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.     

Visualization of a Berezinskii–Kosterlitz–Thouless Topological Phase Transition in a Josephson Medium: Detection of an Anomalous Temperature Dependence of the Magnetoresistance of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7–δ</Subscript> Granular High-Temperature Superconductors

The density of states of the valence band of a p-GaAs layer formed on an n-GaAs surface owing to the bombardment by 2500-eV Ar⁺ ions has been studied by photoelectron spectroscopy. A number of peaks have been detected in the spectrum of the edge of the valence band in the binding energy range E_V < 1.2eV. Their number and energy positions correspond to the quantum confinement levels calculated for a hole quantum well on the surface with the width about the ion penetration depth R_p = 3.6nm. Electronic transitions from these levels to the bottom of the conduction band have been revealed in the spectrum of characteristic energy losses of electrons reflected from the surface. Thus, it has been shown that the action of the argon ion beam on n-GaAs results in the formation of a quantum well on the surface.     

Valence band density of states of amorphous and trigonal selenium determined by X-ray and U.V. photoemission

The densities of states of the entire valence bands of amoprhous and trigonal selenium have been measured by X-ray (hv = 1486.6 eV) and u.v. (hv = 21.2 eV) photoemission. It is found that the primary differences in the density of states of the two forms occurs in the p-like valence bands, in particular the lower bonding set. The density of states calculated by Kramer et al. for the amorphous form is shown to be in better agreement with the experimental density of states of the trigonal form than with that of the amorphous form.     

Effective 4ƒ energy level in virtual excitation and in photoemission processes in Ce-pnictides

Effects of relaxation of occupied band electrons to the ?-hole state through the hybridization between ? and band states are studied based on a detailed model for Ce-monopnictides. The effective 4? level is shifted about 1–2 eV to shallow energy side from the unrenormalized bare level in processes in which the 4? electron is only virtually excited, such as in excitation to the vacant p band states through the p-? mixing. Photoemission spectra show two peaks, one near the Fermi energy and the other about 3 eV below it. The latter is shifted to deep energy side about 0.5–1 eV from the bare level when it lies near the bottom of the valence band. The discrepancy between the 4? level estimated from the low energy phenomena and that from photoemission is resolved.     

Resonant photoemission spectroscopy of Cu(InGa)Se2 materials for solar cells

The electron structure of CuIn_{1 ? x} Ga _x Se₂ single crystals is determined via resonant photoemis-sion and the main regularities of its transformation upon varying concentration x from 0 to 1 are established. The dependence of the shape of valence band spectra on the photon energy is studied. Integral photoemission intensities are shown to be determined by atomic photoionization cross sections. Processes of the direct and two-step creation of photoelectrons accompanying photoemission and the participation of internal states in the spectra of electrons from valence bands are studied. Two-hole final states in photoemission are obtained upon threshold excitation of the Cu 2p level. The strong interaction of holes leads to the multiplet splitting of these states. Partial densities of the components’ states are determined using the energy dependence of atomic photoionization cross sections.     

Dipole oscillator strengths for the photoabsorption,photoionization and fragmentation of molecular oxygen

The photoabsorption, photoionization and fragmentation of O₂ have been studied using electron impact coincidence methods to obtain branching ratios and dipole oscillator strengths (cross-sections). The photoabsorption measurements cover the energy range 5–300 eV while the formation of electronic states of O₂⁺ (photoelectron spectroscopy) and the resulting ionic fragmentation (photoionization mass spectrometry) are both measured from close to threshold up to photon energies of 75 eV. The binding energy spectra of O₂ show peaks at 33, 47 and 57 eV in addition to those reported elsewhere in the literature. These peaks are assigned to multiple final ion states arising from photoionization of the inner valence orbitals. Structure in the O₂⁺ electronic state partial oscillator strength curves is in good agreement with recent theoretical work which predicts the existence of several shape resonances. A quantitative picture of the dipole-induced breakdown of O₂ is obtained for the energy range 12–75 eV. The photoionization efficiency is found to be constant above 20 eV.     

Photoionization branching ratios and asymmetry parameters for inner- and outer-valence shells of H2S in the 18–70 eV energy range

Branching ratios, asymmetry parameters and relative partial cross sections have been obtained for photoionization of the outer and the inner valence shells of H₂S. These measurements were made in the photon energy range 18–70 eV using synchrotron radiation. Our results are compared to a set of calculations using a developed extension of the self-consistent field-Xα scattered-wave method to the continuum states. This comparison shows a qualitative agreement between the experimental and calculated β curves of the outer valence shells. The largely predominant sulfur 3p contribution to the outer valence orbitais 2b₁, 5a₁, 2b₂ is revealed in the corresponding β curves by a Cooper minimum also predicted in the same energy range for the β(3p) of the atomic sulfur. This comparison also shows discrepancies in the branching ratios curves and we suggest that this theoretical framework is better adapted to predict photoionization processes in the outer valence shells than in the inner ones.     

Niveaux d'énergie et spectres de rayons X du rutile

The electronic levels of the complex TiO^?8₆ in the D_2h symmetry are determined according to an extended L. C. A. O. method. The results can explain the X-ray spectra of TiO₂. The absoption L_III and K rays are related to transitions from the 2p_3/2 and 1 s levels to the conduction band levels since the emission L_III and K components are explained by the transitions from the valence band levels to the 2p_3/2 and 1 s states. Interband transitions are related to the components of the optical reflexion spectrum of TiO₂ for the energies 0–20 eV. A comparaison is made with the electronic band structures of SnO₂, TiO₂ and BaTiO₃. At the center of the Brillouin zone, we obtain a forbidden gap of 3,01 eV, the corresponding widths of the valence and conduction band are 4,8 and 2,9 eV.     

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.     

Ultraviolet photoemission study of LiF

Photoemission from evaporated films of LiF were measured at photon energies of 10-27 eV. The photoelectron spectra exhibit features that can be identified as density-of-states structures in the valence and conduction bands of LiF. Regions of high density of states can be seen at ca. 3.3 and 7.8 eV above the vacuum level. The valence-band spectrum shows a doublet structure similar to the calculated density of states for the F^?2p band of LiF. The base width of this structure is found to be 4.6 ± 0.3 eV. The photoelectron spectra for photon energies > 15 eV indicate that the highest occupied states of the F^?2p band are located at 11.8 ± 0.3 eV below the vacuum level. The photoelectron spectra in the exciton region, however, show photo-emission from higher occupied states.     

Photoelectronic investigations of semi-insulating p-type GaAs:Cr containing neutral chromium acceptors

A photoelectronic analysis of p-type GaAs:Cr, i.e. measurements of thermally stimulated currents and the dependences of photoconductivity and photo-Hall effect on photon energy, temperature and light intensity, have enabled trap locations and densities as well as properties of neutral chromium acceptors to be determined. Hole traps proved to be located at 0.15 and 0.23 eV above the valence band, and their densities have been estimated to be 10¹⁵ cm^?3 and 5 × 10¹⁶ cm^?3 respectively. Their occurrence is related to the presence of copper in the samples investigated. Neutral chromium acceptors are located at 0.77 eV above the valence band and are at a constant distance from the conduction band. Their photoionization cross-section is 3 × 10^?17 cm² while the photoexcited electron escape cross-section is about 10^?20 cm². The potential of a neutral Cr acceptor is of the delta function type with weak coulombic tails. The maximum radius of the Bohr orbit of an electron in the ground state is 4 atomic units.     

(e,2e) Spectroscopy of ethane

The 400-eV and 1200-eV non-coplanar symmetric (e, 2e) reaction has been used to measure the momentum distributions of electrons in the individual valence orbitals of ethane as well as to measure the complete separation energy spectra in the valence region. The shapes and relative magnitudes of the momentum distributions agree well with those calculated using the plane wave off-shell impulse approximation and double zeta basis molecular orbital wave functions. The ground state of C₂H₆⁺ is shown to be 1e_g^?1, with the vertical ionization potential being 12.25 ± 0.1 eV. Considerable structure due to configuration interaction is observed in the separation energy region 29–55 eV. Much of this structure can be assigned to the 2a_1g orbital.     

Electron energy loss spectroscopy study of the initial stage of lanthanum oxidation

The electron energy loss spectra (EELS) of a pure metallic lanthanum surface and variations in these spectra at the initial stages of surface oxidation were studied. The measurements were performed at primary-electron beam energies E _p from 200 to 1000 eV. A very pronounced peak at a loss energy of about 7.5 eV arises due to transitions from the La4d electronic states of the valence band into the empty La4f electronic states of the conduction band at 5.0–5.5 eV above the Fermi level. Marked changes are observed in the EELS during the oxidation of lanthanum: the peak at an energy of 7.5 eV disappears, and the peak at 13.5 eV corresponding to bulk collective energy loss in lanthanum oxide becomes more pronounced. The results obtained are discussed in terms of the electronic structure of lanthanum and lanthanum oxide.     

Energy band structure and refractive properties of LiRbSO4 crystals

The energy band structure of mechanically free and compressed LiRbSO₄ single crystals is investigated. It is established that the top of the valence band is located at the D point of the Brillouin zone [k = (0.5, 0.5, 0)], the bottom of the conduction band lies at the Γ point, and the minimum direct band gap E _g is equal to 5.20 eV. The bottom of the conduction band is predominantly formed by the Li s, Li p, Rb s, and Rb p states hybridized with the S p and O p antibonding states. The pressure coefficients corresponding to the energies of the valence and conduction band states and the band gap E _g are determined, and the pressure dependences of the refractive indices n _i are analyzed.     

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).     

On the structure of wave fields in the region of linear interaction between ordinary and extraordinary waves in two-dimensionally inhomogeneous magnetoactive plasmas

The spectra of the low-temperature photodissociation (photoionization) of Landau-Pekar polarons are calculated using the theory of quantum-coherent states and a new method of variation with respect to the parameters of phonon vacuum deformation. It is shown that the final polaron states upon photodissociation may have different numbers of phonons produced in a single dissociation event and different momenta of charge carriers. The spectrum of optical absorption related to the photodissociation of polarons exhibits a superposition of bands corresponding to various numbers of phonons formed as a result of dissociation of a single polaron. Due to a large width of the energy region corresponding to the final states of charge carriers, the halfwidth of each band is on the order of the energy of polaron coupling and is much greater than the phonon energy. For this reason, the individual phonon bands exhibit strong overlap. The very broad and, probably, structureless band formed as a result of the superposition of all these components begins at an energy equal to the sum of the polaron coupling energy (E _p) and the phonon energy. This band has a maximum at a frequency of about 5.6E _p/? and a halfwidth on the order of 5.6E _p/? at a unit effective mass (m* = m _e) of band electrons. For an effective charge carrier mass within m* = (1–3)m _e, the energy of the polaron band maximum can be estimated as 5E _p with an error of about 10%, and the halfwidth falls within 3.4E _p < ?Ω_1/2 < 5.6E _p. The multiphonon character of this band is related to a decay of the phonon condensate after the escape of charge carrier from a polaron. Such polarons are likely to be observed in the spectra of complex metal oxides, including high-temperature superconductors. Examples of such polaron bands in the reported absorption and photoconductivity spectra of nonstoichiometric cuprates, manganites, nickelates, and titanates are presented. A theory of the formation of Landau-Pekar polarons with the participation of branches of the polarization oscillations of the medium is developed. It is shown that, under certain conditions, such a multiphonon-dressed polaron can possess a coupling energy on the order of 0.2–0.3 eV, so that the maximum of the corresponding absorption band may occur at 1–1.5 eV.     

Dissociative multiple photoionization of Br2, IBr,and I2 in the VUV and X-ray regions: a comparative study of the inner-shell processes involving Br(3d,3p,3s) and I(4d,4p,4s,3d,3p)

Dissociative multiple photoionization of the bromine, the iodine monobromide, and the iodine molecules in the Br(3d,3p,3s) and I(4d,4p,4s,3d,3p) inner-shell regions has been studied by using time-of-flight (TOF) mass spectrometry coupled to synchrotron radiation in the ranges of 90∼978 eV for Br₂, 60∼133 eV for IBr, and 86∼998 eV for I₂. Total photoion and photoion–photoion coincidence (PIPICO) yields have been recorded as functions of the photon energy. Here, giant shape resonances have been observed beyond the thresholds of the inner-shells owing to the Br(3d¹⁰)→Br(3d⁹ϵf), I(4d¹⁰)→I(4d⁹ϵf), and I(3d¹⁰)→I(3d⁹ϵf) transitions. The dissociation processes of the multiply charged parent ions have also been evaluated from variations of photoelectron–photoion coincidence (PEPICO) and PIPICO spectra with the photon energy. From each Br(3p_3/2) (189.9 eV) and I(4p_3/2) threshold (129.9 eV), quintuple ionization of the molecules begins to play important roles in the photoionization, subsequently yielding ion pairs of X³⁺–X²⁺ (X=Br, I). From the I(3d_5/2) threshold (627.3 eV), loss of six electrons from iodine molecule additionally begins to play a minor role in the multiple photoionization, giving rise to the formation of ion pairs of either I³⁺–I³⁺ or I⁴⁺–I²⁺. A direct comparison of the strengths and the ranges of the I(4d) and Br(3d) giant resonances was successfully made from dissociative photoionization of IBr. Over the entire energy range examined, 60<E<133 eV, biased charge spread relevant to the specific core-hole states of IBr is observed, presumably reflecting the fact that charge localizes mostly in the excited atoms, which can be accounted for mainly by a two step decay via a fast dissociation followed by autoionization upon the VUV absorption.     

An ab initio molecular orbital study of the electronically excited and cationic states of the ozone molecule and a comparison with spectral data

A number of valence and Rydberg, singlet and triplet excited states for ozone in the excitation energy range 1–12eV have been calculated by large scale CI methods preceded by MCSCF studies. A comparison of the theoretical intensity envelope with the VUV + EELS spectrum has been made. The present work supports the assignments for the Huggins + Hartley bands as having two electronic origins, 2 ¹A₁ and 1 ¹B₂. The experimental ～ 9.3eV and ～ 10.2eV bands of the VUV spectrum must have adventitious superposition of valence states on Rydberg transitions, because the high oscillator strengths of the valence states cannot be attributed to the 8.8eV broad band. A number of new valence and Rydberg states have been calculated, and these lead to the conclusion that the experimental 9–11 eV VUV spectral range in particular must yield more experimental states than the few so far identified. This suggests a major need for more sophisticated methods of experimental study for the excited state manifolds. The use of various MCSCF/CI studies of the vertical cationic states, supports the IP order as ²A₁ < ²B₂ < ²A₂. A re-analysis of the 12–13.4eV range of the UV-photoelectron band has been performed, with a view to determining the adiabatic IPs more accurately. The present work suggests that the adiabatic IP₂ lies at 12.86eV, slightly lower than has been assumed, with consequential effect on the analysis of the VUV spectrum near 9.4eV.     

Multiphoton mass spectra of XeKr molecules in the range of excited Xe*6<Emphasis Type="Italic">p</Emphasis>[5/2]<Subscript>2, 3</Subscript> atoms

Data on excited states of XeKr molecules in the energy range 78280–77600 cm^?1 are obtained. Using the method of multiphoton laser photoionization of molecules in a supersonic jet, five vibrational progressions of XeKr molecules are obtained, which are attributed to five electronic-vibrational transitions from the ground state of the XeKr molecule of the symmetry 0⁺ to excited states of the symmetry Ω = 0⁺, 1, 2 with the dissociation limit Kr¹ S ₀ + Xe*6p[5/2]₂ and of the symmetry Ω = 1, 2 with the dissociation limit Kr + Xe*6 p [5/2]₃. The molecular constants of the corresponding excited states of the XeKr molecule are estimated.     

Giant nonlinear absorption in the NiO antiferromagnet

A study of the spectrum of nonlinear two-photon and two-step absorption in NiO single crystals, carried out in the energy region ?ω₁ + ?ω₂ = 2.45–4.575 eV, showed it to have a complex shape and consist of very strong peaks (from 0.05 to 2.7 cm/MW). Within the energy interval 2.45–3.3 eV, the spectrum is due to d-d transitions in the Ni²⁺ ion. The band gap width was determined to be E _g =3.466 eV. The spectral features seen above this energy originate from interband transitions from three valence subbands to the conduction band bottom.