Charge-transfer transitions and optical spectra of vanadates

Specific features of the charge-transfer states and O2p → V3d transitions in the (VO₆)^9? octahedral complex are studied using the cluster approach. The reduced matrix elements of the electric-dipole transition operator are calculated for many-electron wave functions corresponding to the initial and final states of a charge-transfer transition. Using a parameterization of the results, the relative intensities of the allowed charge-transfer transitions are calculated disregarding the mixing of different configurations of the same symmetry. The Tanabe-Sugano theory is used with inclusion of this mixing to calculate the energies of many-electron charge-transfer transitions and their actual intensities. Modeling of the optical spectrum of LaVO₃ reveals a complicated charge-transfer transition band consisting of 81 lines. The main maxima of the band are in the range 6.3–7.3 eV. There are also additional maxima in the regions of ≈3 and ≈8–9 eV. The bandwidth is ≈10 eV. The results of model calculations are in agreement with experiments and demonstrate the weakness of the widely used assumption that the spectrum of charge-transfer transitions has a simple structure.     

Excitation spectra of coupled metallic slabs

The polarisability of the system consisting of two parallel thin metallic slabs within the jellium model is obtained within the assumption that the electron densities of the slabs do not overlap. It is then used to calculate, for the first time, the excitation spectra and dispersion relations of the collective surface excitations of that system. The dispersion relations are compared with the classical ones to enable the classification of the excitation modes.     

Excitation spectra of ferromagnets with crystal fields

We have investigated the ferromagnetic phase transition and excitation spectrum of systems, such as the rare earths, where the crystalline fields are significant in determining the magnetic properties. Using the full level scheme appropriate to a system ofJ-4 ions in a crystal field of cubic symmetry and assuming a Heisenberg interaction, we have calculated the full temperature- andq-dependence of the dynamic susceptibility within the RPA, both in the paramagnetic and ordered regimes. Variations in behavior, both with changes in crystal field parameter and with changes in magnetic interaction strength are discussed, and emphasis is placed on the limiting case which corresponds to current experiments in the light rare earths.     

Excitation and fluorescence spectra of disulfur monoxide

Excitation and fluorescence spectra of the 3400-Å system of S₂O are reported for the first time at low resolution. Fluorescence quantum yields for the upper vibronic states are measured as a function of absorption peaks (bands). Because of the unusually small quantum yields of some absorption peaks and because of the broader bandwidths of some fluorescence bands, multiple transitions are assumed to occur within each “single” absorption peak. Thirty-one fluorescence peaks are identified and grouped into five v″₂ progressions. Although v″₂ up to 8 was assigned, no new vibrational constants are derived because of the low spectral resolution.     

Excitation spectra of ZnS phosphors at low temperatures

IR stimulation spectra are recorded for ZnS phosphors made in various ways. Bands at about 1.0 and 0.55 eV are the most characteristic; some specimens give bands at about 0.36 and 0.30 eV. The 1 eV band usually has a substructure, whose details vary in thermal stability. All bands in the 0.85–2.3m range behave identically in response to intensity variation. The mechanism of IR stimulation is discussed.We are indebted to T. Mauring for much assistance in the measurements.     

Excitation spectra of Chromium(III) complexes

77 K excitation spectra of Cr(oxolate)^3-₃ in several crystal lattices are reported. The components of the ²T₁ and ²T₂ states can be located quite easily by this technique and advantages of this technique over absorption measurements are discussed.     

Excitation spectra of donor-acceptor pair photoluminescence in ZnTe

Photoluminescence excitation spectra of donor-acceptor pairs have been observed in phosphorus doped ZnTe. Binding energies of the donor and the acceptor is estimated from the spectra. Emission spectra measured under below-band-gap excitation showed sharp bands superimposed with a broad emission band. The result indicates the existence of two decay processes under the below-band-gap excitation: (1) recombination of an electron and a hole trapped on a given D-A pair, and (2) recombination of D-A pairs with various distances for which bound electrons are formed via conduction band.     

Excitation of metal spectra in combined glow discharge

Consideration is given to the special features of the excitation of spectra of metals (Ag, Au, Fe, Hg, Pt, and Sb) that manifest themselves in investigating spectral sources with combined glow discharge (glow discharge and capillary discharge). It is established that the proposed method for introducing a substance into a plasma makes it possible to realize two fundamentally different regimes: without evaporation and with complete evaporation and matrix atomization. It is shown that a spectral source of combined glow discharge with a nonequilibrium plasma has significant advantages for the most important analytical characteristics over quasi-equilibrium combined discharge at atmospheric pressure. The gold and platinum sensitivity in combined glow discharge is 10⁻⁷%, and the reproducibility of spectral determinations is 0.001–0.100, depending on the measurement range. A fractional separation of elements that makes it possible to analyze the phase composition of substances, particularly metals, with high sensitivity is revealed in glow discharge. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 2, pp. 143–147, March–April, 1997.     

Excitation lines and multiple ionization in Auger spectra

Satellite lines of a new type and excessively large continuous distributions of energy degraded electrons are observed in theKLL Auger spectra of potassium, chlorine, and sulfur. The lines are tentatively interpreted as due to a process that is characteristic of the atom more than of the entire solid and the excessive number of electrons in the continuous distributions is interpreted as due to multiple ionization.     

Excitation spectra of the intrinsic luminescence of solid neon

The excitation spectra are measured for the atomic and the hot molecular self-trapped exciton (STE) luminescence in solid Ne. The atomic STE component is enhanced in comparison with the molecular STE at 17 and 19 eV of excitation energies, which are located at the low energy side of the bulk exciton transitions. On the other hand, the branching ratio into the atomic and the hot molecular STE's is almost constant at the other excitation energies.     

Excitation wavelength dependence of luminescence spectra of a-Si:H

We report large shifts in the photoluminescence spectrum of a-Si:H as the excitation energy is reduced below ～1.8 eV. The shifts in the position of maxima are equal to approximately half the change in excitation energy. Another interesting feature of the data is a sharp high energy cutoff of the spectrum, also at ～1.8 eV, when excited above that energy.     

Connections BetweenA utocorrelat ion Function and Double-Pulse Excitation spectra

Formulas are presented that describe the general connections between the autocorrelation functions and the double-pulse excitation probabilities. The formulas are derived using the time-dependen t perturbation theory and the rotating-wave approximation for the laser pulses. These formulas can be used to obtain the autocorrelation function from the doublepulse excitation probability or vice versa in many systems.     

Excitation spectra of a system of two coupled quantum wells

Quasi-particle spectra, reconstructed by e-h pairing, have been calculated for a system of spatially separated electrons (e) and holes (h) in ground state. The regions of strong pairing interaction and significant correlation effects are reached by abandoning the BCS approximation and using instead interpolation expressions for correlation energies, which depend functionally on the coefficients of the u-v transformation, with subsequent minimization of the total energy of the reconstructed state with respect to the parameters of the u-v transformation. The dependence of the spectra on quantum-well separation and particle concentration in a system of two coupled quantum wells is discussed. Fiz. Tverd. Tela (St. Petersburg) 40, 2226–2228 (December 1998)     

Excitation spectra of self-activated luminescence in ZnSe crystals under pressure

A narrow excitation band observed for S-A luminescence in ZnSe crystal is attributed to free exciton absorption. Some overlap with a higher characteristic band of low intensity is considered for low temperature spectra.This band shifts under pressure toward higher energies with a coefficient dE/dP = (7.5±0.3) meV Kb^-1 at 300 K and (7.4±0.5) meV Kb^-1 at 85 K.The pressure shift of the excitation due to edge absorption is (7.0± 0.5) meV Kb^-1.     

Excitation spectra of light-induced electric responses in doped ferroelectric crystals

On the basis of studies of light-induced responses of doped lithium niobate crystals, a new approach to the spectroscopy of polar materials is developed. An experimental technique is described and possible applications and advantages of the approach are discussed.     

N-lines in the luminescence spectra of Cr3+-doped spinels: IV. Excitation spectra

Excitation spectra (T = 75–300 K; λ_exc = 450–630 nm) which were measured for the R-lines of Cr³⁺-doped oxides (α-Al₂O₃, β-Ga₂O₃) and for different luminescence lines (R-lines, N-lines) of Cr³⁺-doped spinels (MgAl₂O₄, ZnAl₂O₄, ZnGa₂O₄) are reported. The excitation maxima observed for different luminescence lines of a given compound exhibit considerable differences: 530 nm ? λ^max_exc ? 565 nm for MgAl₂O₄; 530 nm ? λ^max_exc ? 580 nm for ZnAl₂O₄; 545 nm ? λ^max_exc 555 nm for ZnGa₂O₄. According to the interpretation of N-lines to arise from different classes of Cr³⁺ ion swith different short range orderd, the excitation maximum of one distinct line should entirely correspond to the transition Δ : ⁴T₂ ← ⁴A₂ of that Cr³⁺ class from which the line arises. By this method spectroscopic data about the different kinds of Cr³⁺ ions present in a given sample can, therefore, be obtained which are not available from absorption measurements. The experimetal data were found to be in agreement with the results of model calculations. Restrictions which limit the accuracy and relevance of the data are discussed.     

Excitation and emission spectra of zinc-cadmium sulphides containing silver and nickel

Experimental studies have been made to investigate the influences of ZnS and CdS mixed crystal formations as well as nickel dopes on the spectral distributions of excitation and emission of ZnS-CdS-Ag-Ni phosphors. Shifts of the spectra to lower energies occur when the cadmium content of the matrix lattice is increased; the presence of nickel has no measurable effect. This gives evidence that the silver impurity centres are not affected by the nickel content. The spectral shifts observed agree quantitatively with previous results. Possible effects of mixed crystal formations on the temperature-dependence of fluorescence and on thermoluminescence are discussed in the light of the present results.