Dependence of the fundamental absorption edge of CdInGaS4 on hydrostatic pressure

The optical absorption of CdInGaS₄ single crystals has been measured over a hydrostatic pressure range up to 40 kbar in the 2.0–3.0 eV photon energy range at room temperature. The interband gap for the indirect allowed transition was found to have a pressure coefficient, dEg/dP, of +6.4 × 10^-6ev/bar.     

Non-stoichiometric metals with strongly varying electron concentration: Gd-monochalcogenides and LaS

The reflectivity of single crystals of Gd monochalcogenides and of LaS has been measured at 300 K in the spectral region between 0.03 and 12eV. Special attention has been given to the chemical analysis of the materials and the determination of the deviation from stoichiometry. The optical constants have been determined by means of a Kramers-Kronig relation. A coupled mode of plasmons with interband transitions has been observed. The separation into interband transitions and free electron behaviour permitted the determination of the number of free carriers and their effective mass. It can be shown that the carrier concentration changes much more drastically than the stoichiometry. By comparison with LaS the position of the 4f⁷ levels of GdS was found to be 9 eV below the Fermi level.     

Photoluminescence of concentration series of CaF2: Mn phosphors excited by VUV radiation

Fluorescent characteristics of a series of powder CaF₂: Mn phosphors (from 0.01 to 2.47 wt. % of Mn in the mixture) excited by VUV radiation with quantum energies up to 14 eV at 293 K and up to 12 eV at 85 K are measured. Narrow excitation bands of Mn²⁺ centers found at 7.9 and 8.6 eV (at 293 K) are assigned to partially forbidden transitions of electrons from the ground state ⁶ S split by the crystalline field (10 Dq=0.71 eV from the literature) in two sublevels to the excited level corresponding to the ⁶ D term of a free Mn²⁺ ion (3d ⁵ → 3d ⁴4s transitions). A wide nonelementary excitation band in the region of 9.1–10.3 eV is interpreted as photogeneration of near-activator D-excitations: allowed transitions of electrons from levels that are split from the top of the valence band under the influence of an impurity ion to the free 4s-orbital of a Mn²⁺ ion. Channels of energy transport in the CaF₂: Mn system are briefly analyzed.     

Reflectivity of Tm monochalcogenides: No evidence of valence fluctuations

The reflectivity of metallic TmS, TmSe and semiconducting TmTe has been measured between 0.03 eV and 12 eV and has been interpreted as 4fⁿ-4f^n?15d and p-d interband transitions and coupled plasmon modes.     

Soft X-ray bands of highly ionized tungsten,gold and lead emitted by the TEXT tokamak plasma

Spectra of highly ionized tungsten, gold and lead emitted from the TEXT tokamak have been recorded in the 20–70 Å range by means of a grazing incidence, time-resolving spectrograph. The narrow and bright emission bands centered around 40, 45 and 50 Å in Pb, Au and W respectively, and bands of lesser intensities at lower wavelenghts, are emitted by a very large number of superimposed lines originating from transitions of the type 4d¹⁰4fⁿ−4d⁹4fⁿ⁺¹, 4d¹⁰4fⁿ−4d⁹4fⁿ5p and 4fⁿ−4fⁿ⁻¹5l (l=d, g). The isoelectronic analysis shows that at the electron temperatures prevailing in the present experiment, T_e≈800 eV, there are no contributions to the bands from ions having a 4p⁶4dⁿ ground state. This, together with the experimental measurement of the electron temperature profile, indicates that the heavy ion impurity distribution in the tokamak plasma is very close to ionization equilibrium.     

Far infrared optical excitation in metallic uranium sulfide

Previous near normal incidence reflectivity measurements on US single crystals from 12 to 0.03 eV have been extended down to 0.0018 eV (15 cm^?1). A broad plateau with a reflectivity of 90±2% is observed between 40 and 400 cm^?1 with a further increase of the reflectivity below 40 cm^?1. A Kramers-Kronig transformation of the data shows the existence of a resonance at 315 cm^?1. From a comparison with recent neutron data and other arguments we deduce that this resonance is due to the excitation of a transverse optical phonon coupled to an f→d or d→f interband transition.     

Pressure dependent optical absorption edge shift and compressibility of CdCr2Se4 single crystals

The pressure shift of the optical absorption edge (dE_g/dp = (1.1 ± 0.1) × 10^?6 eV bar^?1) and the compressibility (κ = (1.3 ± 0.) × 10^?6 bar^?1) of single crystalline CdCr₂Se₄ have been measured at ambient temperature. These data suggest an interpretation of the fundamental absorption in terms of either p → p interband or p → localized d charge transfer transitions, but exclude excitations involving s-band states.     

Luminescence characteristics of the Pr<Superscript>3+</Superscript> ion in SrB<Subscript>4</Subscript>O<Subscript>7</Subscript> and SrB<Subscript>6</Subscript>O<Subscript>10</Subscript>

Spectral and kinetic characteristics of the luminescence and luminescence excitation spectra of polycrystalline SrB₄O₇:Pr (1%) and SrB₆O₁₀:Pr (1%) samples are studied at 150–170 K. The samples show an intense luminescence band in the vicinity of 405 nm (¹ S ₀→¹ I ₆ transitions of Pr³⁺) and shorter wavelength bands also assigned to transitions from the ¹ S ₀ level. The main luminescence decay constant is ～2×10^?7 s. The excitation spectra of the ¹ S ₀ luminescence in these crystals are significantly different. The SrB₄O₇:Pr crystal shows three well-resolved bands at 6.14, 6.55, and 6.91 eV in the region of the 4f ²→4f ¹5d transitions and a complex structure in the region of interband transitions (7.1–20 eV), whereas the SrB₆O₁₀:Pr crystal shows a weakly structured band at 6.31 eV and no excitation in the region of the interband transitions. The physical mechanisms that may be responsible for the observed features of the spectra are discussed.     

Scandium and lutetium surfaces studied by reflection electron energy-loss spectroscopy

The reflection electron energy-loss spectra of the (1 0 0) and (0 0 1) surfaces of Sc single crystals and the (0 0 1) surface of a Lu single crystal have been studied with primary energies in the range 50–2000 eV. Scandium is congeneric with lutetium and the loss spectra of the two elements are very similar in both the collective excitations and the interband transitions. Strong excitations observed at around 41 eV are attributed to 3p → 3d and 5p → 5d transitions in Sc and Lu, respectively. The loss data of Sc fit the characteristic energy-loss data of the other elements of the first group of transition metals. Oxygen adsorption and nitrogen adsorption on the (1 0 0) surface of Sc influence the loss spectra. The observed differences are correlated with density-of-states calculations for Sc, ScO and ScN.     

Proprietes optiques et structure electronique de MnAu2

The optical spectrum of the helical antiferromagnetic compound MnAu₂(t_N = 90°C) has been measured, using a scanning ellipsometric method, in UHV, between 0.47 and 5.7 eV, at temperatures ranging from 88 to 700 K. Below 0.6 eV the experimental data can be fitted to a Drude-like intraband model. The maximum of the interband absorption occurs at 5.1 eV, while the onset of interband absorption may be placed at 0?.4eV as is suggested by the rapid rise of $\text{ε}{}_2\text{(ω)}\text{λ}$. below 0.5 eV. In the absence of theoretical work, the analysis of the optical spectrum leads to a preliminary rough model of the electronic structure; the proposed local density of d states is represented. The 5.1eV peak is attributed to d → E_F transitions (parabolic edge at 2.7 eV similar to Au), originating in the lower part of the band, associated mainly with Au sites. To account for the moment $\text{(}\text{3.6μ}{}_{\mathrm{S}}\text{at Mn}\text{)}$, the upper d band (mainly Mn sites) is split: the d↑ band is below E_F (interband edge at 0.4eV), while the d↓ band contains 1.4 electrons. ESCA measurements tend to confirm this model. An important unusual fact is the sharp anomaly of /~ε(ω) in the infrared, around T_N; attempts to correlate this with magnetic (s-d) interactions have been initiated.     

Piezo optic properties of nickel and platinum

The piezoreflectance spectra of nickel and platinum were measured over the range 1.25–5.2 eV with the direction of incident light polarised parallel and perpendicular to the strain axis. Strain amplitudes of 0.12 × 10^?3 and 0.6 × 10^?4 respectively were employed at a frequency of 68kHz. The results showed enhanced structure in the differential absorption spectra of nickel at 1.75, 3.75 and 5 eV. Weaker structure was also found in the range 2.0–2.5 eV. From the results it is possible to discount the assignment of the higher energy structures as due to symmetry point transitions at the X and L points. A summary of possible assignments for the observed structures is given.The results for platinum show an essentially featureless spectrum in the visible range from which it may be concluded that the earlier prediction of interband transitions in this metal in the visible regions are incorrect.     

Luminescence of LaBr3: Ce,Hf crystals under photon excitation in the ultraviolet,vacuum ultraviolet,and X-ray ranges

This study has been carried out using synchrotron radiation, time-resolved luminescence ultraviolet and vacuum ultraviolet spectroscopy, optical absorption spectroscopy, and thermal activation spectroscopy. It has been found that, in scintillation spectrometric crystals LaBr₃: Ce,Hf characterized by a low hygroscopicity, along with Ce³⁺ centers in regular lattice sites, there are Ce³⁺ centers located in the vicinity of the defects of the crystal structure. It has also been found that the studied crystals exhibit photoluminescence (PL) of new point defects responsible for a broad band at wavelengths of 500–600 nm in the PL spectra. The minimum energy of interband transitions in LaBr₃ is estimated as E _g ～ 6.2 eV. The effect of multiplication of electronic excitations has been observed in the range of PL excitation energies higher than 13 eV (more than 2E _g ). Thermal activation studies have revealed channels of electronic excitation energy transfer to Ce³⁺ impurity centers.     

Proprietes optiques et photoelectriques du GaSb

Reflectivity of single crystals of GaSb were measured between 1,3 and 70 eV; optical and dielectric constants were deduced by Kramers-Kronig's analysis in the interband transition region. An interpretation of observed singularities is given.Optical studies were completed by determination of quantum yield and energy distribution curves between 7,7 and 25 eV. The analysis of these curves gives initial and final energies of excitations and allows transitions assignments.By reflectivity measurements we also observed core transitions from d levels of gallium and antimony.     

Electronic structure of GdP as determined from optical data

The growth of large single crystals of GdP of various stoichiometry has permitted for the first time accurate measurements of the reflectivity for photon energies from 0.03 to 12 eV. By means of a Kramers-Kronig analysis it was possible to separate intra- and interband transitions. The latter ones have provided valuable information about the structure of the 5d conduction band, its crystal field splitting and the position in energy of the 4f levels of Gd³⁺. The variation of the plasma resonance with deviations from stoichio-iometry does not exclude a semimetallic behavior of GdP.     

Oxygen chemisorption and initial oxidation of Cr(110)

The initial stages of the interaction of oxygen with a Cr(110) surface have been investigated at 300 K by LEED, AES, electron energy loss spectroscopy (ELS), secondary electron emission spectroscopy (SES) and work-function change measurement (Δφ). In the exposure region up to 2 L, the clean-surface ELS peaks due to interband transition weakened and then disappeared, while the ～5.8 and 10 eV loss peaks attributed to the O 2p → Cr 3d transitions appeared, accompanied with a work-function increase (Δφ = +0.19 eV at2L). In the region 2–6 L the work function decreased to below the original clean-surface value (Δφ_min = ?0.24 eV at6L), and five additional ELS peaks were observed at ～2, 4, 11, 20 and 32 eV: the 2 and 4 eV peaks are ascribed to the ligand-field d → d transitions of a Cr³⁺ ion, the 11 eV peak to the O 2p → Cr 4s transition, the 20 eV peak to the Cr 3d → 4p transition of a Cr³⁺ ion and the 32 eV peak probably to the Cr 3d → 4f transition. A new SES peak at 6.1 eV, being attributed to the final state for t he 11 eV ELS peak, was observed at above 3 L and identified as due to the unfilled Cr 4s state caused by charge transfer from Cr to oxygen sites in this region. In the region 6–15 L the work function increased again (Δφ_max = +0.32 eV at15 L), the 33 and 46 eV Auger peaks due to respectively the M_2,3(Cr)L_2,3(O)L_2,3(O) cross transition and the M_2,3VV transition of the oxide appeared and the 26 eV ELS peak due to the O 2s → Cr 4s transition was also observed. Above 10 L, the ELS spectra were found to be practically the same as that of Cr₂O₃. Finally, above 15 L, the work function decreased slowly (Δφ = +0.13 eV at40L). From these results, the oxygen interaction with a Cr(110) surface can be classified into four different stages: (1) dissociative chemisorption stage up to 2 L, (2) incorporation of O adatoms into the Cr selvedge between 2–6 L, (3) rapid oxidation between 6–15 L leading to the formation of thin Cr₂O₃ film, and (4) slow thickening of Cr₂O₃ above 15 L. The change in the Cr 3p excitation spectrum during oxidation was also investigated. The oxide growth can be interpreted on the basis of a modified coupled current approach of low-temperature oxidation of metals.     

Fundamental reflection spectrum and the electronic band structure of chlorine-doped CdTe

The effect of chlorine impurity on the fundamental reflection spectrum and the electronic band structure of cadmium telluride crystals has been studied. At the impurity concentration N _Cl>5.0×10¹⁹ cm^?3, a peak appears in the reflectance spectra. This peak is due to electron transitions at the X point of the Brillouin zone from the upper split valence band to Cl levels lying 0.05 eV above the Γ minimum of the conduction band. The other features in the reflectance spectra and band structure are explained as being due to the effect of spin-orbit splitting at the X point and to indirect electronic transitions from the Cl levels to the Γ minimum.     

Direct and modulated reflectance spectra of MnO,CoO, and NiO

Direct reflectance, thermoreflectance, and electroreflectance have been measured for MnO, CoO, and NiO above the fundamental edge. Spectra of all three materials support a model containing both localized and one-electron band states. In MnO peaks with temperature coefficients of ～ 10^?3 eV/K were observed at 5.7 and 6.9 eV, temperature-independent structure occurred at 5.4, 6.3, and 7.2 eV, and spectral features with indeterminate temperature dependence were seen at 4.6 and 5.5 eV. The temperature-dependent structure was assigned to one-electron interband transitions associated with anion 2p and cation 4s states; the temperature independent structure was assigned to crystal field split localized interionic transitions between the 3d-states of neighboring Mn ions. Thermoreflectance spectra for CoO exhibited temperature dependent structure (9.5 × × 10^?4 eV/K at 5.0, 6.0, and 7.2 eV. A strong, temperature dependent electroreflectance oscillation was seen in NiO near 6.2 eV. On the basis of these data the interband gap between the anion 2p and cation 4s bands was determined to be 5.7 eV in MnO, 6.0 eV in CoO, and 6.2 eV in NiO.     

Europium luminescence in fluorite upon high-energy excitation

The reflection and luminescence excitation spectra of CaF₂ crystals containing europium ions in divalent (Eu²⁺) and trivalent (Eu³⁺) states were measured in the range from 4 to 16 eV. It was established that, in CaF₂ : Eu³⁺ crystals, luminescence of Eu³⁺ ions (the f-f transitions) is effectively excited both in the charge-transfer band (at ～8 eV) and in the region of the 4f–5d transitions (at ～10 eV) but is virtually not excited in the fundamental region of the crystal (at an energy higher than 10.5 eV). Luminescence of Eu²⁺ ions (the 427-nm band) in CaF₂ : Eu³⁺ is effectively excited in the fundamental region of the crystal; i.e., luminescence of divalent europium ions occurs through the trapping mechanism. Emission of Eu²⁺ ions in CaF₂ : Eu²⁺ crystals is characterized by the excitation band at an energy of 5.6 eV (the 4f → 5d,t _2g transitions), as well as by the exciton and interband luminescence excitations. The results obtained and data available in the literature are used to construct the energy level diagram with the basic electron transitions in the CaF₂ : Eu crystals.     

Optical absorption and electronic structure of intermetallic compound RuIn3

The optical properties of intermetallide RuIn₃ are investigated by ellipsometry in the spectral range of 0.22–10 μm. The experimental data point to the existence of an energy gap of about 0.5 eV in the electronic spectrum of the compound. The density of the electron states and interband optical conductivity are calculated in terms of the density functional theory. The experimental and theoretical spectra of the optical conductivity are compared. It is found that the formation of basic absorption bands is caused by interband transitions of electrons of the d-band of Ru and p-band of In.     

Optical properties of ternary β electronphases based on NiAl

Optical constants of several ternary β Hume-Rothery phases have been investigated as a function of the valence electron concentration and defect structure. The composition of the alloys was based on the β phase NiAl, the absorption spectrum of which is dominated by two maxima at 2·5 and 4 eV. The intensity of the 2·5 eV-ε₂ peak is considerably increased with increasing valence-electron concentration, whereas that of the other peak is decreased. The valence electron concentration is varied by substituting Cu for Ni in Ni_1?yCu_yAl or Si for Al in NiAl_1?ySi_y. The 2·5 eV absorption peak disappears when Co is substituted for Ni in Ni_1?yCo_yAl. The absorption peaks are attributed to interband transitions of electrons and are discussed according to the rigid band model. The absorption in the infrared is explained by the scattering of electrons from lattice defects and phonons. The position of the d-band relative to the Fermi level is discussed in connection with s-d band scattering.