Fine band structure of the vibrational spectra of fullerite C<Subscript>60</Subscript> and enhancement of intermolecular interaction in high-temperature phase

The fine structure of the fundamental vibrational bands and some combination tones of fullerite C₆₀ in its IR absorption and reflection spectra, as well as in Raman spectra, has been studied. This structure is due to the overlapping components of Davydov and isotopic splittings and the removal of vibrational degeneracy with symmetry lowering. It is shown that for IR F _u (i) bands (i = 1–4) and low-frequency H _g (1) and A _g (1) bands in the Raman spectrum the splittings at room temperature exceed those for the low-temperature phase. The enhancement of intermolecular interaction at elevated temperatures is explained by the nonequilibrium vibrational excitation of the medium as a result of nonlinear interaction of vibrational modes and by the change in the electronic states.     

A comparative study on diode-pumped continuous wave Tm:Ho:YVO<Subscript>4</Subscript> and Tm:Ho:GdVO<Subscript>4</Subscript> lasers

In this paper, we presented experimental results concerning on the laser characteristics of two microchip lasers emitting in the 2 μm range, Tm:Ho:YVO₄ microchip laser and Tm:Ho:GdVO₄ microchip laser. At a heat sink temperature of 283 K, the maximum output power of Tm:Ho:YVO₄ laser and Tm:Ho:GdVO₄ laser is 47 and 34 mW under absorbed pump power of 912 mW, respectively. High efficiency can be achieved for both lasers at room temperature. Nevertheless, compared with Tm:Ho:GdVO₄ laser, Tm:Ho:YVO₄ laser can operate on single frequency with high power easily. At the heat sink temperature of 288 K, as much as 16.5 mW of 2052.3 nm single-longitudinal-mode (SLM) laser was achieved for Tm:Ho:YVO₄ laser. Under the same condition, only 8 mW of 2048.5 nm SLM laser was achieved for Tm:Ho:GdVO₄ laser.     

Growth and Study of Scintillation Properties of Orthovanadate Crystals Ca:GdVO<Subscript>4</Subscript> and Ca:YVO<Subscript>4</Subscript>

New scintillation orthovanadate crystals Ca:GdVO₄ and Ca:YVO₄ featuring intrinsic luminescence are grown. The spectra of pulsed cathodoluminescence of new scintillators and their luminescence decay time are studied. Using γ-rays with an energy of 662 keV from the ¹³⁷Cs source, total absorption spectra (so-called photopeaks) for new orthovanadate crystals are measured. It is shown that the light yield of Ca:YVO₄ and Ca:GdVO₄ crystals is 28100 photon/MeV and 14000 photon/MeV, respectively.     

Bandgap-width correction for luminophores CaMoO<Subscript>4</Subscript> and CaWO<Subscript>4</Subscript>

Submicron-powder luminophores CaMoO₄ and CaWO₄ obtained via solid-phase reactions have been studied using diffuse-reflection (DR) spectroscopy and photoluminescence (PL) spectroscopy. It is found that the diffuse-reflection spectrum in the range of a fundamental absorption edge of <300 nm is distorted by PL overlapping, so that subsequent calculations of optical band gap E _g of luminophores CaMoO₄ and CaWO₄ result in an overestimation of this value. An algorithm for the correct processing of diffuse-reflection spectra is described. It is based on a subtraction of the photoluminescence spectrum in the range of fundamental absorption. The correct E _g values and energy values for the defect levels in the bandgap of CaWO₄ and CaMoO₄ are determined to amount to 4.78, 4.83, and 4.86 ± 0.01 eV and 3.97, 4.07, 4.16 ± 0.01 eV, respectively.     

Comparison of <Emphasis Type="Italic">c</Emphasis>-Nd:YVO<Subscript>4</Subscript>/YVO<Subscript>4</Subscript> raman lasers and <Emphasis Type="Italic">c</Emphasis>-Nd:YVO<Superscript>4</Superscript> self-Raman lasers

Raman lasers based on c-Nd:YVO₄ crystals can generate 1178 nm Stokes line, which can be frequency-doubled to realize 589 nm sodium lasers. We make comparative experimental studies of c-Nd:YVO₄/YVO₄ Raman lasers and c-Nd:YVO₄ self-Raman lasers. About these two kinds of lasers, the output characteristics of power, center wavelength and beam quality are measured and compared.     

Vibrational spectra of monoisotopic SiH<Subscript>4</Subscript> and GeH<Subscript>4</Subscript> in low-temperature matrices

IR absorption spectra of monoisotopic ²⁸SiH₄ and ⁷⁶GeH₄ are studied in Ar and N₂ matrices at 10 K. It is shown that the absorption spectra of silane and germane are similar in the regions of the stretching ν₃ and bending ν₄ vibrations. Four groups of bands can be separated out in the spectra of each molecule: (1) narrow bands characteristic of the matrix isolation studies, (2) broad bands, (3) diffuse absorption with a large value of the spectral moment M ₂^* the intensity of which increases upon annealing, and (4) bands of dimers the intensity of which increases quadratically with concentration. The spectra of ²⁸SiH₄ and ⁷⁶GeH₄ in nitrogen matrices contain a triplet in the stretching region and a doublet in the bending region, which is explained by the change in the molecular symmetry from T _d to C _3V on passage from the gas phase to solid nitrogen.     

The neutrino mass matrix and (selected) variants of <Emphasis Type="Italic">A</Emphasis><Subscript>4</Subscript>

Recent neutrino oscillation experiments have measured leptonic mixing angles with considerable precision. Many theoretical attempts to understand the peculiar mixing structure, observed in these measurements, are based on non-Abelian flavour symmetries. This talk concentrates exclusively on models based on the non-Abelian symmetry A ₄. A ₄ is particularly well suited to describe three family mixing, and allows to explain the near tri-bimaximal mixing observed. Special emphasis is put here on the discussion of the neutrinoless double beta decay observable 〈:m _ν 〉. Different models based on A ₄ with very similar predictions for neutrino angles can yield vastly different expectations for 〈m _ν 〉. Neutrinoless double beta decay can thus serve, in principle, as a discriminator between different neutrino mass models.      

Nanoclusters in mixed crystals Hg<Subscript>2</Subscript><Emphasis Type="Italic">Hal</Emphasis><Subscript>2</Subscript>

The Raman spectra of mixed Hg₂(Br,I)₂ and Hg₂(Cl,Br)₂ crystals were investigated. The multimode behavior of optical vibrations induced by presence of three types of molecules (Hg₂ Hal′₂, Hg₂ Hal″₂ and Hg₂ Hal′Hal″) in the crystals was observed experimentally and is discussed theoretically. Phase transition effects such as soft modes, density of states, IR vibration branches, and anti- and ferroelectric nanoclusters induced by polar Hal′-Hg-Hg-Hal″ molecules and their immediate environment, were observed in the Raman spectra.     

Effects of the interaction between R and Fe modes of the magnetic resonance in RFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> rare-earth iron borates

Resonance modes that are due to magnetic excitations in the exchange-coupled subsystems of rare-earth ions (R = Nd³⁺, Sm³⁺, and Gd³⁺) and Fe³⁺ ions have been detected in submillimeter transmission spectra (0.1–0.6 THz) of RFe₃(BO₃)₄ iron borate-multiferroic single crystals. The strong interaction between spin oscillations of the Fe and R subsystems has been revealed, which determines the behavior of the modes depending on the anisotropy of the exchange splitting of the ground doublet of the R ion. It has been shown that the intensities of coupled modes (contributions to the magnetic permeability) depend strongly on the difference between the g factors of Fe and R ions. This dependence makes it possible to determine the sign of the latter g factor. In particular, a noticeable intensity of exchange Nd modes in NdFe₃(BO₃)₄ is due to an increase in their contribution at g _{⊥, ‖}^Nd < 0, while in GdFe₃(BO₃)₄ with g _Gd ≈ g _Fe ≈ 2, the Fe and Gd contributions compensate each other and the exchange (Gd) mode is not observed. In spite of the weak interaction of Sm ions with the magnetic field, SmFe₃(BO₃)₄ exhibits resonance modes, which are attributed to the excitation of Sm ions through the Fe subsystem.     

A computer study of the Raman spectra of the (GaN)<Subscript>129</Subscript>, (SiO<Subscript>2</Subscript>)<Subscript>86</Subscript>, and (GaN)<Subscript>54</Subscript>(SiO<Subscript>2</Subscript>)<Subscript>50</Subscript> nanoparticles

The Raman spectra of the (GaN)₁₂₉, (SiO₂)₈₆, and (GaN)₅₄(SiO₂)₅₀ nanoparticles were calculated using the molecular dynamics method. The spectrum of (SiO₂)₈₆ had three broad bands only, whereas the Raman spectrum of (GaN)₁₂₉ contained a large number of overlapping bands. The form of the Raman spectrum of (GaN)₅₄(SiO₂)₅₀ was determined by the arrangement of the GaN and SiO₂ components in it. The nanoparticle with a GaN nucleus had a continuous fairly smooth spectrum over the frequency range 0 ≤ ω ≤ 600 cm⁻¹, whereas the spectrum of the nanoparticle with a SiO₂ nucleus contained well-defined bands caused by vibrations of groups of atoms of different kinds and atoms of the same kind.     

Superhyperfine Structure of EPR Spectra in LiLuF<Subscript>4</Subscript>:U<Superscript>3+</Superscript> and LiYF<Subscript>4</Subscript>:Yb<Superscript>3+</Superscript> Single Crystals

Electron paramagnetic resonance (EPR) spectra of doped paramagnetic crystals LiLuF₄:U³⁺ and LiYF₄:Yb³⁺ have been investigated at a frequency of about 9.42 GHz in the temperature range of 10–20 K. The U³⁺ ion spectrum is characterized by g-factors g _∥ = 1.228 and g _⊥ = 2.516, and contains the hyperfine structure due to the ²³⁵U isotope with nuclear spin I = 7/2 and natural abundance of 0.71%. The observed hyperfine interaction constants are A _∥ = 81 G and A _⊥ = 83.8 G. Moreover, the spectrum reveals the well-resolved superhyperfine structure (SHFS) due to two groups of four fluorine ions forming the nearest surrounding of the U³⁺ ion. This SHFS contains up to nine components with the spacing between components being about 12.7 G. The SHFS is observed also in the EPR spectrum of the LiYF₄:Yb³⁺ crystal; up to 17 components with spacing of about 3.7 G may be traced. Some parameters of the effective Hamiltonian of the SHF interaction are estimated, the contribution of covalent bonding of f-electrons with ligands into these parameters is discussed. Authors' address: Igor N. Kurkin, Kazan State University, Kremlevskaya ulitsa 18, Kazan 420008, Russian Federation     

Influence of cetyltrimethylammonium bromide on the morphology of AWO4 (A = Ca, Sr) prepared by cyclic microwave irradiation

AWO₄ (A = Ca, Sr) was prepared from metal salts [Ca(NO₃)₂·4H₂O or Sr(NO₃)₂], Na₂WO₄·2H₂O and different moles of cetyltrimethylammonium bromide (CTAB) in water by cyclic microwave irradiation. The structure of AWO₄ was characterized by X-ray diffraction (XRD) and selected area electron diffraction (SAED). Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) revealed the presence of nanoparticles in clusters with different morphologies; spheres, peaches with notches, dumb-bells and bundles, influenced by CTAB. Six Raman vibrational peaks of scheelite structure were detected at 908, 835, 793, 399, 332 and 210 cm⁻¹ for CaWO₄ and 917, 833, 795, 372, 336 and 192 cm⁻¹ for SrWO₄, which are assigned as ν₁(A_g), ν₃(B_g), ν₃(E_g), ν₄(B_g), ν₂(A_g) and ν_f.r.(A_g), respectively. Fourier transform infrared (FTIR) spectra provided the evidence of W-O stretching vibration in [WO₄]²⁻ tetrahedrons at 793 cm⁻¹ for CaWO₄ and 807 cm⁻¹ for SrWO₄. The peaks of photoluminescence (PL) spectra were at 428-434 nm for CaWO₄, and 447-451 nm for SrWO₄.     

Influence of magnetic ordering on electronic structure of Tb<Superscript>3+</Superscript> ion in TbFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> crystal

Optical absorption spectra of trigonal crystal TbFe₃(BO₃)₄ have been studied in the region of ⁷F₆ → ⁵D₄ transition in Tb³⁺ ion depending on temperature (2–220 K) and on magnetic field (0–60 kOe). Splitting of the Tb³⁺ excited states, both under the influence of the external magnetic field and effective exchange field of the Fe-sublattice, have been determined. Landé factors of the excited states have been found. Stepwise splitting of one of the absorption lines has been discovered in the region of the Fe-sublattice magnetic ordering temperature. This is shown to be due to the abrupt change of equilibrium geometry of the local Tb³⁺ ion environment only in the excited state of the Tb³⁺ ion. In general, the magnetic ordering is accompanied by temperature variations of the Tb³⁺ local environment in the excited states. The crystal field splitting components have been identified. In particular, it has been shown that the ground state (in D ₃ symmetry approximation) consists of two close singlet states of A ₁ and A ₂ type, which are split and magnetized by effective exchange field of the Fe-sublattice. Orientations of magnetic moments of the excited electronic states relative to that of the ground state have been experimentally determined in the magnetically ordered state of the crystal. A pronounced shift of one of absorption lines has been observed in the vicinity of the TbFe₃(BO₃)₄ structural phase transition. The temperature interval of coexistence of the phases is about 3 K.     

Exciton absorption spectra of Cs<Subscript>2</Subscript>CdI<Subscript>4</Subscript> and Rb<Subscript>2</Subscript>CdI<Subscript>4</Subscript> ferroelastic solid solutions

The exciton absorption spectra of thin films of (Cs_{1 − x} Rb _x )₂CdI₄ solid solutions have been investigated and the refractive index n(λ) in their transparency window in the concentration range of 0 ≤ x ≤ 1 has been measured. The exciton-band parameters and optical permittivity ɛ_∞(x) have been found to linearly depend on the concentration. It is established that excitons are incorporated into the CdI₂ sublattice of the solid solutions and belong to intermediate-coupling ones. The characteristics of excitons in ferroelastics are compared with the corresponding parameters for CdI₂, RbI, and CsI, which are used as components to synthesize ternary compounds.     

Photoexcitation of rare-gas neon and argon clusters doped with <Emphasis Type="Bold">H</Emphasis><Subscript><Emphasis Type="Bold">2</Emphasis></Subscript><Emphasis Type="Bold">O</Emphasis>

We have studied the fluorescence of electronically excited OH^*, H^* and H₂O^+* dissociation fragments after VUV excitation ( h ν≥11.6 eV) of rare-gas clusters (Rg = Ne, Ar) doped with H₂O molecules. In contrast to a free molecule, where Balmer H-series dominate the UV-visible spectra, only the OH ^* ( A ² Σ ⁺ ↦ X ² Π) emission band is observed in neon clusters. No emission of excited water ions has been observed. We find that while higher excitation energies (Ne vs. Ar) induce higher vibrational excitation of the OH^* ( A ) fragment, the rotational temperature is lower. This effect is attributed to the difference in the geometric position of the H₂O molecule on the surface or inside the Rg-cluster. The rotational relaxation in neon clusters is rapid while the vibrational relaxation is slow because of the coupling with the low energy matrix phonons. Received 7 March 2002 / Received in final form 27 May 2002 Published online 19 July 2002     

An analysis of a superweak absorption spectrum of the SO<Subscript>2</Subscript> molecule using the variational procedure

The high resolution structure of a superweak 2ν₁+ν₂+ν₃−ν₂ band of sulfur dioxide, SO₂, is for the first time recorded with a Bruker IFS-120 HR Fourier spectrometer and analyzed using a specially derived procedure and a relevant computer code. As a result of the analysis, 115 lines have been found in the recorded spectrum, which allowed us to obtain high-accuracy values of the vibrational energy and rotational parameters of the highly excited vibrational state, (211).     

High-pulse-stability double-end continuous-grown YVO<Subscript>4</Subscript>/Nd:YVO<Subscript>4</Subscript>/YVO<Subscript>4</Subscript> acousto-optically Q-switched laser at high repetition rates

High pulse amplitude stability of 0.62% (rms) is achieved at 60 kHz repetition rate in fundamental mode with double-end continuous-grown YVO₄/Nd:YVO₄/YVO₄ composite crystal. The average output power and pulse peak power are 32.9 W and 27.7 kW, respectively, with 19.8 ns pulse width and 548 μJ pulse energy. The pulse amplitude stability is investigated experimentally. The stability gets improved with the decrease of repetition rate and output transmission. From theoretical analysis, the reason of pulse instability at high repetition rates is that the initial population inversion doesn’t saturate and the final population inversion doesn’t approach zero. With the decrease of repetition rate and output transmission, the final population inversion decreases and the interaction between two adjacent pulse periods is weakened. Therefore, pulse stability improves.     

Structural and magnetic phase transitions in Pr<Subscript>0.7</Subscript>Ca<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> at high pressures

The crystal structure and Raman spectra of Pr_0.7Ca_0.3MnO₃ manganite at high pressures of up to 30 GPa and the magnetic structure at pressures of up to 1 GPa have been studied. A structural phase transition from the orthorhombic phase of the Pnma symmetry to the high-pressure orthorhombic phase of the Imma symmetry has been observed at P ∼ 15 GPa and room temperature. Anomalies of the pressure dependences of the bending and stretching vibrational modes have been observed in the region of the phase transition. A magnetic phase transition from the initial ferromagnetic ground state (T _C = 120 K) to the A-type antiferromagnetic state (T _N = 140 K) takes place at a relatively low pressure of P = 1 GPa in the low-temperature region. The structural mechanisms of the change of the character of the magnetic ordering have been discussed.     

Manganese doping effects on interband electronic transitions,lattice vibrations,and dielectric functions of perovskite-type Ba<Subscript>0.4</Subscript>Sr<Subscript>0.6</Subscript>TiO<Subscript>3</Subscript> ferroelectric ceramics

The Ba_0.4Sr_0.6−x Mn _x TiO₃ (BSMT) ceramics with different Mn composition (from 1% to 10%) have been prepared via the conventional solid-state reaction sintering. The X-ray diffraction analysis shows that the ceramics are polycrystalline with the single perovskite phase. The lattice vibrations and optical properties have been investigated using Raman scattering, spectroscopic ellipsometry (SE), and infrared reflectance spectra. It was found that the optical bandgap for the BSMT ceramics is varied between 3.40 and 3.65 eV. The three first-order Raman-active phonon modes can be observed, and the frequency of the A ₁(LO₃)/E(LO) mode shows a blue shift of 8 cm⁻¹ with the Mn composition, which can be attributed to the distortion of the TiO₆ octahedron. With increasing Mn composition, the frequency of the infrared-active TO₄ mode decreases from 532 to 520 cm⁻¹, owing to the local variation of the lattice constant induced by the Mn incorporation. Moreover, the optical functions of the ceramics from the far-infrared to ultraviolet region are obtained based on the SE and reflectance spectra, which is useful for the potential applications in ferroelectric-based optoelectronic devices.     

Passively Q-switched <Emphasis Type="Italic">a</Emphasis>-cut Nd:YVO<Subscript>4</Subscript> self-Raman laser

A passively Q-switched a-cut Nd:YVO₄ self-stimulating Raman laser using a Cr:YAG saturable absorber has been demonstrated for the first time. The maximum average output power of the self-Raman laser at 1176 nm is 347 mW at the incident pump power of 10 W with a pulse repetition frequency (PRF) of 66 kHz. The pulse width, pulse energy of the 1176 nm are found to be 10 ns and 5.6 μJ. The conversion efficiency from diode laser input power to Raman output power is 3.47%.