Stark spectroscopy of Langmuir-Blodgett monolayers and multilayers

The electrical (capacity and conductivity) and electro-optical (Stark spectra) properties were investigated for monolayers of an anthraquinone dye. The polar structure of the monolayers results in the appearance of the linear Stark effect with a field-induced change δT in the optical transmission proportional to the difference δμ of the dipole moments for an excited and the ground molecular states. Multilayers were shown to be non-polar owing to the effects of molecular association. In the latter case only the quadratic Stark effect is observed with δT proportional to the difference δ of the polarizabilities of an associated excited state and the ground state. It was also shown that the electric field distribution across an ultrathin Langmuir-Blodgett film is extremely non-uniform.     

Heterogeneity in mixed-valence manganites

Recent studies focus on the static and dynamic character of microscale electronic phase segregation in manganese perovskites, along with associated magnetic polaron effects. Interest in the magnetoresistance of manganese perovskites has shifted from the colossal magnetoresistance observed near the Curie temperature to low-field magnetoresistance, a micromagnetic phenomenon found in many sorts of granular material.     

Fluorescence and infrared spectroscopy of electrochemically self assembled ZnO nanowires: evidence of the quantum confined Stark effect

We report room temperature fluorescence (FL) and infrared absorption (IR) spectra of spatially ordered two-dimensional arrays of vertically standing ZnO nanowires. The wires are produced by selective electrodeposition of Zn in 10-, 25- and 50-nm pores of a porous anodic alumina film, followed by chemical oxidation. Wires of different diameters show distinctly different FL emission characteristics associated with either deep level traps, or exciton recombination. The intensity of the peak caused by exciton recombination is larger than that caused by deep level traps, which is unusual in nanostructures, and attests to the high structural purity. We also see an anomalous red-shift in the FL emission spectrum which appears to be evidence of quantum confined Stark shift caused by built-in electric fields in the alumina template. The IR absorption spectra are mostly featureless and show no significant peaks indicating the absence of shallow level traps.     

Millikelvin studies of mixed-valence HgSe:Fe

Resistivity and magnetic susceptibility measurements of HgSe:Fe have been performed down to 30 mK and in the magnetic field range 50 mOe-3 kOe. Previously observed high conductivities of HgSe:Fe in the mixed valence region of Fe donors have been found to remain unchanged down to the lowest measured temperatures, indicating a minor influence of the resonant scattering and Kondo effect. This points to a weak k-d hybridization and substantiates classification of HgSe:Fe as an inhomogeneous mixed-valence system in which intersite Coulomb interactions impose spatial correlations in positions of charged centers. A resistive and inductive indication of superconductivity has been detected in some samples. The phenomenon is assigned to the presence of Hg-related superconducting inclusions and the associated proximity effect. This interpretation explains the field and temperature dependence of the resistivity and magnetic susceptibility, as well as being consistent with a sensitivity of the effect on deviations from stoichiometry and on annealing in Se vapor.     

ESCA studies of some mixed-valence rare-earth intermetallics

ESCA (or XPS) studies of some rare-earth intermetallics were carried out using ESCA (3) Mark II and ‘ESCALAB’ machines from V G Scientific. The mixed valence nature of EuCu₂Si₂ and EuNi₂P₂ intermetallic compounds has been established. The 3d core-level photoemission spectra of Eu in EuCu₂Si₂ and EuNi₂P₂ exhibit two sets of structures corresponding to two different valence states of the rare-earth ions. Each of these sets shows simple spin-orbit splitting. The replicate splitting measured by us agrees well with those calculated by Herbst and Wilkins. The 3d core-level photoemission spectra of Eu is free from the multiplet splitting and satellite structure. The importance of 3d core-level photoemission studies in the understanding of valence fluctuation phenomenon has been emphasized.     

Dynamic mechanical spectroscopy of plastic crystalline states in n-alkane systems

Dynamic mechanical spectroscopy (DMS) and differential scanning calorimetry (DSC) were used to study the plastic crystalline region of several model n-alkane systems and a commercial paraffin wax. Results indicate that DMS provides a valuable complement to existing tools for locating plastic crystalline or rotator states and characterizing the pre-melting region containing these mesophases. DMS measurements demonstrate that the mechanical properties of plastic crystalline states in n-alkanes are difficult to isolate, and that observed viscoelasticity surrounding the mesophases is in great part due to transitions between phases, not the rotator phases themselves. This is indicated from DMS curves that show the dynamic moduli of rotator phases are inhibited from achieving equilibrium values due to the close proximity of successive transitions. Furthermore, study of a system in which stability of a single rotator phase has been extended through strategic blending of n-alkanes shows that dynamic mechanical properties return to near pre-mesophase values when sufficient opportunity following a transition is provided. These results demonstrate that many n-alkane blends can possess a short temperature interval over which the material oscillates between Hookean and viscous behavior, controlling their performance and possibly providing for new applications requiring changes in viscoelastic properties in a narrow temperature span.     

碳原子跃迁的Stark加宽

运用直流壁稳弧等离子体辐射,测量了碳原子３ｓ－３ｐ 和３ｐ－３ｄ 跃迁的多重谱线的Ｓｔａｒｋ 加宽参数,并与Ｇｒｉｅｍ 的理论计算值进行了比较。实验所使用的等离子体,经诊断,其电子密度为７．５～８．６×１０１６ｃｍ － ３,温度为１２２１０～１２３８０Ｋ。     

Fabrication of heterostructures of halogen-bridged quasi-one-dimensional mixed-valence transition metal complexes

Single-heterostructure crystals of a series of halogen-bridged quasi-one- dimensional mixed-valence transition metal complexes (HMMCs) were fabricated by selective coordination epitaxial growth (SCE growth). The overgrowth crystals of the HMMC were grown on the columnar crystals of the substrate HMMC in solution. The single-heterostructure crystals were epitaxially grown only when the two HMMCs had similar crystal structures. If their crystal structures were different, no heterostructure crystal was grown. This result suggests that the degree of mismatch between the crystal structures of the substrate and the overgrowth is the predominant factor which determines the SCE growth of the HMMCs.     

Photon correlation spectroscopy on flowing polydisperse fluid-particle systems: theory

We develop an analytical expression for the homodyne autocorrelation function of laser light scattered by a laminar flow of a polydisperse particle-fluid system. In contrast to the already existing literature on the development of autocorrelation functions, we explicitly begin with the effects of the finite linewidth of the light source, the spatial and temporal intensity averaging that is due to the detection process, the Brownian particle movement on the amplitudes of the scattered light waves as well as on the degree of resolution that we introduce in this paper, and a general system velocity v = (v(x), v(y), v(z)). One main result is a new physical interpretation of the well-known, generally empirically introduced coherence factor. Quantities that are comparable to the well-known degree of coherence, coherence area, and number of coherence areas have also been obtained. Finally the investigations are simplified to an autocorrelation function that can be used for the analysis of fluid-particle systems in the low Knudsen number regime. It is shown that in this case particle size or size distribution, system velocity, and particle concentration can be obtained simultaneously. The developed autocorrelation function is related to frequently analyzed special cases and compared with expressions from the literature.     

Dynamics of the Optical Stark Effect in Semiconductors

We present in this paper a comprehensive study of the optical Stark effect of excitons in semiconductors. After a review of our experimental results, and of some theoretical interpretations, we develop a new treatment of the optical Stark effect in the framework of nonlinear optics. This allows one to take into account transient phenomena such as spectral and temporal oscillations and leads to a proper interpretation of the very surprising dynamics experimentally observed.     

Stark level analysis for Er-doped ZBLAN glass

From the absorption and emission spectra recorded in a conventional way at 13 K, the Stark levels in manifolds from ⁴I_15/2 to ²H_9/2 for Er³⁺-doped ZBLAN (ZrF₄–BaF₂–LaF₃–AlF₃–NaF) glass have been investigated by means of the effective crystal field model previously proposed to describe the average of local symmetries occupied by rare earth ions in oxide glasses [1]. This model, based on the principle of descending symmetry, used the symmetry group chain scheme. Crystal field strength, homogeneous and inhomogeneous widths of levels have been determined, and the former models and results about optical properties for rare earth ions in heavy metal fluoride glasses are discussed. In fluoride glasses, the rare earth ions appear more evenly distributed than in oxide glasses, and the crystal field strength is weaker than in oxide glasses.     

A comparison of different neutron spectroscopy systems at the reactor facility VENUS

The VENUS facility is a zero-power research reactor mainly devoted to studies on LWR fuels. Localised high-neutron rates were found around the reactor, with a neutron/gamma dose equivalent rate ratio as high as three. Therefore, a study of the neutron dosimetry around the reactor was started some years ago. During this study, several methods of neutron spectroscopy were employed and a study of individual and ambient dosemeters was performed.A first spectrometric measurement was done with the IPSN multisphere spectrometer in three positions around the reactor. Secondly, the ROSPEC spectrometer from the Fraunhofer Institut was used. The spectra were also measured with the bubble interactive neutron spectrometer. These measurements were compared with a numerical simulation of the neutron field made with the code TRIPOLI-3. Dosimetric measurements were made with three types of personal neutron dosemeters: an albedo type, a track etch detector and a bubble detector.     

Comparative study of laser-induced breakdown spectroscopy measurement using two slurry circulation systems

The experimental conditions associated with slurry measurements to achieve good precision by using laser-induced breakdown spectroscopy (LIBS) are examined. LIBS analysis was applied to a special waste slurry sample that contains 85.4% water, 2.5% ferric oxide Fe(2)O(3), 1.7% alumina Al(2)O(3), and small quantities of oxides of boron and chromium. While liquids add challenge to LIBS measurements, the analysis was successfully performed on iron and aluminum. Two slurry circulation systems were devised to overcome the major technical problems associated with LIBS measurements of slurry samples, namely, sedimentation and change in the lens-to-sample distance during measurement. LIBS slurry measurements using both circulation systems are compared. The results show that the experimental configuration plays a crucial role for online slurry analysis.     

Stark effect in semiconductor nanocrystals: tight-binding approach

The influence of an external uniform electric field on the electron energy structure and charge densities of semiconductor nanocrystals is investigated. We study both uniform and multilayer nanocrystals as well as artificial molecules, i.e. systems built of two coupled quantum dots. We work in the framework of an empirical tight-binding theory, that allows the study of effects not accessible within the effective mass approach. The electric field can destroy the delocalized character of bonding-like and antibonding-like states of artificial molecules but can also lead to the formation of other field-tunable states.     

The quantum confined Stark effect in silicon nanocrystals

The quantum confined Stark effect (QCSE) in Si nanocrystals embedded in a SiO(2) matrix is demonstrated by photoluminescence (PL) spectroscopy at room and cryogenic temperatures. It is shown that the PL peak position shifts to higher wavelengths with increasing applied electric field, which is expected from carrier polarization within the quantum dots. It is observed that the effect is more pronounced at lower temperatures due to the improved carrier localization at the lowest energy states of the quantum dots. Experimental results are shown to be in good agreement with phenomenological model developed for the QCSE model.     

Diffuse reflectance spectroscopy of particulate systems: a comparative study of discrete models

A comparative study of statistical particle model theory of diffuse reflectance has been made. Theories have been applied to six particulate samples having different optical characteristics and average particle sizes that vary from 42 to 106 mum. We made an overall assessment of each theoretical model by determining the CIELAB color difference using experimentally measured and theoretically predicted diffuse reflectance spectra in the visible spectral range. We discuss the performance ratings of the models of other researchers and discovered numerous typographical errors in Fresnel reflection coefficient expressions. We provide the correct versions for these expressions.     

Solitons, polarons and their dynamics in mixed-valence halogen-bridged MX-chains

This article describes the photo-generation processes of elementary excitations such as solitons and polarons, and their dynamics in the one-dimensional (1D) halogen-bridged Pt compound [Pt(en)2Br](ClO4)2. Spin-solitons were photo-generated via relaxation processes of CT excitons and self-trapped excitons, made evident by photo-induced absorption and photo-induced electron spin resonance spectra. Polarons were not generated from CT excitons. Diffusion of spin-solitons on the 1D chain was studied quantitatively by analysing 1H NMR spin-lattice relaxation times (T1).