Investigation of the optical absorption and emission spectra of CdCl2: Ni2+ single crystals

The optical absorption, emission, and excitation spectra of CdCl₂: Ni²⁺ single crystals are investigated. The absorption bands around 19,200 cm^-1 are reassigned and a few new bands above 22,000 cm^-1 are observed and assigned to singlet states. The emission bands at 77 K are examined in more detail and the previous assignments seem to be not correct.     

Optical and structural characterization of KBr crystals doped cadmium bromide (CdBr2)

In this paper we demonstrate the presence of CdBr₂ and cadmium aggregates in KBr matrix during Czochralski growth of KBr crystals. The chemical decomposition of CdBr₂ due to high temperature of crystallisation and reformation of cadmium bromide seems to be responsible for this effect.     

Optical absorption of isotopically enriched Li2B4O7 single crystals irradiated by thermal neutrons

Induced absorption spectra in the range 200–900 nm at 77 and 290 K for Li₂B₄O₇ single crystals, isotopically Li and B enriched are presented after irradiation of these crystals by thermal neutrons with fluence 1.8×10¹⁶ cm⁻². The dependence of induced absorption spectra on the isotope composition was revealed: for ⁶Li₂¹⁰B₄O₇ and ⁷Li₂¹⁰B₄O₇ crystals intensive band in the region of 280–294 nm was observed. Under substitution of ⁷Li isotope by ⁶Li in the lithium tetraborate lattice no changes in the absorption spectra were observed. The nuclear reaction ¹⁰B(n,)⁷Li is proposed to be the main mechanism of formation of the radiation defects.     

Fundamental absorption of AgGaS2 single crystals and thin polycrystalline films

AgGaS₂ single crystals show an absorption edge at 300 K which follows Urbach's rule for both directions of polarized light, parallel and vertical to the optical c-axis. The UV absorption curve of a polycrystalline thin film exhibited several maxima at photon energies which are in good agreement with previous results on optical reflectance and electroreflectance measurements.     

Optical absorption study of CuInTe2 crystals grown from near-stoichiometric compositions

From the optical absorption study of CuInTe₂ crystals grown from non-stoichiometric compositions, it has been shown that the sharp increase in the absorption coefficient observed around 0.90 eV corresponds to the transition between the shallow acceptor level and the conduction band and not due to the energy gap as reported earlier. The origin of this shallow acceptor state has been attributed to Te vacancy, which confirms the covalent bonding model for CuInTe₂.     

Optical studies in gamma irradiated Mg doped CaF2 single crystals

Pure and Mg doped CaF₂ single crystals grown by the Bridgman method were irradiated with gamma rays (γ-rays) for doses ranging from 97 Gy to 9.72 KGy. The pristine samples showed minimal absorption indicating the purity of the samples. The γ-irradiated pure CaF₂ crystals showed prominent and strong absorption with a peak at ~ 374 nm besides three weak ones at ~ 456, 523 and 623 nm. However γ-rayed Mg doped crystals showed a prominent absorption with a strong peak at ~ 370 nm and a broad one at ~ 530 nm. The absorption indicated the generation of F and F-aggregate centers in the irradiated crystals. The photoluminescence (PL) emission spectrum of both pure and Mg doped crystals showed prominent emission at ~ 390 nm when they were excited at ~ 250 nm. Also, when the samples were excited at 323 and 363 nm strong emissions were observed at ~ 430 and 422 nm respectively. The optical absorption and PL intensities were found to increase with increase in dose.     

UV-induced two-photon absorption in BiB3O6 single crystals

We show that BiB₃O₆ (BiBO) crystals, well known for their excellent second harmonic generation (SHG) properties, may also be of interest for third-order optical phenomena, particularly for two-photon absorption (TPA). Photoinduced TPA measurements were performed under illumination of excimer Xe–F laser (λ = 217 nm) as a photoinducing (pumping) beam. It created a thin surface layer (about 85 nm) that was a source of the observed photoinduced TPA. Raman shifted Nd-YAG laser radiation (λ = 1.9 μm) as well as its second and fourth harmonics (λ = 950 and λ = 475 nm, respectively) were used as fundamental (probing) beams of the TPA. The highest values of the TPA β coefficient were achieved for a polarization of the pumping light directed along crystallographic axis b. Quantum chemical simulations indicate on substantial contribution of UV-induced electron–phonon anharmonicity to the observed TPA. The obtained values of TPA coefficients indicate a possibility of using BiBO crystals as UV-operated optical limiters in a wide spectral range.     

EPR parameters and local lattice structure study of V ions in CdCl2 and CsMgCl3 crystals

The local lattice structure and EPR parameters (D, g_‖, g_⊥) have been studied systematically on the basis of the complete energy matrix for a d³ configuration ion in a trigonal ligand field. By simulating the calculated optical and EPR spectra data to the experimental results, the local distortion parameters (ΔR, Δθ) are determined for V²⁺ ions in CdCl₂ and CsMgCl₃ crystals, respectively. The results show that the local lattice structure of CdCl₂:V²⁺ system exhibits a compression distortion (ΔR=−0.0868 Å) while that of CsMgCl₃:V²⁺ system exists an elongation distortion (ΔR=0.0165 Å). The different distortion may be ascribed to the fact that the radius of V²⁺ ion is smaller than that of Cd²⁺ ion or larger than that of Mg²⁺ ion. Moreover, the relationships between EPR parameter D and local structure parameters (R, θ) as well as the orbital reduction factor k and g‐factors (g_‖, g_⊥) are discussed.     

Optical and EPR study of BaY2F8 single crystals doped with Yb

Optical and electron paramagnetic resonance study have been carried out on BaY₂F₈ single crystals doped with Yb ions at 0.5 and 10 mol%. The crystals have been obtained using the Czochralski method modified for fluoride crystal growth. Optical transmission measurements in the range of 190-3200 nm and photoluminescence measurements were carried out at room temperature. Absorption spectra of BaY₂F₈ single crystals doped with Yb due to the ²F_7/2→²F_5/2 transitions have been observed in the 930-980 nm range. To analyze the possible presence of Yb²⁺ ions in the investigated crystals, irradiation with γ-quanta with a dose of 10⁵ Gy have been performed. The observed photoluminescence bands show usual emission in IR and other one in VIS, being an effect of cooperative emission of Yb³⁺ ions and energy up-conversion transitions of photons from IR to UV-vis(visible) due to hoping process between energy levels of paired Yb³⁺ and Er³⁺, where Er³⁺ ions are unintentional dopants. The EPR spectra of BaY₂F₈:Yb 10 mol% consist of many overlapping lines. They have been analyzed in terms of spin monomers, pairs, and clusters. The angular dependence of the resonance lines positions have been studied also to find the location of coupled ytterbium ions in the crystal structure.     

Photoluminescence of PbBr2, PbCl2 and β-PbF2 single crystals

Photoluminescence measurements on a PbBr₂ single crystal at 4.2 K revealed a violet and a yellow emission band, in addition to the UV, blue and red band which were already known. The violet and blue luminescence are ascribed to emission of an exciton bound by a defect. The UV emission is quenched at much lower temperature than reported previously, because of a more effective energy transfer from the exciton to defect centres. Luminescence results on PbCl₂ and β-PbF₂ are in fair agreement with those of other authors.     

Luminescence centers in SnO2 single crystals

The luminescence spectrum observed in SnO₂ has been analyzed based on the vibroelectronic model. However, additional measurements of the photoconductivity and the spectral distribution of recombination probability cannot be explained by this model, which only takes into account strong phonon coupling. Arguments are presented in favor of Birman's model which is more consistent with all the present results.     

Edge emission in AgGaS2 single crystals

This paper reports on a study of the luminescence of single crystals of silver thiogallate (AgGaS₂). Cathodoluminescence measurements were made between liquid helium temperature and 250°K at two voltages, 25 kV and 10 kV and for 6 and ⊥ polarizations at the c axis. The cathodo-excitation technique used revealed the self-absorption phenomena, which are considerable near the bandgap.It is shown that the excitonic emission observed at liquid helium temperature remains up to 150°K and at higher temperatures there is band to band transition. In addition, a free to bound type of luminescence is indicated. This transition would appear to involve a level at 75 meV of the transport band.     

Optical absorption in MnIn2S4 single crystals

Optical absorption in MnIn₂S₄ single crystals has been studied. Direct and indirect optical transitions are found to occur at photon energies of 1.90?C2.16 eV in the temperature range of 80?C342 K. The temperature dependence of the band gap is determined; its temperature coefficients E _gd and E _gi are found to be ?4.84 × 10^?4 and ?6.33 × 10^?4 eV/K, respectively. The electron-phonon interaction is the main mechanism of the temperature shift of the intrinsic-absorption edge. MnIn₂S₄ single crystals exhibit anisotropy in polarized light at the absorption edge in the temperature range of 90?C190 K; the nature of this anisotropy is explained.     

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.     

Optical absorption spectra of synthetic CoCO3 single crystal

New results have been obtained on absorption spectra of synthetic CoCO₃ single crystal in the spectral range from the 11,500 to 34,500 cm^-1. Some vibration frequencies of the (CO₃)^2- ions have deduced from the absorption spectrum and zero-phonon lines have been identified. In the regions 22,900 and 24,500 cm^-1, a shift is observed of absorption frequencies with antiferromagnetic order for temperatures increasing from 4.2 to 20°K.     

Optical reflectivity of CdCr2Se4 single crystals in the visible and ultra-violet spectral region

The specular reflectivity of CdCr₂Se₄ single crystals has been measured within the spectral energy interval E = 0.5 ? 12.0 eV at room temperature. The spectral dependence of the real and imaginary part of the dielectric constant has been obtained from a Kramers-Kronig analysis of the experimental data. The observed optical transitions and the origin of the red-shifting band are discussed with respect to the semiempirical ionic model of the spinel structure, based on the recently reported increase of the structure parameter u of the spinel during cooling (Göbel, J. Magn. Magn. Mat. 3 (1976) 143). Contradicting results of the optical measurements in the region of the red-shifting edge may be attributed to the sensitivity of the parameter u on the internal stress.     

Electrical properties of CuGaSe2 single crystals

Hall effect and resistivity data have been obtained from melt grown single crystals of CuGaSe₂. A shallow accepter state with an ionisation energy of approximately 30 meV has been identified and the results are discussed and compared with similar data obtained from other Cu-III-VI₂ compounds.     

Magnetization behaviour of DyAl2 single crystals

We report the theoretical interpretation of the magnetization and the magnetocrystalline anisotropy of ferromagnetic DyAl₂ single crystals between 4.2 and 60 K and magnetic fields up to 15 T. Good agreement between theory and experiment is obtained by using three temperature independent parameters: the two crystal field parameters B₄ = (?0.50 ± 0.05) × 10^?4 meV, B₆ = ? (0.51 ± 0.05) × 10^?6 meV and the Curie temperature T_c = (62 ± 2) K.