5 to 10 eV. Interpretation covers these results and also includes: attenuation of the photon flux in the gas phase; PI in the gas phase; changes of temperature and solute concentration caused by evaporation. The PI spectrum of indole in glycol at ?7 °C exhibits a band for bound-continuum transitions (rising segment up to ≈ 7.0 eV) and a superimposed band (maximum at ≈ 7.6 eV) for PI via autoionization of excited bound states. Systematic development of the photoionization spectroscopy of solutions up to 10 eV is made possible by this approach. Application of the rotating disk methodology to higher photon energies (HeI, ESCA) may be of interest.     

Valence band of molybdenum by photoelectron spectroscopy

Experimental photoelectron spectra of a clean polycrystalline Mo surface excited by monochromatized Al K _α X-rays are presented. The spectra are compared with valence bands obtained by UPS and by band structure calculations within the 5 eV region below the Mo Fermi level. All results mentioned above display peaks at 0.3, 1.7, 2.8 and 4 eV belowE _F. The energy distribution of the valence band does not vary with photon energy and electron emission angle for the four different polycrystalline Mo surfaces compared. It is concluded that the four peaks representing the Mo valence band are predominantly of bulk origin.     

Angular resolved photoemission from intrinsic surface states on Al (100)

Using photon energies of 11.7, 16.8 and 21.2 eV, we have recorded angular resolved photoemission spectra from clean Al (100). A dominant, surface sensitive peak is interpreted as emission from a two-dimensional band of surface states. The band mass is $\text{m}{}^1\text{=(1.03±0.10)m}$, and the band minimum, (2.8±0.2) eV below E_F for surface momentum k₆=0, is located within the bulk band gap. The observed existence of the peak for large values of k₆, indicates a transition from a true surface state to a surface resonance.     

Thermo- and photostimulated luminescence in LiF : Mg,Ti single crystals irradiated by ions and VUV light

A coordinated study of the relaxation of optical absorption induced by vacuum ultraviolet radiation, x-rays, and α-particles, as well as of photo- and thermostimulated luminescence (TSL) of LiF : Mg, Ti crystals (TLD-100) in the 295–750-K interval, has revealed that TSL regions characterized by activation energies E _a = 2.2–2.4 eV and anomalously high frequency factors p ₀ = 10²¹–10²² s^?1 alternate with regions where E _a = 1.5 eV and p ₀ = 10¹²–10¹⁴ s^?1. The relative intensities of the TSL peaks produced by UV illumination (10–17 eV) differ strongly under the conditions of selective photon-induced generation of anion excitons, free electrons and holes, or near-impurity electronic excitations. The latter are responsible for the high efficiency of tunneling radiative (involving titanium centers) or nonradiative (involving hydroxyl ions) recombination. The analysis of TSL peaks of LiF: Mg, Ti and LiF took into account two-step processes, namely, thermal dissociation of three-fluorine F ₃ ^? molecules and recombination of the products of their decay (V _K and V _F centers, H interstices).     

K+3P-core excitons in potassium halides studied at low temperatures with high resolution

Reflection spectra of KF, KCl, KBr and KI single crystals have been measured at photon energies of approximately 20 eV with a bandwidth of 10 meV using synchrotron radiation in order to study fine structure and temperature dependence of the excitonic transitions associated with the K⁺3p core level. The crystals were cleaved under ultrahigh vacuum and cooled down to 20 K. Information on energy positions, halfwidths and line shapes for the K⁺3p-core excitons and their temperature dependence has been obtained. A new exciton predicted by the ligand field model was observed. For KI we evaluate an electron-hole exchange energy of only 30 (± 7) meV.     

Valence band structure of single crystal titanium carbide,studied by soft X-ray and ultraviolet photoelectron spectroscopy

Valence states of single crystal titatium carbide (TiC_x, X?0.88) have been studied with photon energies ranging from far ultraviolet (u.v.) to soft X-ray. The valence band consists of two peaks located at 3 and 10 eV below the Fermi level. This is in good agreement with recent APW band structure calculations that predict a strong hybridization of the Ti 3d and C 2p bands and a C 2s band at lower energy.     

UV-photoemission study of barium,europium and ytterbium

Measurements are reported of the photoemission spectra and absolute yield of thin films of Eu, Ba and Yb for the photon energy range 2 to 21 eV. Transitions from the occupied 4f-states in Eu and Yb have been observed and the binding energies deduced. The excitation probability of the 4f electrons is found to be very low and an explanation based on their atomic-like nature is given. Transitions involving valence band states are compared with predictions based on published energy band schemes and the density of states for the unoccupied 5d band in Yb is deduced. The effect of chemisorbed oxygen on Ba and Eu surfaces is reported.     

The electronic structures of Mg,Al and Si in octahedral coordination with oxygen from SCF Xα MO calculations

Molecular orbital calculations performed using the SCF Xα Scattered Wave Cluster method are presented for the octahedral oxyanions MgO₆^?10, AlO₆^?9 and SiO₆^?8. The AlO₆^?9 results are used to assign and interpret the X-ray photoelectron spectra (XPS), X-ray emission (XES) and u.v. spectra of Al₂O₃. Agreement between calculation and experiment is good for valence band and fair for conduction band orbitais. The SCF Xα results for MgO₆^?10 are also in good agreement with observed valence band energies in MgO, but in this case the lowest energy features in the u.v. spectrum are not assignable in terms of either the calculations or the X-ray spectral results. The substantial increase in covalency expected between the Mg and Si oxides is evidenced in the calculations by an increase in valence region width from 2.6 to 5.3 eV and an increase in valence-conduction band separation from 5.2 to 10.0 eV. The calculated trends are in reasonable agreement with u.v. spectral data and with absolute valence orbital binding energies derived from X-ray spectra. A comparison of the SiO₆^?8 calculation with the analogous tetrahedral SiO₄^?4 calculation shows the valence band in the octahedral oxyanion to be much simpler in structure and somewhat narrower than that in the tetrahedral oxyanion. Using the orbital structure calculated for the valence bands of tetrahedral and octahedral oxides, a method is presented for calculating atomization energies directly from X-ray spectral data for SiO₂, Al₂O₃ and MgO. Results are in good agreement with experiment but the method involves an empirical parameter which is not presently understood in detail. Studies of trends in p-type bonding orbital binding energies derived from experimental data provide a qualitative explanation for the preferred coordination numbers in the Mg, Al and Si oxides.     

An interpretation of the Auger LVV transitions from oxides of third-row elements

The principal features in the LVV Auger spectra from the oxides of third-row elements are semi-empirically derived for the XO₄^n? species of Si, PS and Cl, and the XO₆^n? species of Mg and Al. Electron molecular orbital energies are derived from X-ray photoelectron and X-ray emission spectra; the central atom 3p electron density of states is taken from the Kβ X-ray emission. Two principal peaks, separated by ca. 14 eV, are predicted for the central atom LVV Auger spectra and are experimentally confirmed for the XO₄^n? species. Similar features are observed in published spectra for oxides of Mg and Al. These peaks correspond to central atom 3p electrons in orbitals whose energy is dominated by the atomic oxygen 2s and 2p electron levels. An examination of the total LVV line-shape shows that a self-convolution of the Kβ spectra does not reproduce the more subtle features, which are probably a result of the contributions of other electron orbitals and final-state effects. The possibility of using the LVV Auger spectra to discriminate between various oxide stoichiometries, i.e. sulfate, sulfite, etc., and between various ligand species, i.e. carbide, nitride, oxide, fluoride, is discussed.     

Synchrotron-radiation photoemission study of the ultrathin Ba/3C–SiC(111) interface

Electronic structure of the Ba/3C–SiC(111) interface has been detailed studied in situ in an ultrahigh vacuum using synchrotron radiation photoemission spectroscopy with photon energies in the range of 100–450 eV. The 3C–SiC(111) samples were grown by a new method of epitaxy of low-defect unstressed nanoscaled silicon carbide films on silicon substrates. Valence band photoemission and both the Si 2p, C 1s core level spectra have been investigated as a function of Ba submonolayer coverage. Under Ba adsorption two induced surface bands are found at binding energies of 2 eV and 6 eV. It is obtained that Ba/3C–SiC(111) interface can be characterized as metallic-like. Modification of both the Si 2p and C 1s surface-related components were ascertained and shown to be provided by redistribution effect of electron density between Ba adatoms and both the Si surface and C interface atoms.     

Observation of p- and d-like surface states on CuCl(100) by angle-resolved photoemission

In the course of a systematic ultraviolet photoemission study of the electronic band structure of CuCl, we have identified two occupied surface states on CuCl(100), situated at 0.25 and 3.0 eV below valence band maximum in normal emission spectra. They essentially show pure p- and d-like orbital symmetry, respectively. We interpret them as a chlorine p_x,y-like occupied antibonding resonance and a copper Γ₁₂-derived state split off from the bulk orbitals by the surface potential. We also present critical point energies along Γ-X and Γ-L.     

Virtual bound states in AuTi alloys

We have measured the optical absorption of dilute AuTi alloys for photon energies from 0.1 to 4.5 eV. The extra absorption we observe below the Au interband threshold can be accounted for if the Ti impurities form virtual bound states with half-width at half height δ ～ 0.6 ± 0.2 eV centered E_d ～ 0.5 ± 0.2 eV above the Fermi level and containing 2.7 ± 0.3 electrons.     

Relation between interface states and temperature behavior of the barrier height of silver contacts on clean cleaved n-type silicon

The properties of silver-silicon interfaces formed by cleaving n-type silicon in ultra high vacuum (UHV) in a stream of evaporating silver atoms were studied. The barrier heights of these contacts were measured at different temperatures by using C-V techniques. All measurements were performed in UHV. The dependence of the barrier height upon temperature did not follow the temperature dependence of the Si band gap as it is usually found. The measured temperature behavior depended on the roughness of the Si surface. The temperature behavior can be explained by assuming a specific band structure of the interface states. For Ag contacts on atomically smooth n-type Si, the interface states were found to be arranged in two bands, one band 4 × 10^?3 eV wide with acceptor type states 0.18 eV below the intrinsic level E_i and a density of 10¹⁷ states/cm² eV, and the other 1 eV wide with donor type states with its upper edge 0.28 eV below E_i, and a density of 4 × 10¹⁴ states/cm²eV.     

Optical properties of dangling-bond states at cleaved silicon surfaces

The spectral dependence of surface photovoltage and surface photoconductance both under continuous illumination as well as LEED I/V spectra were studied with cleaved Si(111)-2 × 1 surfaces at 130 K. Between 0.23 and 0.5 eV a doubly peaked absorption band was found with opposite sign compared to the SPV and SPC signals at higher photon energies. This band is due to electronic transitions from occupied to empty dangling-bond states located at the raised and the lowered rows of atoms in the 2 × 1 reconstruction, respectively. This absorption shows a pronounced dependence on the polarization of the incident light which correlates with the spatial symmetry of the dangling-bond states. Anneals at up to 500 K remove the low-energy absorption peak and equalize the 2 × 1 reconstruction: The homogeneous Si(111)-2 × 1 structure exhibits a buckling of 0.3 Å and a dangling-bond absorption with a threshold at 0.42 eV and a maximum at 0.47 eV. An anneal at 750 K, forming the 7 × 7 structure, destroys the peak of opposite sign in SPV and SPC and only leaves a broad tail with a threshold of 0.32 eV.     

Emission spectra of the cations of some fluoro-substituted phenols in the gaseous phase

Emission spectra of the cations of 2,5- and 3,5-difluorophenol, of 2,3,4- and 2,4,5-trifluorophenol, of 2,3,5,6-tetrafluorophenol and of 2,3,4,5,6-pentafluorophenol have been obtained in the gas phase using low-energy electron beam excitation. The band systems are assigned to the B?(π^?1) → X?(π^?1) electronic transitions of these cations by reference to photoelectron spectroscopic data. The He(Iα) photoelectron spectra and the ionisation energies of ten fluoro-substituted phenols are reported. The symmetries of the four lowest electronic states of these cations are inferred from the radiative decay studies. The lifetimes of the lowest vibrational levels of the B?(π^?1) state of the six fluoro-substituted phenol cations above have also been measured.     

Photoluminescence of nanostructured Zn<Subscript>2</Subscript>SiO<Subscript>4</Subscript>:Mn<Superscript>2+</Superscript> ceramics under UV and VUV excitation

The photoluminescence of Zn₂SiO₄:Mn²⁺ ceramics with a particle size of 120 ± 10 nm, which is excited in the range of 3.5–5.8 eV and subjected to synchrotron radiation with photon energies of up to 20 eV, is investigated. Nanoscale Zn₂SiO₄:Mn²⁺ ceramics possesses intense luminescence with a maximum of 2.34 eV, the position and half-width of the band are independent of the excitation energy. It is found that the photoluminescence at 2.34 eV decays nonexponentially upon ultraviolet excitation. In the case of nanoscale ceramics is irradiated by vacuum ultraviolet, an additional photoluminescence-excitation channel is likely to occur due to interaction of band states and intrinsic vacancy-like defects of the Zn₂SiO₄ matrix.