Bound and many-particle states in polariton Raman spectra of NH4Cl crystals

The properties of polariton Raman spectra for the internal vibration region of NH₄Cl crystal are discussed. These spectra have been recorded by means of a copper laser ( $\text{λ = 5105 and 5782}\text{A}\text{?}$) for sample temperature T=80 and 10°K.According to the theory a polariton singularities in the region of a polar bound state has been obtained. The two-particle bands have been studied and a very complicate structure inside of these bands has been revealed. Moreover, multiphonon singularities at polariton curves have been observed. The experimental polariton law has been compared to the theory and a conclusion has been made as to the necessity of a further development of the theory.     

Exciton polariton in an organic–inorganic multiple-quantum-well crystal (C4H9NH3)2PbBr4

Exciton polariton in an organic–inorganic multiple-quantum-well (MQW) single crystal (C₄H₉NH₃)₂PbBr₄ at low temperature has been investigated by photoluminescence excitation (PLE), reflection, and time-resolved photoluminescence measurements. Since (C₄H₉NH₃)₂PbBr₄ has ideal two-dimensional excitons with an extremely large oscillator strength and forms self-organized MQW with a very short well-period (d～10 Å), polaritonic coupling among excitons is strong and extends over a large number of wells. Therefore, observed MQW polariton features were the same as those of bulk polaritons. We have also investigated relaxation dynamics of the MQW polariton in the same framework as discussions on bulk polaritons.     

Exciton polaritons in monoclinic zinc diphosphide

A classical theory based on excitons is insufficient to explain the reflectivity spectra of βZnP₂. Instead an exciton polariton picture is invoked and the polariton parameters are determined. Photoluminescence spectra are consistent with the polariton interpretation. Higher energy states of the exciton have been observed and from these the band gap and exciton binding energy in βZnP₂ have been deduced.     

Kinetics of electron tunneling transfer in KH<Subscript>2</Subscript>PO<Subscript>4</Subscript> and NH<Subscript>4</Subscript>H<Subscript>2</Subscript>PO<Subscript>4</Subscript> crystals with hydrogen bonds

A model of electron transfer by tunneling between trapped electron and hole centers in crystals with hydrogen bonds under the conditions of thermostimulated mobility of one carrier type in the recombination process has been developed. The proposed model describes all features in the kinetics of induced optical density relaxation observed in nonlinear optical crystals of KH₂PO₄ (KDP) and NH₄H₂PO₄ (ADP) on a wide temporal scale (10⁻⁸–10 s) under pulsed irradiation. The results of model calculations have been compared with experimental data on the photoinduced transient optical absorption (TOA) in KDP and ADP crystals in the visible and UV ranges. The nature of the radiation-induced defects, which account for the TOA, and the dependence of the TOA decay kinetics on the temperature, excitation power, and other experimental conditions have been considered.     

Raman scattering in sodium nitrite crystals near the phase transition

Optical Raman spectra of a ferroelectric sodium nitrite crystal have been detected in a wide spectrum range at various temperatures, including the region of the ferroelectric phase transition. A manifestation of a transverse soft polar mode of the A ₁(z) type responsible for the ferroelectric phase transition has been discovered in the spectrum at room temperature. This mode has been found to become overdamped even far from the ferroelectric phase transition temperature. This mode also appears as a central peak under heating. It has been found that the pseudoscalar mode of the A ₂ type has the highest intensity in the Raman spectrum of sodium nitrite. The frequency corresponding to the maximum intensity of this mode in the Raman spectrum varies from 130 cm^–1 at 123 K to 106 cm^–1 at T = 513 K. A fair agreement of the experimental data for the A ₁(z) mode with the Lyddane–Sachs–Teller relation has been established. The polariton curves for the A ₁(z) polar mode and the dispersion curves for axinons has been plotted.     

Double Fluorescence Conversion in Ultraviolet and Visible Region for Some Praseodymium Complexes of Aromatic Carboxyates

Four praseodymium complexes of aromatic carboxylates (benzoate, 4-tert-butylbenzoate, 2-benzoylbe-noate, and benzimidazole-5-carboxylate) have been synthesized and characterized, whose photophysical properties have been studied with ultraviolet spectra, phosphorescence spectra, and fluorescence spectra. The fluorescent emission spectra of all praseodymium complexes show two emission peaks under the excitation band of 245 nm at about 395 and 595 nm respectively, while one peak under 415 nm at about 595 nm, which attributed to be ¹S₀ → ¹I₆ (395 nm) transition and the characteristic emission ¹D₂ → ³H₄ (595 nm) transition of Pr³⁺ ion. The ¹S₀ → ¹I₆ transition can be ascribed as the transition of charge transfer state, and the ¹D₂ → ³H₄ can be further proved that there exists an antenna effect in the fluorescence of praseodymium with aromatic carboxylic acids. In conclusion, the praseodymium complexes systems can realize the double fluorescent conversion in both ultraviolet and visible region and can be further studied the application of this conversion.     

Temperature‐dependent Raman scattering of KDP:Mn (0.9% weight of Mn) crystal

Low temperature polarized Raman scattering measurements of KDP:Mn (0.9% weight of Mn) were performed at temperatures ranging from 14 to 300 K, over the spectral range 50–1250 cm⁻¹. In the present results we can see that the spectra of undoped and doped samples at room temperature are very different. Doped samples maintain the KDP structure as tetragonal, with the same factor group D_2d but with a different class of the space group, different from the original 12. The results show that the crystal undergoes a phase transition at temperature between 115 and 97 K, which is much lower than the phase transition temperature of undoped KDP that occurs at 122 K, where the crystal changes from the para‐electric to the ferroelectric phase. Further, at very low temperature (14 K) we can see that the spectra of KDP:Mn (0.9% weight of Mn) present a behavior very different from the behavior presented by the spectra of KDP doped with low Mn concentration. Copyright © 2010 John Wiley & Sons, Ltd.     

Manifestation of directional dispersion of overdamped optical phonons in Raman spectra

This study is devoted to the analysis of the features of the manifestation of the directional dispersion of strongly damped (overdamped) optical phonons in Raman spectra. The results of the experimental study of the directional dispersion of the overdamped soft mode of symmetry B ₂(z) of the KDP crystal (KH₂PO₄) are presented.     

Ferromagnetic resonance in K2CuF4 by means of quasi-optical methods in the millimeter wave range

We have studied ferromagnetic resonance in the millimeter wave range between 35 and 100 GHz with external fields ranging from 1 to 4 T. The results show that it is advantageous to employ quasi-optical methods in this range. Moreover, our results demonstrate also that propagation and polariton effects cannot be neglected in analyzing the spectra. The experiments have been performed on K₂CuF₄, a transparent quasi-two-dimensional ferromagnet.     

Temporal characteristics of picosecond continuum as revealed by a two-dimensional analysis of streak images

A two-dimensional analysis has been made for streak images of picosecond continua generated in D₂O, CCl₄, saturated aqueous solution of KDP, H₃PO₄ and quartz block. Their pulse width and distribution of arrival time at the streak camera were determined as a function of the wavelength.     

On the drastic temperature broadening of hard mode Raman lines of ferroelectric KDP type crystals near Tc

The drastic broadening of a few A₁ Raman lines of ferroelectric KH₂PO₄(KDP) and KH₂AsO₄(KDA) crystals have been measured at various temperatures near T_c. The results in KDP are in fair agreement with two different simplified expressions of linewidths, which are based on pseudospin-phonon interaction arguments. No such agreement is found between the results and a simplified linewidth expression derived from a theory based on soft mode-hard mode interaction arguments. The linewidths and the reciprocal of the proton spin-lattice relaxation time in KDP exhibit a very similar temperature dependence below T_c but behave differently above T_c. This phenomena is presented in detail, and discussed.     

KDP晶体固-液界面吸附行为的分子模拟研究

通过构建晶体表面-KDP分子界面吸附结构模型, 采用分子动力学和密度泛函计算方法研究KDP分子在(001)和(010)面吸附的物理化学过程, 考察了温度对物理吸附行为的影响. 研究表明: KDP晶体表面的吸附过程和生长习性主要由化学吸附主导, 化学吸附能的计算表明[K-O₈]基元在(001)界面的结合能是(010)界面结合能的2.86倍; 在饱和温度附近, [H₂PO₄]^-阴离子在KDP界面的物理结合能随温度的变化呈现振荡特征, 溶液中有较多的离子团簇形成, 溶液变得很不稳定; 当温度从323 K降低至308 K时, 水分子在界面的结合能总体呈下降趋势, 而KDP分子在界面的吸附能总体呈上升趋势, 脱水过程是水分子和[H₂PO₄]^-阴离子在固液界面边界层竞争吸附的结果. 研究结果对确足晶体生长界面动力学过程发展和完善晶体生长理论有重要意义. 关键词： 分子动力学 双层结构模型 结合能     

Structural studies of the monoclinic dihydrogen phosphates: A neutron-diffraction study of paraelectric CsH2PO4

The crystal structure of paraelectric CsH₂PO₄ at room temperature is presented. As suggested by the strong deuteration-dependence of the transition temperature and the distribution of quasielastic scattering, one of the hydrogen bonds is found to be very probably (97.5%) disordered, with H-H=0.48(4)Å. This work is set in the context of the interest now being shown in the group of monoclinic dihydrogen phosphates-deuterated KDP, NaDP, the ferroelectric CsDP and the presumed ferroelectric T1DP (DP denotes H₂PO₄). Recent work on these other compounds is surveyed briefly.     

Band-edge photoluminescence of PbI2

Photoluminescence spectra of the hexagonal layered semiconductor PbI₂ have been measured at 4.2°K on various single crystalline specimens. They are interpreted in relation with the anisotropic polariton model. The emitted radiation is assumed to originate from free and bound exciton recombination, with and without phonon emission. A peak polarized E ∥ c about 4 meV above the excitonic line is related to the extraordinary polariton mode.     

Hypersensitivity in the 4f–4f absorption spectra of tris (acetylacetonato) neodymium(III) complexes with imidazole and pyrazole in non-aqueous solutions. Effect of environment on hypersensitive transitions

The optical absorption spectra of [Nd(acac)₃(H₂O)₂]·H₂O, [Nd(acac)₃(im)₂] and [Nd(acac)₃(pz)₂] (where acac is the anion of acetylacetone, im is imidazole and pz is pyrazole) complexes in the visible region have been analyzed. The transition ⁴G_5/2←⁴I_9/2 located near the middle of the visible region (17,500 cm^?1) is hypersensitive. Its behavior is in sharp contrast to many other typically weak and consistently unvaried, normal 4f–4f transitions. It is overlapped by a less intense transition ²G_7/2←⁴I_9/2. The band shapes of the hypersensitive transition show remarkable changes on passing from aqueous solution to various non-aqueous solutions, which is the result of changes in the environment about the Nd(III) ion in the various solutions and suggests coordination of a solvent molecules. Pyridine has been found especially effective in promoting 4f–4f electric-dipole intensity. The DMSO invades the complexes and replaces the water molecules and heterocyclic amines from the coordination sphere. Two DMSO molecules coordinate and the complexes acquire similar structure, [Nd(acac)₃(DMSO)₂] in solution. The oscillator strength and the band shape of the hypersensitive transition of all the complexes remains the same in this solvent. The IR spectra and the NMR spectra of the complexes have also discussed.     

Anomalous wave interference at Z3-exciton resonance of CuCl

The transmission and the reflection spectra of a thin CuCl single crystal of 0.15μ thickness have been measured in the Z₃-exciton resonance region at 1.6K by using a weak dye-laser light as a light source. Well resolved interference fringes have been obtained over the exciton resonance. In the higher energy region than the longitudinal exciton's energy, the separation of adjacent fringes cannot be explained by interference of the lower branch polariton waves (LBP) or the upper branch polariton waves (UBP). These structures have been explained by the mutual interference effect between the UBP and LBP waves, anomalous waves. This has been confirmed by the measurements of two-photon absorption due to the excitonic molecule via respective polariton states.     

Linear and nonlinear optical properties of dye-doped KDP crystals: Effect of thermal treatment

Potassium dihydrogen phosphate crystals (KDP, KH₂PO₄) doped with the organic xylenol orange (XO) dye are grown, the XO concentration in the crystal matrix is about 10 ppm. The spectral and luminescent properties of nominally pure, dye-doped and dye-doped/annealed at 150 °C crystals (KDP, KDP:XO and KDP:XO_an) were measured. The annealing temperature effect on the degree of dye protonation in the crystal matrix is established. Analysis of the IR-absorption spectra reveals a strong interaction between the incorporated dye molecules and the hydrogen subsystem of the matrix. The nonlinear optical (NLO) properties of KDP, KDP:XO and KDP:XO_an crystals are studied within the self-action effect of picosecond laser pulses at 532 nm. The mechanism of photoinduced bleaching and the effects of laser beam self-focusing (in KDP) and self-defocusing (in KDP:XO and KDP:XO_an) are supposed to be due to resonance excitation of the subsystems of intrinsic defects and dye molecules, correspondingly. For KDP:XO_an it is shown that thermal annealing of intrinsic crystal defects leads to domination of more effective NLO response of the subsystem of dye molecules that is correlated with photoluminescence data.     

Synthesis, structure, and dielectric properties of KH2PO4 fractal aggregates

Macroscopic fractal aggregates of KH₂PH₄ (KDP) measuring up to 500 μm have been obtained. The fractal structure forms as a result of the precipitation of KDP particles from a supersaturated aqueous solution in the presence of a temperature gradient followed by a diffusioncontrolled mechanism of aggregation. The electron-microscopic analysis performed has shown that the fractals are formed predominantly from crystallites of the tetragonal modification measuring ∼1 μm. The dielectric constant (ɛ) of fractal KH₂PO₄ has been measured in the temperature range 80–300 K. A characteristic anomaly has been discovered on the ɛ(T) curve in the vicinity of 122 K, which attests to a ferroelectric phase transition. The absolute value of ɛ is significantly smaller than the components ɛ ₁₁ and ɛ ₃₃ for KH₂PO₄. Fiz. Tverd. Tela (St. Petersburg) 41, 2059–2061 (November 1999)     

Influence of crystal structure,ligand environment and morphology on Co L‐edge XAS spectral characteristics in cobalt compounds

The electronic structure of a material plays an important role in its functionality for different applications which can be probed using synchrotron‐based spectroscopy techniques. Here, various cobalt‐based compounds, differing in crystal structure, ligands surrounding the central metal ion and morphology, have been studied by soft X‐ray absorption spectroscopy (XAS) at the Co L‐edge in order to measure the effect of these parameters on the electronic structure. A careful qualitative analysis of the spectral branching ratio and relative intensities of the L₃ and L₂ peaks provide useful insight into the electronic properties of compounds such as CoO/Co(OH)₂, CoCl₂.6H₂O/CoF₂.4H₂O, CoCl₂/CoF₂, Co₃O₄ (bulk/nano/micro). For further detailed analysis of the XAS spectra, quantitative analysis has been performed by fitting the spectral profile with simulated spectra for a number of cobalt compounds using crystal field atomic multiplet calculations.