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.     

Optical absorption spectra of linear and cyclic thiophenes—selection rules manifestation

We theoretically study the size-dependent relation between absorption spectra of thiophene-based oligomers and the corresponding cyclothiophenes. In our approach based on a Frenkel exciton Hamiltonian, we demonstrate that the geometry and selection rules determine the observed relations between the spectra.     

Assessment of the coherent potential approximation for the absorption spectra of a one-dimensional Frenkel exciton system with a Gaussian distribution of fluctuations in the optical transition frequency

We investigate the accuracy of the coherent potential approximation (CPA) for the optical absorption spectra of a one-dimensional Frenkel exciton system with nearest-neighbor interactions and a Gaussian distribution of fluctuations in the optical transition frequency (diagonal Gaussian disorder). Our earlier studies have established that the CPA gives highly accurate results for the integral of the density of states of this system. In this paper we compare the CPA results for the normalized optical absorption with the finite-array calculations of Schreiber and Toyozawa and Schreiber for the same model. It is found that the CPA results for the absorption are in good agreement with their findings. It is pointed out that an expansion of the density of states in terms of the eigenstates of the ideal (no disorder) array contains a term proportional to the normalized absorption. Since the density of states is accurately approximated by the CPA, the presence of this term explains the success of the CPA in reproducing the absorption spectra. Our findings support the use of the Gaussian disorder model to interpret the absorption spectra of one and quasi-one dimensional exciton systems.     

Kramers-Kronig analysis of the reflectance spectra of Pb-doped Bi-4334 type glass and the corresponding glass-ceramic superconductor

The room temperature reflectance spectra in UV-VIS-NIR region (energy range of 0.6 to 6.2 eV) for glassy, partially crystalline and its fully crystalline superconducting ceramic phases of Bi_3.9Pb_0.1Sr₃Ca₃Cu₄O _x have been studied by Kramers-Kronig (KK) analysis. A comparative study of the energy loss function [− Im (1/ε)] and the absorption coefficient [α(E)] has been done. Excitions in the superconducting phase hitherto evidenced by the authors are located in the polarizable layers of the superconducting cuprate and their implications for superconductivity have been pointed out. An estimate of the optical band gap energy (E _g) has also been made from the linear fit ofα ² vs.E curve for the superconducting phase. Jezierski’s method ofR-extrapolations in the higher energy has been used to show that both methods yield results that agree quantitatively and can be relied upon.     

Dielectric tensor and optical spectra of quaterthiophene crystals

We present an approach to the microscopic calculation of the dielectric tensor near the resonances of mechanical Frenkel excitons in a vibronic coupling regime with one effective mode. The model, applied to the prototypical quaterthiophene crystal, is based on a discrete distribution of charges for the evaluation of the molecular transition moment, whilst the coulombic interactions responsible for the exciton delocalization in the crystal are calculated by the Ewald method. From the calculated tensor we simulate absorbance and reflectance spectra and by a direct comparison with the experimental ones, we identify the origin of some shoulders present in the spectra.     

叶黄素聚集体吸收光谱的实验分析与理论模拟

分子聚集体表现出单分子所不具备的特有功能,利用吸收光谱对聚集体分子结构特性的研究是理解其电子和能量转移功能的基础。实验中利用紫外-可见分光光度计,检测了叶黄素在乙醇溶液中的单体吸收谱和在1∶1乙醇水溶液中的聚集体吸收谱。并通过对叶黄素单体吸收谱的高斯分解,获得了激发态的振动能级结构参数。理论上采用时间相关函数描述的吸收谱线和Frenkel激子模型,通过模拟单分子吸收光谱,获得了叶黄素分子的激发能、特征模振动频率、Huang-Phys因子等参数。再利用这些参数进行了聚集体分子光谱的模拟,分析了叶黄素聚集体的分子结构影响光谱变化的原因。结果表明,(1) 分子间的相互作用是决定吸收光谱峰位移动的主要原因,实验中叶黄素H-聚集体的吸收光谱较单体蓝移了77 nm,模拟显示相互作用在2 000 cm-1附近;(2) 聚集体分子个数越多,聚集协作效应越大,吸收光谱半高宽变小,同时吸收峰进一步蓝移;(3) 环境的无序度对吸收光谱的半高宽也存在较大的影响,无序度越大,吸收光谱半高宽越大。实验结果为进一步研究聚集体在生物系统和材料系统中的功能提供了理论依据。     

Stray-light induced artefacts in absorption spectra of crystalline oligothiophenes

The absorption spectra of oligothiophene single crystals and thin films are discussed in terms of stray light induced experimental artefacts. After typical experimental problems have been overcome, absorption spectra can be collected over a wide spectral range in the UV-Vis region that has often not been included in the literature data. A clear similarity is found between the absorption spectra of single crystals and highly ordered thin films. Received 10 April 2002 Published online 13 August 2002     

Photoluminescence spectra of rare earth doped CaGa2S4 single crystals

Six kind CaGa₂S₄ single crystals doped with different rare earth (RE) elements are grown by the horizontal Bridgman method, and their photoluminescence (PL) spectra are measured in the temperature range from 10 to 300 K. The PL spectra of Ce or Eu doped crystals have broad line shapes due to the phonon assisted 4f-5d transitions. On the other hand, those of Pr³⁺, Tb³⁺, Er³⁺ or Tm³⁺ doped samples show narrow ones owing to the 4f-4f transitions. The assignments of the electronic levels are made in reference to the reported data of RE 4f multiplets observed in same materials.     

Theoretical study of X-ray photoelectron spectra of mono- and polycrystalline samples

Recently Sixl and Sanwald¹ reported on measurements of fluctuation conductivity in very thin superconducting Al films. We reinterpret their data and find good agreement with the Aslamazov-Larkin and Maki-Thompson theories with a thickness dependent pairbreaking parameter.     

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.     

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.     

Localized self-trapping in the two-dimensional discrete molecular lattice with the interaction between Frenkel excitons and phonons

We investigate the interactions of lattice phonons with Frenkel exciton, which has a small radius in a two-dimensional discrete molecular lattice, by the virtue of the quasi-discreteness approximation and the method of multiple-scale, and obtain that the self-trapping can also appear in the two-dimensional discrete molecular lattice with harmonic and nonlinear potential. The excitons' effect on the molecular lattice does not distort it but only causes it to localize which enables it to react again through phonon coupling to trap the energy and prevent its dispersion.     

Normal- and oblique-incidence reflection of semi-infinite crystals: Adiabatic approach with excitonic effects

Summary Excitonic reflection spectra of semi-infinite crystals are calculated within Stahl's coherent wave approach to band gap dynamics. Stahl's constitutive equations are solved by an adiabatic approximation in a surface region of the thickness z₀ and exactly in the bulk. The depth z₀ determines a region where the repulsive surface potential plays a decisive part for the motion of electrons and holes. The method yields simple analytical expressions for the reflectivity and for the bulk polariton amplitudes both for the normal and for the oblique incidence. Calculated reflectivity curves are compared with experimental data for various semiconductors and various angles of incidence showing a fairly good agreement.

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Riassunto Si calcolano gli spettri di riflessione eccitonica di cristalli semi infiniti entro l'approccio d'onda coerente di Stahl alla dinamica dell'intervallo di banda. Le equazioni costitutive di Stahl si risolvono con un'approssimazione adiabatica in una regione di superficie di spessore z₀ ed esattamente nel volume. La profondità z₀ determina una regione in cui il potenziale di superficie repulsivo ha un ruolo decisivo per il moto degli electtroni e delle lacune. Il metodo rende semplici espressioni analitiche per la riflettività e per le ampiezze del polaritone di volume sia per l'incidenza normale che per quella obliqua. Si confrontano le curve di riflettività calcolate con i dati sperimentali per vari semiconduttori e vari angoli d'incidenza mostrando un accordo abbastanza buono.

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Резюме Вычисляются экситонные спектры отражения для полубесконечных кристаллов в рамках подхода когерентных волн Сталя к динамике ширины запрещенной зоны. Решаются конститутивные уравнения Сталя с помощью адиаватического приближения в поверхностной области для толщины z₀ и точно в объеме. Глубина z₀ определяет область, где поверхностный потенциал отталкивания играет суцественную роль для движения злектронов и дырок. Предложенный метод дает простые аналитические выражения для овоих случаев нормального и наклонного падения. Вычисленные кривые отражательной срособности сравниваются с экспериментальми данными для раэличных полупроводников и различнях углов падения. Обнаружено довольно хорошее соответствие.

     

Investigation of doping in Si crystals by means of electroreflectance

Summary In this work the importance of electroreflectance is evidenced in two specific applications concerning studies of semiconductor surfaces: i) determination of junction depths and carrier profiles, ii) mapping of inhomogeneity curves of the carrier distribution on Si surfaces. New information in both cases is obtained and briefly discussed. Work partially supported by Progetto Finalizzato Energetica (Sottoprogetto Energia Solare) of consiglio Nazionale delle Ricerche.     

Absorption and propagation of light in quaterthiophene crystals

We examine absorption of light in quaterthiophene crystals for different directions of the ray and wave-normal vector after refraction on the vacuum/crystal interface. Experimental non-normal incidence spectra are reported where different absorption bands show different behaviour as a function of the angle of incidence. This allows to identify those originating from the b_u Frenkel exciton state.     

Evidence of polarized charge-transfer transitions by probing the weak dielectric tensor components of oligothiophene crystals

The absorption spectra of different oligothiophene single crystals without saturation effects in a wide spectral range are reported and compared. The origin of the main ac polarized broad band is discussed in terms of Frenkel excitons. Exploiting experimental configurations which correspond to the excitation of the weak dielectric tensor components, bands of different nature are also detected at different positions for orthogonal polarizations and attributed to transitions of charge-transfer character.     

NIR-VIS reflectivity spectra of some transition metal thiophosphates

Summary Room temperature optical-reflectivity measurements on some transition metal thiophosphates were carried out in the near infrared and visible regions. The resulting spectra, interpreted on the basis of the ?transition metal weakly interacting? model, agree well with earlier optical transmission measurements. Below the fundamental absorption edge, the observed features have been assigned to 3d–3d transitions occurring on the transition metal ion, while those observed at photon energies greater than the absorption threshold have been attributed to transitions from the valence bands to discrete 3d orbital levels or to the conduction bands. A more detailed information on the metal ion 3d levels energy distribution with respect to the valence band states belonging to the (P₂S₆)^4- cluster and a more precise determination of the MPS₃ absorption edge energy position have been obtained.     

Optical spectra and energy levels of the Cr ions in MWO4 (M=Mg, Zn, Cd) and MgMoO4 crystals

Single crystals of MWO₄ (M=Mg, Zn, Cd) and MgMoO₄ doped with Cr³⁺ have been grown by the flux growth method. Their optical spectra have been systematically measured and assigned on the basis of the classical Ligand Field Theory. The exchange charge model of the crystal field has then been applied to calculate the crystal field parameters (CFPs) and the energy levels of the Cr³⁺ ion in all studied crystals. These are in reasonable agreement with the experimental data. Systematic trends in the CFPs values, crystal field splittings and Racah parameters have been evidenced and their relation with sizes and symmetry properties of the host cavities occupied by Cr³⁺ has been pointed out.     

Effect of surface roughness on the optical constants of bulk polycrystalline gold samples

Four bulk polycrystalline samples of gold were subjected to different polishing treatments using diamond pastes of grain size 10, 6, 3 and 1 μm. The effect of surface roughness on the optical constants n and k is studied by 45° angle-of-incidence ellipsometry at 632.8 nm. Results for n and k are extrapolated to the case of an ideal surface which we believe to be highly representative of gold. Comparison with published results for the optical constants of gold thin films is presented.