Exciton Reflection Spectra and the Structure of the Valence Band of Arsenic Chalcogenides

The polarized reflection spectra of arsenic sulfide crystal have been measured with high accuracy. The identical character of the triplet structures of the reflection spectra of As₂S₃ and As₂Se₃ has been established. On the basis of the well-known theoretical calculations of the bands of As₂Se₃, it is suggested that the three uppermost valence bands of both compounds are caused by the p states of arsenic, rather than sulfur, and are split into 0.1 and 0.2 eV. Underneath (about 0.4 eV) there are bands attributable to spin-orbital splitting of the p states of arsenic.     

Ab initio phonon dispersion relations of $\mathsf{\alpha}$-Ga

We present the ab initio phonon dispersion relations of -Ga. The calculations are carried out within density functional perturbation theory by using either norm-conserving pseudopotential and 4s and 4p electrons in the valence or ultrasoft pseudopotential and 3d electrons in the valence as well. The inclusion of 3d electrons in the valence turned out to be necessary to better reproduce the experimental frequencies of the stretching modes of the Ga₂ dimers present in the -Ga structure.Received: 29 July 2003, Published online: 19 November 2003PACS: 63.20.Dj Phonon states and bands, normal modes, and phonon dispersion - 71.15.Nc Total energy and cohesive energy calculations - 71.15.Mb Density functional theory, local density approximation, gradient and other corrections     

Electronic structure and calculated x-ray photoemission spectra of substoichiometric cubic molybdenum nitride

Total and partial densities of states of substoichiometric cubic MoN _x have been calculated for several concentrationsx by means of the KKR-CPA method. There are significant differences to the results of Papaconstantopoulos and Pickett [1] obtained by the LCAO-CPA method. In particular, the concentration dependence of the DOS at the Fermi energy is quite different. Using the single-scatterer final-state approximation, Al–K-XPS spectra have been calculated. It is found that the first peak in the valence band spectra is due mainly to metal-p rather than nonmetal-p states. The vacancy states in the DOS of substoichiometric MoN _x give also rise to an additional peak in the XPS spectra.     

Local electronic structure of Cu2O,CuO and YBa2Cu3O7−δ

Cu 3d and O 2p electronic states of Cu₂O, CuO, and the highT _c compound YBa₂Cu₃O_7– have been probed by means of high resolution x-ray emission spectroscopy (XES). The CuL and OK XES bands are compared in detail with recently reported x-ray photoelectron and ultraviolet photoelectron spectroscopy (XPS and UPS) measurements and densities of states obtained by local density functional (LDF) theory. The XES data show that the hybridization between Cu 3d and O 2p states is completely modified in CuO and YBa₂Cu₃O_7–, whered-d correlation energy is large, as compared to LDF predictions. Such is not the case for Cu₂O where agreement between theory and experiment is good.The Cu 3d states are found to be highly localized in YBa₂Cu₃O_7– (though less so than in CuO). The O 2p states lie at lower binding energies than in the simpler oxides and are mainly situated above the Cu 3d states. The respective positions of the centre of gravity of the OK emission bands on an x-ray energy scale indicate that the oxygen sites are less well screened by the O 2p states in the highT _c compound. This provides indirect evidence for the presence ofd-like states at the oxygen sites.     

Forthcoming papers

Kβ X-ray spectra of Si and SiO₂ have been measured accurately with a double crystal spectrometer. The measuredKβ spectrum of silicon element was compared with calculations of the electronic density of states. Observed intensity distribution shows that thep-electrons predominate at the top of the valence band, and somep-like states extend to the middle of the valence band. According to MO calculations the most intensiveKβ line of SiO₂ is 4t ₂ (100), the 3t ₂ (16) line is 17.9 eV lower, and 5t ₂ (5) line 6.3 eV higher. In our measurements the energy differences are 13.0 and 4 eV, respectively, and intensities 30% and 3% from the main line.     

Influence of the special features of atomic structure of Si24 and MeSi24 (Me = Na or K) nanoparticles on their electron properties

The influence of atomic structure of silicon nanoparticles on the distribution of total and partial state densities in the valence and conductivity bands as well as on the valence band and gap widths is analyzed based on the calculated geometrical and electron structure of the Si ₂₄ and MeSi ₂₄ (Me = Na or K) clusters. Semi-empirical AM1 and PM3 methods are used for calculations. The adequacy of calculations is confirmed by their comparison with the available experimental data. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 46–51, February, 2006.     

Formation of impurity bands in iodine cation substitutionally doped TiO2 and its effects on photoresponse and photogenerated carriers

First-principles calculations based on the plane-wave pseudopotential (PWP) method were performed to investigate the electronic structure of iodine cation substitutionally doped anatase TiO₂ (referred to as I_sTiO₂). A 2×2×1 anatase supercell, in which one Ti atom was substituted by one iodine atom, was constructed for calculations. The calculated results reveal that iodine dopant contributes part of its 5s states to form one isolated impurity band in band gap and part of its 5p states to form some impurity bands at the bottom of conductive bands. The visible light response of I_sTiO₂ should be the consequence of the excitation between the two types of impurity bands (2.77 eV, about 450 nm). According to the distribution of impurity states, iodine dopant is considered to serve as a recombination center or an efficient trap for photogenerated carriers. This deduction was partly approved by water photosplitting under visible light irradiation.     

The structure of theL 2,3 VV-auger line of silicon and silicon compounds

TheL _2,3 VV Auger transitions of Si, SiO₂, and SiC have been measured and compared with the self-fold electron density of states. The data indicate that Auger matrix effects must be included to explain the structure of the Auger lines. A comparison with soft X-ray measurements of Wiech shows, that the measured Auger line shape is nearly identical with the self-foldK _β emission band. The selection rules for X-ray emission lead then to the conclusion that mostlyp-like valence electrons are involved in the Auger transition. This result indicates the relative importance ofs andp states in Auger transitions which is in accordance with theoretical calculations of Feibelman et al.     

Nature of the electronic states involved in the chemical bonding and superconductivity at high pressure in SnO

We have investigated the electronic structure and the Fermi surface of SnO using density functional theory calculations within recently proposed exchange-correlation potential (PBE + mBJ) at ambient conditions and high pressures up to 19.3 GPa where superconductivity was observed. It was found that the Sn valence states (5s, 5p, and 5d are strongly hybridized with the O 2p states, and that our density functional theory calculations are in good agreement with O K-edge X-ray spectroscopy measurements for both occupied and empty states. It was demonstrated that the metallic states appearing under pressure in the semiconducting gap stem due to the transformation of the weakly hybridized O 2p-Sn 5sp subband corresponding to the lowest valence state of Sn in SnO. We discuss the nature of the electronic states involved in chemical bonding and formation of the hole and electron pockets with nesting as a possible way to superconductivity.     

Electronic states near fermi level in superconductors La2 − x Sr x CuO4 by means of x-ray absorption spectra near Cu-K and La-L edges

The electronic states of La_{2? x} Sr _x CuO₄ with 0.00 ≤ x ≤ 0.20 are studied by means of X-ray absorption spectroscopy (XANES, EXAFS) near the K-edge of Cu²⁺ ion and the L-edges of La³⁺ ion. It is found that characteristic white lines occurring near L _II and L _III edges of La³⁺ ion show a slight energy shift depending on substituted Sr²⁺ ions, x and temperature. The white lines suggest that unoccupied high-density 5dπ and 5dδ bands of La³⁺ ion just above a Fermi level transform to a hybridized single band of 5dπδ at 78?K in the superconductors with x = 0.10, 0.16 and 0.20. On the other hand, the XANES spectra near the Cu-K edge including a pre-edge region do not depend on x and temperature in the region of 0.00 ≤ x ≤ 0.20. It is considered that there is no reconstruction of electronic states at the Fermi level in a Mott–Hubbard band gap between an O 2p valence band and a Cu 3d conduction band. The electronic states at the Fermi level are probably consisted of the unoccupied 5dπδ band and an empty charge-transfer 3d?⁹ L band at low temperature, bands of which occur in a band gap between a filled O 2p valence band and an unoccupied O 2p conduction band. The insulator–superconductor–metal transitions in La_{2? x} Sr _x CuO₄ are related to the 5dπδ and 3d?⁹ L bands and holes, which site at a top region of the O 2p valence band near the Fermi level produced by a substitution of La³⁺ with Sr²⁺ ions.     

The XPS valence band spectra of NbC

The x-ray photoemission valence band spectra of NbC have been measured and are compared with the x-ray emission spectra and with the results from band structure calculations. This comparison leads to a large enhancement of theC2 _p photoabsorption cross section (at=1,487 eV) compared to the value calculated for the free atom. The effect of the nonmetal vacancy in the valence band can be described very well with vacancy cluster calculations.     

Zur Interpretation der Röntgenemissionsbanden von AIII BV-Verbindungen

The experimental data on the x-ray emission bands of AIII BV compounds corresponding to transitions with valence band states as initial states are compiled. The comparison of the observed emission band widths and line shapes with band structure data and also with the simple semiconductor model due to PENN shows that most of the observed structure can be explained if one assumes that the K-emission bands reflect the p-like part and the L-emission bands the s-(and d-) like parts of the total valence band state density.     

Resonant photoemission and absorption spectroscopy of the Cu<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>TiSe<Subscript>2</Subscript> compound

Single crystals of the Cu _x TiSe₂ compound with x = 0.05, 0.09, and 0.33 have been grown. Resonance photoelectron Cu 3p-3d and 2d-3d spectra of the valence bands, the spectra of the core levels, and the L absorption spectra for titanium and copper have been obtained. It is shown that the degree of oxidation of titanium atoms is +4 and the state of copper atoms is close to the state of free copper ions. It is found that the spectra of the valence bands obtained under the Cu 3p and 2p resonance conditions radically differ. For the spectra in the Cu 2p excitation regime, several bands corresponding to different decay channels of the excited state are observed. According to calculations of the density of states, the 3d states of copper are filled incompletely; the occupancy of the 3d band of copper is 9.5 electrons per atom.     

The plasmon density of states of a layered electron gas

The plasmon density of states (DOS)p() of a layered electron gas (LEG) is studied theoretically. It is shown thatp() is a linear function of frequency as goes to 0, and then increases more rapidly as tends to the screened, three dimensional plasma frequency _p. We also study the partial densities of state for constantK andq _z , whereK andq _z are the magnitude of the transverse and perpendicular momentum transfers. A possible experimental probe for the plasmon DOS is discussed.     

Electron energy-loss spectroscopy studies on C60 and C70 fullerite

The electronic structure of the crystalline fullerites C₆₀ and C₇₀ has been investigated by high-energy electron energy-loss spectroscopy in transmission. From valence band excitations and from core-level excitations of the C 1s level information on occupied and unoccupied and bands has been obtained.     

Electronic structure of amorphous Nb1-x Si x films studied by X-ray emission and X-ray photoelectron spectroscopy

We present valence band spectra of the amorphous system Nb_1–x Si _x (0.2x0.8), of bcc-Nb and of a-Si obtained by X-ray photoelectron spectroscopy (XPS, Al K) and X-ray emission spectroscopy (XES, Si K-emission bands). The samples were prepared as thin films by sputtering. The origin of all prominent spectral features was identified and consistently correlated to Si 3s-, Si 3p-and Nb 4d-derived states. The Nb4d-Si3p coupling is stable in binding energy over a wide concentration range. There is strong experimental evidence that the short range order changes considerably within the concentration interval 0.4x0.7, whereas the partial density of states of the Si 3p-electrons is clearly altered in the small concentration range 0.50x0.57.     

Chemical Bond in Metal Azides and Their Reactivity

Results of experimental investigations into the processes of slow and explosive decomposition of metal azides and the corresponding phenomenological models are analyzed. The energy-band structure and the valence electron density for actual crystallographic planes as well as the hypothetical electron density of the first unoccupied band of KN₃ and AgN₃ are calculated by the density functional method with the use of pseudopotentials. It is demonstrated that the anion-cation chemical bond in KN₃ is predominantly ionic in character, whereas a significant covalent component in AgN₃ is caused by Ag d-states. From the character of distribution of the valence and hypothetical electron densities it follows that the exciton and vibron decomposition mechanisms are improbable. The chain process is also improbable for alkali metal azides, because the gap energy for them is E _g 8 eV, which is close to the energy of azide ion decomposition Q 9–12 eV; the chain process is probable for AgN₃, because E _g(AgN₃) 3 eV. The chain process in AgN₃ is also favored by intense delocalization of the valence electron density through the hybridization of the Ag d-state and the N p-state, which causes a relatively high hole mobility.     

A Theoretical Description of the Photodissociation Spectrum of the H2O...HCl Complex

The effect of association of the water molecule and hydrogen chloride on the UV absorption bands is studied. Complete ab initio calculations for the H₂O...HCl complex in the S ₁ and S ₂ states are performed. A mathematical model using Airy functions is developed to describe the absorption cross-sections in a continuous spectrum. The form of the potential is determined by accurate ab initio calculations. The cross-sections of potential surfaces of lower electron states are found from ab initio calculations using the Hartree–Fock, configurational interaction, and multi-configurational interaction techniques. A complete vibrational analysis and an analysis of the change in the electron density for the S ₀ S ₁ transition on moving along the reaction coordinate allow a conclusion to be made on the feasibility of applying the model proposed to the H₂O...HCl complex. The results obtained in the framework of the model using Airy functions show reasonably good agreement with the experiment. For the H₂O...HCl heterodimer, the absorption band has the same structureless form as for the water monomer. The absorption band (peaking at 161 nm) is seen to shift towards short wavelengths as compared with the water monomer H₂O ( 167 nm).     

Optical absorption spectra of K41-atoms isolated in argon matrices

High purity argon polycrystals were doped with 0.2 ppm of K⁴¹ through the nuclear reaction Ar⁴⁰(n, ^–)K⁴¹. Optical transmission was measured as a function of the annealing temperature between 300 nm and 800 nm. Absorption bands corresponding to the transitions 4² p _{1/2, 3/2}4² s _1/2 and 5² p _{1/2, 3/2}4² s _1/2 of free potassium atoms could be observed. Annealing dependence of these absorption bands was studied and may be understood by a simple model.Research supported by Bundesministerium für Forschung und Technologie under FV. 4MT/1L     

Energy band structure and X-ray spectra of phenakite Be<Subscript>2</Subscript>SiO<Subscript>4</Subscript>

The electronic structure of crystalline phenakite Be₂SiO₄ is investigated using x-ray emission spectroscopy (XES) (Be K _α XES, Si L _{2, 3} XES, O K _α XES) and x-ray absorption spectroscopy (XAS) (Be 1s XAS, Si 2p XAS, O 1s XAS). The energy band structure is calculated by the ab initio full-potential linearized augmented-plane-wave (FLAPW) method. The total and partial densities of states and the dispersion curves for the Be₂SiO₄ compound are presented. It is shown that the top of the valence band and the bottom of the conduction band of the Be₂SiO₄ compound are predominantly formed by the oxygen 2p states. According to the results obtained, the electron transition with the lowest energy supposedly can occur at the center of the Brillouin zone. The effective masses of electrons (0.5m _e ) and holes (3.0m _e ) for the Be₂SiO₄) compound are estimated.