Excitation spectroscopy,Raman scattering and the temperature dependence of the luminescence in highly excited red HgI2

The luminescence of red HgI₂ is investigated as a function of temperature, excitation intensity and wavelength. At high excitation intensity and low temperature an “M-band” emission dominates. This M-band is assigned to biexciton decay and bound exciton scattering with acoustic phonons (“acoustic wing”), this assumption being supported by the results of excitation spectroscopy. The energy of the biexciton is determined to be (4661 ± 1) meV. From the evaluation of Raman spectra, the phonon energies (1.9, 3.1 and 14.0 ± 0.2) meV are found. At higher temperatures two lines are observed, one of which is ascribed to exciton-free carrier scattering. Position and line shape are in good agreement with theoretical results. The other emission line is found to be due to scattering involving excitons or carriers bound to lattice defects.     

Anisotropic polariton dispersion in red HgI2

We have measured resonant Brillouin Scattering near the A-exciton region of a layer-semiconductor, red-HgI₂. The Brillouin shift is interpreted in terms of a scattering process between the two polariton states by an acoustic phonon. The estimated exciton masses for the translational motion are (0.68 ± 0.1)m₀ and (1.2 ± 0.1)m₀ for the (100 and (001) directions, respectively.     

Thermally stimulated current measurements on vapour-grown HgI2

Two ways of performing the Thermally Stimulated Current (TSC) experiment have been used to investigate vapour grown HgI₂:i) under light excitation, four peaks are observed at 104, 158, 210, 240 K; ii) under bias excitation, two types of spectra have been obtained with peaks around 160, 240K (type I) and 182, 280 K (type II), respectively. The activation energies have been estimated to be 0.38 eV for the peak showing up at 182 K, and 1.05 eV for the one at 280 K.     

Cooperative fluorescence by excitonic resonant pumping in PbI2 and HgI2

The behaviour of the cooperative emission from PbI₂ and HgI₂ at 80 K has been studied at high exciton density by means of a tunable high intensity dye laser pumping. From a comparative analysis of the observed M and S stimulated emission lines from PbI₂ and HgI₂ we find that they strongly depend in two different ways on the pumping photon energy. Then, it is also possible to conclude that exciton-exciton and exciton-electron inelastic scattering are responsible for stimulated emission in PbI₂ and HgI₂ respectively.     

Phonon-assisted recombination of free electron-hole pair in HgI2

The emission spectrum due to the indirect recombination of free electrons and holes has been observed in the layer-type red-HgI₂ in coexistence with the spectrum caused by the recombination of free direct exciton. The characteristics in emission spectrum is caused by the presence of weak interlayer van der Waals forces.     

The solid state reaction between AgI and HgI2

The kinetics of the solid state reaction between AgI and HgI₂ is investigated experimentally by measuring the thickness of the layer of the α-Ag₂HgI₄ product phase as a function of the annealing time and the reaction temperature. A theoretical expression is derived which allows the calculation of the variation of the chemical potentials of the binary components across the product layer considering the variation of the product phase composition. A value of A₀ = 6 kJ/mol is estimated for the standard affinity for the formation of stoichiometric Ag₂HgI₄ from the binary components AgI and HgI₂.     

Raman scattering from mixed crystals (HgBr2)1−x(HgI2)x

The behavior of long wavelength optic phonons in mixed system HgBr₂-HgI₂ has been investigated in Raman scattering. Different types of behavior, i.e. ‘persistent type’ and ‘amalgamation type’ were observed. Three strong Raman bands were observed for persistent type mode. These correspond to stretching vibration of HgBr₂, HgBrI, HgI₂ molecules. Intensity of these bands showed striking concentration dependence. A model based on nearly free molecular approximation have been proposed to explain the dependence. The analysis indicated that the distribution of anions on the sublattices is essentially random. For amalgamation (one-mode) type behavior, it is suggested that the intramolecular force relevant to the vibration is comparable to the intermolecular force and that the band width of the corresponding phonon branch in the parent crystal is broad enough to be degenerate with impurity mode at the low concentration limit.     

“Thermal barrier” effect in energy relaxation of polaritons in HgI2

The paper presents the results of measurements of polariton radiation spectra, polariton band excitation spectra and spectra of resonance Raman scattering HgI₂. These are registered at different temperatures and units of energetic derangement between the exciting radiation quantum energy and that of transverse exciton. The threshold value of the temperature (T=12 K) starting from which there is attained the equilibrium between the polariton and phonon subsystems is found. The scattering mechanism that guides the formation of “thermal barrier” favouring the polaritons thermalization, is revealed.     

Al2O3-Y2O3-ZrO2三相复合陶瓷的介电谱研究

采用传统的固相反应法,在1400–1500 ℃下烧结,制备得到Al₂O₃-Y₂O₃-ZrO₂三相复合陶瓷.样品的结构、形貌和电性能分别用X射线衍射(XRD)、扫描电子显微镜(SEM)及介电谱表征.XRD表明此三相复合体系无其他杂相,加入Y₂O₃及ZrO₂后使得Al₂O₃成瓷温度降低;SEM表明此体系晶粒直径为200–500 nm,并且样品随烧结温度的升高而变得更加致密,晶界更加清晰;介电损耗谱中出现峰值弛豫现象,根据Cole-Cole复阻抗谱得出其为非德拜弛豫. 关键词： 2O₃-Y₂O₃-ZrO₂三相陶瓷')" href="#">Al₂O₃-Y₂O₃-ZrO₂三相陶瓷 介电弛豫 阻抗谱 热导率     

Opto-galvanic spectroscopy of some molecules in discharges: NH2, NO2, H2 and N2

The change of the discharge voltage when laser light crossing the discharge is tuned to a molecular transition has been measured. Experiments have been performed in the wavelength region between 570 nm and 620 nm with discharges in NH₃, NO₂, H₂, N₂, O₂ and argon. Transitions from the ground states of NH₂ and NO₂ and transitions from metastable states of N₂ and H₂ have been detected. The spacial dependence of the opto galvanic in a low pressure dc-discharge of H₂ and N₂ has been studied.     

Mass spectroscopy of HCN-laser plasmas with H2 and D2

Mass spectroscopic studies reveal the role of H₂ in HCN-laser plasma. This is confirmed by laser experiments with different gas mixtures containing H₂ and D₂.     

Excitation spectra and fluorescent lifetime measurements of Mn2+ in CaF2 and CdF2

The excitation spectrum of the Mn²⁺ emission has been measured in CaF₂ and CdF₂. The observed excitation bands have been assigned to transitions of the Mn²⁺ ions in a cubic environment. The calculated values for the crystal field (Dq) and Racah parameters (B,C) are Dq = 425 cm^-1 for CaF₂, Dq = 500 cm^-1 for CdF₂ and, B = 770 cm^-1 and C / B = 4.48 for both compounds. The lifetime of the fluorescent level ⁴T_1g(⁴G) has been measured in both compounds at different temperatures in the range from 10 to 500 K. The lifetime thermal dependence is explained taking into account different mechanisms (purely radiative, phonon assisted, and radiationless transitions) for the decay of excited Mn²⁺ ions.     

Submillimeter-wave spectroscopy of HN2 and DN2 in the excited vibrational states

The pure rotational transitions of HN₂⁺ and DN₂⁺ in the first excited vibrational states for all the fundamental vibrational modes have been observed in the range of 300-750 GHz. The molecular constants determined are much more accurate compared with those obtained from the infrared spectroscopy. The equilibrium rotational constants, B_e = 46832.45 (71) MHz for HN₂⁺ and B_e = 38708.38 (58) MHz for DN₂⁺, have been determined by correcting for the higher-order vibration-rotation interaction effects, γ_ij, obtained by an infrared investigation. The equilibrium bond lengths are derived from these equilibrium rotational constants: r_e(H-N) = 1.03460 (14) Å and r_e (N-N) = 1.092698 (26) Å.     

Core level spectroscopy of MoS2

X-ray photoelectron spectroscopy has been used to study mineral molybdenite, MoS₂. The fitted core level spectra of sulphur 2p and molybdenum 3d states reveal several photon energy sensitive components. The high binding energy component in both spectra is proposed to originate from the uppermost sulphur or molybdenum atoms of an S–Mo–S sandwich layer of the hexagonal structure, respectively. The other features are suggested to be caused by the edge structures formed during the sample cleavage. The edge facets have much stronger chemical properties than the basal planes and they are known as the active sites of MoS₂ when it is used as a catalyst. The spectral features and the effect of the structure of UHV cleaved MoS₂ on them are discussed.     

Brillouin spectroscopy in K2Ba(NO2)4

The temperature dependence of elastic stiffnesses of K₂Ba(NO₂)₄ is investigated in the range - 140 to + 160°C, in which two phase transitions have been reported. Marked anomalies appear at the lower transition, in contrast with continuous variations through the upper transition. Results are compared to those obtained near the upper transition by an ultrasonic method and they are qualitatively discussed.     

Diode laser spectroscopy of the HO2ν2 band

The vibration-rotation spectrum of the HO₂ν₂ bending fundamental band was observed by a semiconductor diode laser spectrometer with a Zeeman modulation technique. The wavelength of the laser was measured by a high-precision λ meter. Of 153 lines which were observed by Zeeman modulation, 137 lines were assigned. A least-squares analysis was carried out on 131 observed lines with 1 ≦ N ≦ 13 and 0 ≦ K_a ≦ 4, to determine the rotational constants, the centrifugal distortion constants, and the spin-rotation interaction constants in the ν₂ state. The band origin, which was also derived, is 1391.7540 (2) cm^?1 [the value in the parenthesis denotes the standard error]. The force field of the HO₂ molecule is briefly discussed using molecular constants obtained in previous works and in the present work.     

Raman spectroscopy of RuSr2(Eu1.5Ce0.5)Cu2O10 magneto-superconductor

Raman spectroscopy studies are reported for the RuSr₂Eu_1.5Ce_0.5Cu₂O₁₀ (Ru-1222) compound at various temperatures of 300, 250, 200 and 90 K. Three distinct vibrational bands: the first at 110, 140, and 160 cm⁻¹, the second at 295 and 347 cm⁻¹, and third one at 651 cm⁻¹ are seen in Raman spectra of the compound at room temperature. These bands are attached to the Cu atoms’ c-direction, the Ru atoms’ ab-plane stretching and Ru atoms’ c-direction anti-stretching modes. Below 200 K, an extra vibrational mode is also seen at 260 cm⁻¹. Also, with a decrease in temperature, though the Cu vibrational modes remain intact, the Ru atoms’ ab-plane stretching (295 cm⁻¹) and c-direction anti-stretching (651 cm⁻¹) modes shift gradually to higher wave number positions. The frequencies of modes at 260 and 651 cm⁻¹ showed anomalous softening and line-width broadening below 100 K that corroborates well with the spin ordering seen in susceptibility studies. The studied compound is a ferromagnetic superconductor with magnetic ordering of the Ru spins at 200 K and superconductivity below 30 K. A magnetic and electrical transport characterization of the compound is also presented briefly.     

Electronic structures of MRhO2, MRh2O4, RhMO4 and Rh2MO6 on the basis of X-ray spectroscopy and ESCA data

X-ray O Kα, Rh Mγ and a series of M Lα emission spectra, ESCA spectra of the valence and inner levels, and O K and Rh M_III quantum-yield spectra for X-ray photoemission of the rhodium double oxides MRhO₂ (M = Li, Na, K), MRh₂ O₄ (M = Be, Mg, Ca, Sr, Ba, Co, Ni, Cu, Zn, Cd, Pb), RhMO₄ (M = V, Nb, Ta) and Rh₂MO₆ (M = Mo, W) have been measured and the dependence of electronic structure on the metal M analysed. For all compounds the inner part of the valence band corresponds to O 2p_σ + O 2p_π + Rh 4d states, while the outer part corresponds to Rh 4d. The valence band is separated from the conduction band by a narrow gap of width less than 1 eV. The first empty band, near the bottom of the conduction band, is formed by Rh 4d states, followed by a band due to vacant O 2p states.     

Soft X-ray appearance-potential spectroscopy studies of DyFe2 and ErFe2 intermetallics

The soft X-ray appearance-potential spectra (SXAPS) of the M_4,5 levels of Dy and Er and the L_2,3 levels of Fe in pure metals and their intermetallics DyFe₂ and ErFe₂ have been obtained in the energy range of 700–1500 eV. SXAPS probes the binding energies of the core levels and the distribution of local unfilled conduction-band states. Both the constituents of DyFe₂ show negative shifts with respect to the pure metals while those of ErFe₂ show a negative shift for Er and a positive shift for Fe. The widths of the Dy and Er peaks increase and that of the Fe decreases on alloying. The results are interpreted in terms of the Fermi level changes and the charge transfer from the rare earth to Fe. The discrepancy in Fe chemical shifts has been correlated to the electronic specific heat capacity coefficient reported for these intermetallics.     

Optical properties of Mn-activated Na2SnF6 and Cs2SnF6 red phosphors

Alkaline hexafluorostantanate red phosphors Na₂SnF₆:Mn⁴⁺ and Cs₂SnF₆:Mn⁴⁺ are synthesized by chemical reaction in HF/NaMnO₄ (CsMnO₄)/H₂O₂/H₂O mixed solutions immersed with tin metal. X-ray diffraction patterns suggest that the synthesized phosphors have a tetragonal symmetry with the space group D_4h¹⁴ (Na₂SnF₆:Mn⁴⁺) and a trigonal symmetry with the space group D_3d³ (Cs₂SnF₆:Mn⁴⁺). Photoluminescence (PL) analysis, PL excitation (PLE) spectroscopy, and the Raman scattering techniques are used to investigate the optical properties of the phosphors. The Franck-Condon analysis of the PLE data yields the Mn⁴⁺-related optical transitions to occur at ∼2.39 and ∼2.38 eV (⁴A_2g→⁴T_2g) and at ∼2.83 and ∼2.76 eV (⁴A_2g→⁴T_1g) for Na₂SnF₆:Mn⁴⁺ and Cs₂SnF₆:Mn⁴⁺, respectively. The crystal field parameters (Dq) of the Mn⁴⁺ ions in the Na₂SnF₆ and Cs₂SnF₆ hosts are determined to be ∼1930 and ∼1920 cm⁻¹, respectively. Temperature-dependent PL measurements are performed from 20 to 440 K in steps of 10 K, and the obtained results are interpreted by taking into account the Bose-Einstein occupation factor. Comprehensive discussion is given on the phosphorescent properties of a family of Mn⁴⁺-activated alkaline hexafluoride salts.