Evidence of orbital reconstruction at interfaces in ultrathin La0.67Sr0.33MnO3 films

Linear dichroism (LD) in x-ray absorption, diffraction, transport, and magnetization measurements on thin La(0.7)Sr(0.3)MnO(3) films grown on different substrates, allow identification of a peculiar interface effect, related just to the presence of the interface. We report the LD signature of preferential 3d-e(g)(3z(2)-r(2)) occupation at the interface, suppressing the double exchange mechanism. This surface orbital reconstruction is opposite to that favored by residual strain and is independent of dipolar fields, the chemical nature of the substrate and the presence of capping layers.     

Torsion induced circular dichroism in hexagonal CdS

The influence of the torsion on the absorption properties of hexagonal CdS has been studied in our experiments. The undeformed CdS does not possess circular dichroism (CD) but it manifests linear dichroism (LD). An appearance of CD and a change of LD can be observed in the torsion deformed CdS crystals. The mentioned absorption phenomena occur in the vicinity of the isotropic point (i.p.), the crossing point of the curves of the refractive indicesn ₀() andn _e (). The sign of CD is connected with the direction of the torsional momentM. The magnitude ofM influences the shape of CD spectra as well as the values of LD. The appearance of CD and the change of LD are explained by the torsion induced gyrotropy in the CdS crystals, i.e. by the created chirality in the hexagonal CdS.     

Linear Dichroism of Chloroplasts and Subchloroplast Fractions Oriented by Flow

Abstract

Chloroplasts as well as subchloroplast vesicles show linear dichroism (LD) when subjected to a hydrodynamic gradient. LD is positive for the ππ? in-plane polarized bands indicating that the chlorophyll porphyrin ring is in average oriented parallel1 to the elongated membrane surfaces. The vesicle fraction enriched inphotosystem I1 shows lower LD/A at 660 nm indicating lower average orientation of chlorophyll and the fraction enriched in photosystem I shows higher LD/A at 700 nm indicating that P700 is more oriented than the bulk chlorophyll.     

On the response of infrared absorption spectra of polymers to large inelastic deformations

It is well known that the refractive index of a material and its coefficient of absorption are coupled quantities. Within the theory of light propagation they appear as a single complex eigenvalue. Thus, birefringence and in particular induced birefringence as the physical base of stress optics, have their counterpart in the absorption spectrum. This phenomenon is called dichroism or induced dichroism. Dichroism is recording absorption spectra using light with two different planes of polarisation (generally, the orientation of the planes is perpendicular to each other) resulting in two different absorption spectra of the same specimen at the same amount of stretch. It can be shown that the dependence of absorption spectra on the orientation of the polarisation plane of the light comes out of two roots. A (small) detuning off the resonance frequency of the molecular oscillators of the material and a (dramatic) change of absorption, the latter being a consequence of change of orientation of the molecular polarisation vectors. Both, the detuning and the orientation effects, can be related to the macrodeformation of the specimen. Some possible applications of these effects in experimental mechanic are discussed.     

Picosecond coherence coupling in the pump and probe technique

A general expression for the coherent artifact in photoinduced absorption measured by coherent polarized pump and probe beams is derived. The conditions for its reduction in isotropic media by using cross-polarized beams is discussed. Examples of experimental data illustrating the conclusions of the theory are shown.     

Laser-Diode Phase-Shifting Joint-Transform Correlator with Multiple-Object Recognition

A joint-transform phase correlation is made by an extracted phase from the joint power spectrum using a phase-shifting Mach-Zehnder interferometer with a wavelength-shifted laser diode (LD), and then by numerically Fourier-transforming a measured phase. The algorithm in six steps is insensitive to the power changes in an LD. The phase correlator gives the best discrimination performance. Discriminative multiple-object recognition is performed with no intermodulation noise and artifact noise-free correlation by arranging the multiple objects in a regularly equal-spaced array. The experimental and numerical results are shown.     

k-space inherited parallel acquisition (KIPA): application on dynamic magnetic resonance imaging thermometry

In this study, a novel method for dynamic parallel image acquisition and reconstruction is presented. In this method, called k-space inherited parallel acquisition (KIPA), localized reconstruction coefficients are used to achieve higher reduction factors, and lower noise and artifact levels compared to that of generalized autocalibrating partially parallel acquisition (GRAPPA) reconstruction. In KIPA, the full k-space for the first frame and the partial k-space for later frames are required to reconstruct a whole series of images. Reconstruction coefficients calculated for different segments of k-space from the first frame data set are used to estimate missing k-space lines in corresponding k-space segments of other frames. The local determination of KIPA reconstruction coefficients is essential to adjusting them according to the local signal-to-noise ratio characteristics of k-space data. The proposed algorithm is applicable to dynamic imaging with arbitrary k-space sampling trajectories. Simulations of magnetic resonance thermometry using the KIPA method with a reduction factor of 6 and using dynamic imaging studies of human subjects with reduction factors of 4 and 6 have been performed to prove the feasibility of our method and to show apparent improvement in image quality in comparison with GRAPPA for dynamic imaging.     

Absorption in the fractional quantum Hall regime: trion dichroism and spin polarization

We present measurements of optical interband absorption in the fractional quantum Hall regime in a GaAs quantum well in the range 0相似文献   

The action of the electric field on absorption spectra of solutions: Electrochromism spectra,their properties,and operations with them

Theoretical foundations of molecular absorption spectroscopy of solutions in external electric fields are considered. The action of the field varies the energies of the ground and the excited states of a molecule participating in a vibronic transition, which induces specific deformations in the spectra, which are called the electrochromism spectra. The dichroism of solutions appearing in the field also belongs to them. One can determine from electrochromism the spectra parameters of molecules in the ground and excited electronic states, study slow thermal mobility in solutions and the direction of displacement of the electron density of reacting molecules, and determine their other properties, which are difficult to measure by usual methods. An interpretation of electrochromism based on clear-cut physical principles is proposed. Electrochromism is considered as a natural branch of absorption molecular spectroscopy.     

光诱导辣根过氧化物酶催化活性变化及机理研究

辣根过氧化物酶(HRP)是广泛应用的一种生物催化剂,其光照后HRP酶活性变化的机理还未见报道。利用紫外-可见(UV-Vis)、同步荧光、圆二色(CD)光谱研究了光对HRP催化活性及酶活变化机制。实验结果表明：光照射HRP可促使其发生还原反应,同时其催化活性会发生变化。UV-Vis吸收光谱数据显示,光照射后HRP的Soret带403 nm最大吸收峰红移、Q带498 nm处氧化峰强度下降,说明经光照射HRPFe(Ⅲ)被还原为HRPFe(Ⅱ)。不同波长对光照还原影响程度为280 nm>254 nm>498 nm>403 nm;酶活为403 nm>498 nm>254 nm>280 nm;280 nm波长光照射下,Phe,Trp,Tyr和Cys四种游离氨基酸以及谷胱甘肽的加入均对光照还原有促进作用。酶活性与光照还原程度紧密相关,因Fe(Ⅱ)无法提供空轨道与H₂O₂配位,所以光诱导Fe(Ⅲ)还原导致酶活下降。同步荧光和圆二色光谱揭示了光照还原后HRP血红素的构象发生了变化,构象的变化也是光诱导酶活变化的原因之一。紫外光影响酶活性下降的因素为光照引发活性中心Fe(III)还原,蛋白质构象变化的贡献。研究结果对进一步了解光对含血红素蛋白(酶)结构和功能的影响有重要的理论和实际意义。     

Use of UV-Vis Spectrometry to Gain Information on the Mode of Binding of Small Molecules to DNAs and RNAs

Abstract: Ultraviolet-visible (UV-Vis) spectrometries (absorbance, linear dichroism [LD], and circular dichroism [CD]) are relatively simple techniques that can provide important information on the mode of binding of small molecules (dyes, drugs) to DNA and RNA helices. In this article the spectral characteristics of three families of dyes upon interaction with polynucleotides are reviewed. Each of them is representative for one of the major binding modes: acridines for intercalation, Hoechst dyes for groove binding, and porphyrins for external dye stacking. The purpose is to help the reader who is not accustomed to using these systems to gain a simple picture of what the predominant UV-Vis features of each binding mode are. These data are discussed and tables are provided to collect and compare the absorbance, LD, and CD spectral features of several examples of the three dye families chosen.     

Calculation of the intensity and dichroism of the IR absorption bands of oriented polymers

An algorithm for calculating the intensity and dichroism of polarized IR absorption bands of oriented polymers is proposed. The absorption curves in the IR spectrum of amorphous cis-1,4-polyisoprene and the dichroism of its absorption bands have been calculated. The polarization IR spectra of oriented amorphous cis-1,4-polyisoprene for a stretching ratio equal to 3 have been constructed. The calculated values of the absorption-band dichroism are in fairly good agreement with the experimental data.__________Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 1, pp. 129–132, January–February, 2005.     

Linear optical anisotropy and domain structures at two successive antiferromagnetic phase transitions in α-MnS

Two successive antiferromagnetic phase transitions of α-MnS are confirmed to occur at T_C1 = 152.7 K and T_C2 = 129.5 K, respectively, by microscopic observations of T domain formation and rearrangement, linear birefringence (LB), optical absorption and linear dichroism (LD) measurements. Stress experiments and X-ray data reveal a rhombohedral lattice contraction below T_C1, which switches into a trigonal elongation below T_C2. The second-order transition at T_C1 (β = 0.40) is proposed to yield the familiar fcc type-2 single axis spin order of NiO and MnO, whereas a multispin axis structure seems to be achieved via a first-order transition below T_C2. Model calculations of the exchange striction essentially agree with the observed lattice distortions, whereas neutron diffraction data are less conclusive with respect to the proposed spin structure transformation. The interdependence of LB and LD is shown via dispersion relations and a Kramers-Kronig analysis involving the d-d transitions of Mn²⁺ in the visible region.     

Analysis of the halo background in femtosecond slicing experiments

The slicing facility FemtoSpeX at BESSY II offers unique opportunities to study photo‐induced dynamics on femtosecond time scales by means of X‐ray magnetic circular dichroism, resonant and non‐resonant X‐ray diffraction, and X‐ray absorption spectroscopy experiments in the soft X‐ray regime. Besides femtosecond X‐ray pulses, slicing sources inherently also produce a so‐called `halo' background with a different time structure, polarization and pointing. Here a detailed experimental characterization of the halo radiation is presented, and a method is demonstrated for its correct and unambiguous removal from femtosecond time‐resolved data using a special laser triggering scheme as well as analytical models. Examples are given for time‐resolved measurements with corresponding halo correction, and errors of the relevant physical quantities caused by either neglecting or by applying a simplified model to describe this background are estimated.     

Theoretical Study into the Dichroism of the Absorption Bands in the IR Spectra of Oriented Polymers

The IR dichroism of the polarized absorption bands of oriented polymers has been considered. An equation has been obtained that makes it possible to calculate the dichroism of the absorption bands for a different degree of the polymer orientation. This equation has been simplified especially for structurally insensitive absorption bands. In this case it allows one to determine the type of polarization of absorption bands.     

A picosecond time-resolved vibrational circular dichroism spectrometer

We present an experimental setup to detect transient vibrational circular dichroism signals. A femtosecond laser system is synchronized to a photoelastic modulator to produce alternating left- and right-handed circularly polarized mid-IR pulses at 1 kHz repetition rate. Transient changes in the circular dichroism of the CH-stretch vibrations of a cobalt-sparteine complex were probed in a proof-of-principle experiment and are clearly distinct from conventional transient absorption changes.     

LD波长啁啾效应与二极管抽运固体激光器性能优化

给出了光谱、时间、空间分辨的激光二极管(LD)抽运动力学模型，包括LD阵列输出的波长啁啾效应、输出功率随时间的瞬态变化，重点研究了不同吸收带宽的激光介质对大功率LD阵列输出的光谱选择吸收特性. 基于该模型，分析了三种典型激光增益介质Yb:YAG，Yb:S-FAP，Nd:YAG在脉冲储能运转方式下的抽运激发性能. 优化了不同增益介质所需的LD阵列的初始中心波长，并对三种介质对LD波长啁啾效应的敏感性进行了详细研究. 关键词： 激光二极管阵列 波长啁啾 抽运动力学 激光材料     

Yb3+离子掺杂钨酸钾钆晶体的光谱参数

从Yb³⁺:KGd(WO₄)₂(KGW)的吸收谱和荧光谱出发,利用倒易理论计算其各个光谱参量,如吸收截面,受激发射截面,辐射寿命等.由于Yb³⁺:KGW晶体较适合用作二极管泵浦的激光器的材料,而激光二极管所发出的光是偏振光,晶体又是双折射晶体,因此研究了泵浦光的不同通光方向和偏振方向对Yb³⁺:KGW晶体荧光光谱的影响.实验表明在不同情况下其对晶体荧光强度影响较大,因此在设计用激光二极管泵浦的激光器时,应考虑合适的泵浦光偏振方向和入射在晶体上的轴向,其中泵浦光的通光方向比泵浦光偏振方向对晶体的荧光强度的影响更大.对按晶轴切割的这两种晶体,相同实验条件下,荧光强度I_c>I_b>I_a.对于按折射率主轴切割的来说,荧光强度I_Np>I_Ng>I_Nm.在通光方向为c轴,偏振方向平行b轴时得到了最大的荧光强度.     

Enhanced orbital magnetism in Fe(50)Pt(50) nanoparticles

X-ray absorption and magnetic circular dichroism spectra at both the Fe and Pt L(3,2) edges were measured on wet-chemically synthesized monodisperse Fe(50)Pt(50) particles with a mean diameter of 6.3 nm before and after complete removal of the organic ligands and the oxide shell covering the particles by soft hydrogen plasma resulting in a pure metallic state. After thermal treatment of the metallic particles, the coercive field increased by a factor of 6, the orbital magnetic moment at the Fe site increased by 330% and is reduced at the Pt site by 30%, while the effective spin moments did not change. A decrease of the frequency of oscillations in the extended x-ray absorption fine structure at the Pt L(3,2) edges provides evidence for crystallographic changes towards the L1(0) phase.     

Applications of linear Dichroism Spectroscopy

Abstract

Linear dichroism is the phenomenon of anisotropic absorption of light. It is shown by materials containing oriented molecules for which the absorption varies with the direction. The absorption intensity is proportional to the square of the scalar product between the electric field vector of the light and a molecule-characteristic transition moment vector, the absorption being maximum when the light vector is polarized parallel to the transition moment and zero when perpendicular to it. Linear dichroism can therefore provide (1) directions of transition moments when the molecule orientation is known (spectroscopic applications), or (2) information on molecular orientation when the transition moments are known (structural applications). Both applications are useful in several chemical systems, but so far the use of linear dichroism has been confined to a relatively small number of specialized laboratories, not least because of a lack of appropriate commercial instruments. Plane-polarized spectra have long been measured on crystals and other well-oriented materials, but systems with less complete orientation have usually been studied by birefringence which has allowed greater sensitivity. Birefringence and linear dichroism are related by the dispersion equations and therefore in principle contain the same basic information. However, linear dichroism is better suited for practical use since it is related in a very simple way to more-or-less well-separated quantal transitions, while birefringence is a complicated average over all transitions in the molecule.