Optical absorption spectra of some transition metal thiophosphates

Near-infrared and visible absorption spectra at room temperature of MnPS₃, CoPS₃, CoPS₃ are reported. These materials show weak structures below their fundamental absorption thresholds, due to the 3d-3d transitions occurring on the transition metal ion. By comparison with other transition metal compounds, such excitations are interpreted on the basis of the ligand field theory. The appearance of well developed crystal field spectra is a further confirmation that these materials are considerably ionic.     

Photoacoustic investigation of the absorption edge of Cd1−x Fe x Te compounds

Summary We have measured at room temperature the photoacoustic spectra of Cd_1−x Fe _x Te(0<x<0.055) around the CdTe fundamental absorption threshold. We identified an absorption band at (1.38±0.01) eV, increasing with the Fe concentration, that we assigned to the⁵ E→³ T ₁ transition within the Fe²⁺ (3d ⁶) manifold. We observed also a broadening and a shift to higher energies of the CdTe absorption edge for increasingx. Work partially supported by the Italian Consorzio INFM and CNR through the GNSM.     

Nonlocal potential and optical properties of solids

Summary The theory of the RPA optical response of a solid has been generalized in order to take into account also the possible presence of spatially nonlocal potentials in the Hamiltonian. Explicit expressions for first- and second-order susceptibilities are given in the new framework. The expressions obtained depend on the matrix elements of operators of the form of a commutator of a component of the position operatorr and an operator that commutes with the lattice translations. The problem of the evaluation of these matrix elements is solved in a simple manner by introducing an auxiliary, periodic position operator,XXXr. In such a way a general formulation is obtained that preserves the gauge invariance. As an application of the new theory, the second harmonic generation (SHG) from a semiconductor in a simple two-band model has been studied. The differences between our correct gauge-invariant results and those obtained in the usual local approximation is an indication of a slow convergence of the expressions obtained in the local approximation.     

Ellipsometry with fourier transform spectrometer: An application to TaSi2 films

Summary Ellipsometric measurements on TaSi₂ polycrystalline films are presented in this paper. The optical functions are directly obtained in the wavenumber range from 400 to 3000 cm⁻¹ using an infrared ellipsometer coupled with a Fourier transform spectrometer. The analysis of the results provides the spectral range where the infrared response has a Drude-like behaviour. Finally, the optical resistivity at zero frequency is evaluated from the Drude parameters.     

Spontaneous-emission spectrum in a cavity: Doublet or triplet?

Summary The Green’s functions formalism together with the new algorithm in operating with ζ-functions has been used to obtain the three-peaked spectrum of spontaneous emission of an atom in a weakly damped cavity. The single-peaked spectrum and exponential decay law with the short (τ_s ~ 1/Γ _c) and long (τ₁ ~Γ _c/g ²) decay times have been shown to exist in the strongly damped cavity with its passive linewidthΓ _c.     

Normal-incidence exciton transmission and reflection spectra of semiconducting plane-parallel slabs

Summary Exciton transmission and reflection spectra of thin semiconducting crystals in the spectral region of excitonic polaritons are calculated by the Stahl's coherent wave approach. The influence of the crystal thickness and of the intrinsic damping is discussed. Calculations are performed for the lowest state of theA-exciton in CdS. Experimental results of Broseret al. (1985) are reproduced by inserting an appropriate energy-dependent damping. The research was supported by the Polish Ministry for Science and Higher Education, Program CPBP01.03.     

Photoelectron spectra of some transition metal pentachlorides

He(I) and He(II) excited photoelectron spectra of the transition metal pentachlorides MCl₅, where M  Nb, Ta, Mo, W and Re, are reported. De     

Reflectance magnetocircular-dichroism spectra and electronic structures of CdCr2Se4

Summary Reflectance magnetocircular-dichroism spectra of a ferromagnetic semiconductor CdCr₂Se₄ were measured for temperatures between 4.2 K and 157 K. Off-diagonal elements of the dielectric tensor were calculated from the measured spectra. A number of fine structures was resolved, the energy position of which was plotted against temperature. It is found that there are two red-shifting structures: one at 1.1 eV (the well-known absorption edge) and the other at 1.9 eV. Other structures show slight blue-shift. These energy positions were compared with the band structure calculated by means of the self-consistentDV-Xα technique. Paper presented at the ?V International Conference on Ternary and Multinary Compounds?, held in Cagliari, September 14–16, 1982.     

Impact broadening and shift of thallium two-photon transition 6P 1/2–9P 1/2,3/2 perturbed by argon and helium

Summary This paper reports pressure broadening and shift coefficients of thallium 6P–9P _1/2,3/2 transitions by Ar and He as perturbers. Two-photon absorption Doppler free technique has been adopted. The perturing pressure ranges from 300 Torr to 7 atm at 978 K. The red-shift due to Ar and the blue-shift due to He are found to vary linearly with pressure. The Van der Waals constants for these transitions are also reported.

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Riassunto Questo lavoro tratta dell’aumento di pressione e dei coefficienti di spostamento delle transizioni 6P–9P _1/2,3/2 con Ar e He come perturbatori. è stata adottata una tecnica libera Doppler di adsorbimento a due fotoni. La pressione che perturba varia da 300 Torr a 7 atm a 978 K. Si trova che lo spostamento verso il rosso dovuto ad Ar e lo spostamento verso il blu dovuto ad He variano liberamente con la pressione. Si presentano anche le costanti di Van der Waals per queste transizioni.

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Резюме В этой статье исследуются коэффициенты уширения под действием давления и сдвига для переходов 6P–9P _1/2,3/2 в таллии, возмущенном Ar и He. Используется техника двух-фотонного поглощения. Рассматриваются давления от 300 торр до 7 атм при 978 К. Получено, что красный сдвиг, обусловленный Ar, и голубой сдвиг, обусловленный, He, изменяются линейно с давлением. Определяются постоянные Ван дер Ваальса для этих переходов.

     

Optical properties of transition metal atom adsorbed graphene: A density functional theoretical calculation

Electronic and optical properties of 3d-transition metal adsorbed graphene system, theoretically studied in the framework of density functional theory, reveals significant modification compared to the pristine system. Due to adsorption of transition metal, the emergence of closely separated electronic bands leads to substantial amount of low energy optical absorption below 2.0 eV photon energy. Very significant enhancement of static dielectric constant and large value of reflectivity in the low optical energy regime has been identified for different adsorbed systems. In the different 3d-transition metal adsorbed systems, particularly up to the half filled d-shell transition metal atom, pronounced emergence of optical absorption line in the deep ultraviolet regime beyond 30.0 eV photon energy is observed.     

Photoacoustic saturation spectra of some semiconducting group IV-A transition metal dichalcogenides

Photoacoustic saturation spectra in some semiconductors as HfSe₂, ZrS₂ and HfS₂ have been measured at energies above the fundamental absorption edge, where the photoacoustic signal is independent of the optical coefficient and the dips detected in the photoacoustic spectra may be ascribed to optical reflection effects inherent to the band structure. The comparison between our experimental data and those obtained by means of conventional and modulatory spectroscopic techniques proves that such method is a useful tool in order to determine the electronic structure of photoacoustically active semiconductors.     

Optical and magneto-optical spectra of europium-doped strontium tetraborate single crystals

Optical absorption, luminescence, magnetic circular dichroism and Faraday rotation spectra of Sr_0.95Eu_0.05B₄O₇ single crystals are studied. The crystal field parameter for europium ions is found to be equal to 790 cm⁻¹. Excitation spectrum of luminescence strongly differs from that of low-doped polycrystalline strontium tetraborate. The Verdet constant in the ultraviolet is comparable to that of terbium gallium garnet in the visible.     

Laser power influence on Raman spectra of ZnO(Co) nanoparticles

Influence of laser power on nanocrystalline samples of ZnO(Co) prepared by commonly used wet chemistry method followed by calcination was investigated. Previous confirmation of the existence of ZnO and Co₃O₄ phases was based on the X-ray diffraction measurements. Here we report the experimental spectra of non-resonant Raman scattering in the range between 100 cm⁻¹ and 1600 cm^–1, for a series of samples irradiated with four different laser power densities. The laser power density has different influence on relative intensity of peaks that belong to ZnO phase than on those corresponding to Co₃O₄ phase. Both peak types show characteristic broadening and red shift toward lower frequencies. The laser power densities used in our study did not cause thermal destruction in any of the investigated samples. Laser-induced local heating effects in samples caused formation of cobalt dimers on the surface of Co₃O₄.     

Optical study of BST films combining ellipsometry and reflectivity

Optical properties of plasma laser-deposited Ba_0.75Sr_0.25TiO₃ (BST) thin films have been investigated using variable angle spectroscopic ellipsometry (VASE) and near-normal spectroscopic reflectivity (NNSR) within a broad spectral range at room temperature. The samples prepared under various deposition conditions and the Si substrate coated with the structure SiO₂/TiO_x/Pt were measured. The X-ray diffraction, atomic force microscopy and alpha step measurement were used for characterization of the samples. A special attention was paid to study sample texture. Both sets of experimental data (VASE and NNSR) were fitted simultaneously to obtain the optical constants (e.g. complex refractive index) and thicknesses of the films. For modeling of the experimental data in the range of transparency the Cauchy and Urbach formulas were used. The direct fit procedure and the Cody-Lorentz model were applied around and below absorption edge. In the entire spectral range the reflectivity spectra were analyzed by Kramers-Kronig analysis. The data around the absorption edge were fitted using the single-wavelength method and the absorption edge features were found up about 3.5 eV. The platinum-coated Si substrate data were fitted as a semi-infinite medium using the Drude and Lorentz oscillators model. The structure model for optical characterization of the sample included not only the BST layers and substrate but also the intermix and surface roughness layers to achieve good agreement with experimental data. The substrate structure was modeled by a simple bulk with surface roughness.     

Optical absorption edge of Cd1−x Mn x Ga2Se4 crystals

Summary We report the absorption edge spectra of the new family of diluted magnetic semiconductors Cd_1−x Mn _x Ga₂Se₄ (0≤x≤1), grown from the vapour phase by chemical transport. Absorption bands observed under the gap of CdGa₂Se₄ are attributed to intra-Mn²⁺ transitions involving excited states of the 3d ⁵ electrons, split by the crystal field. The authors of this paper have agreed to not receive the proofs for correction.     

Symmetry based properties of the transition metal dichalcogenide nanotubes

The full geometrical symmetry groups (the line groups) of the monolayered, 2Hb and 3R polytypes of the inorganic MoS₂ and WS₂ micro- and nanotubes of arbitrary chirality are found. This is used to find the coordinates of the representative atoms sufficient to determine completely the geometrical structure of the tubes. Then some physical properties which can be deduced from the symmetry are discussed: electron band degeneracies, selection rules, general forms of the second rank tensors and potentials, phonon spectra. Received 13 April 2000     

Dichroic property of the H2O-groups in α-quartz, LiNbO3 and LiTaO3 in the infrared range

The absorptive peaks of the H₂O-groups were observed in the infrared absorption spectra in the vicinity of 3480 cm⁻¹ in LiNbO₃ and LiTaO₃, and 3580 cm⁻¹ in α-quartz. The absorptions of these lines were found to depend on the vibration direction of the polarized incident light, showing the dichroic property in these crystals. The absorption of the ordinary ray waves is much larger than that for the E-ray waves in the crystals. This observation indicates that the direction of the bond vibrations for the H₂O-groups is perpendicular to the optic axis of crystals. The dichroic property of the H₂O-groups may serve as the tracer to study the local electric field in piezoelectric crystals.     

Magnetic anisotropy of deposited transition metal clusters

We present results of magnetic torque calculations using the fully relativistic spin-polarized Korringa-Kohn-Rostoker approach applied to small Co and Fe clusters deposited on the Pt(111) surface. From the magnetic torque one can derive amongst others the magnetic anisotropy energy (MAE). It was found that this approach is numerically much more stable and also computationally less demanding than using the magnetic force theorem that allows to calculate the MAE directly. Although structural relaxation effects were not included our results correspond reasonably well to recent experimental data.