Quantum crystallites and nonlinear optics

This is a review and analysis of the optical properties of quantum crystallites, with principal emphasis on the electro-optic Stark effect and all optical third order nonlinearity. There are also introductory discussions on physical size regimes, crystallite synthesis, quantum confinement theory, and linear optical properties. The experiments describe CdSe crystallites, exhibiting strong confinement of electrons and holes, and CuCl crystallites, exhibiting weak confinement of the exciton center of mass. In the CdSe system, neither the Stark effect nor the third order nonlinearity is well understood. The Stark shifts appear to be smaller than calculated, and field inducted broadening also occurs. The third order nonlinearity is only modestly stronger than in bulk material, despite theoretical prediction. Unexpectedly large homogeneous widths, due to surface carrier trapping, in the nominally discrete crystallite excited states appear to be involved. The CuCl system shows far narrower spectroscopic homogeneous widths, and corresponds more closely to an ideal quantum dot in the weak confinement limit. CuCl also exhibits exciton superradiance at low temperature. Surface chemistry and crystallite encapsulation are critical in achieving the predicted large Stark and third order optical effects in II-VI and III-V crystallites.     

Electronic transition dipole moments and excited-state alignment in linear molecules,with application to diatomic rare gas halides

The relationship between the direction of transition dipoles and the symmetry of electronic wavefunctions is investigated. In the case of $\text{Ω}\text{=}\text{1}\text{2}\text{?}\text{Ω′}\text{=}\text{1}\text{2}$ transitions, the dipole direction is not determined by symmetry alone. However, in all other cases the directions (parallel or perpendicular) are specified by simple selection rules based upon $\text{ΔΩ}$. In $\text{Ω}\text{=}\text{1}\text{2}\text{Ω′}\text{=}\text{1}\text{2}$ transitions, the transition dipole may accidentally have spherical symmetry, in which case a linearly polarized plane wave photon beam produces an unaligned excited-state distribution. These results are applied to the ultraviolet charge transfer transitions of the diatomic rare gas halides.     

Photoinduced thermal copper reduction onto gold nanocrystals under potentiostatic control

We study the photoreduction of adsorbed copper ions onto Au nanoparticles, on an indium tin oxide (ITO) electrode in an aqueous electrochemical cell, as a function of applied voltage and laser intensity. The photocurrent is a nonlinear function of laser intensity and increases sharply with cathodic voltage in the underpotential deposition region. The photoreduction is attributed to laser heating of the Au nanoparticles rather than "hot electron" processes. Numerical simulation of the Butler-Volmer kinetic equation using experimental parameters predicts a several orders of magnitude increase in current for a temperature rise of a few Kelvin.     

Tm~(3+)掺杂Ge-Ga-Se玻璃中红外发光特性与多声子弛豫分析

用熔融急冷法制备了系列不同Tm3+掺杂浓度的Ge30Ga5Se65玻璃样品,测试了样品折射率、拉曼光谱、吸收光谱以及800nm激光泵浦下的近红外及中红外波段荧光光谱和荧光寿命。用Judd-Ofelt理论计算了Tm3+在Ge30Ga5Se65玻璃中的强度参数Ωi(i=2,4,6)、自发辐射跃迁概率A、荧光分支比β和辐射寿命τrad等光谱参数。讨论了800nm激光泵浦下掺杂1Wt%Tm3+样品在近红外1.23,1.48和1.8μm处的发光特性及各量子效率,研究了800nm激光泵浦下的样品中红外荧光特性与掺杂浓度之间的关系,计算了常温下该基质玻璃中Tm3+:3H5→3F4跃迁对于3.8μm处的多声子弛豫速率Wmp和Ge30Ga5Se65玻璃基质中多声子弛豫常数W(0)和电子-声子结合常数α值。结果表明Ge30Ga5Se65硒基玻璃较低的声子能量可以大大降低稀土离子能级间跃迁的多声子弛豫概率,从而提高中红外荧光发光效率,因此硒基玻璃作为稀土离子的掺杂基质材料对实现中红外荧光输出是非常有利的。     

On Problems in the Calculus of Variations in Increasingly Elongated Domains

This paper deals with minimization problems in the calculus of variations set in a sequence of domains, the size of which tends to infinity in certain directions and such that the data only depend on the coordinates in the directions that remain constant. The authors study the asymptotic behavior of minimizers in various situations and show that they converge in an appropriate sense toward minimizers of a related energy functional in the constant directions.     

Synthesis and Properties of Hyperbranched Polyimides Combined with Silica

Summary: The novel hyperbranched polyimide - silica hybrid materials containing theoretically 16 wt% of an inorganic phase were prepared via a sol-gel process. An amine terminated polyimide precursor (hyperbranched polyamic acid) was prepared from commercially available monomers 4,4′,4″-triaminotriphenylmethane and 4,4′-oxydiphthalic anhydride in molar ratio 1:1. Tetramethoxysilane and/or 3-glycidoxypropyltrimethoxysilane (also used as a coupling agent) were used as silica precursors. During thermal exposition the polyimide precursor was transformed to hyperbranched polyimide and hydrolyzed alkoxy groups reacted mutually to form silica. The final products were self-standing films, whose structure was characterized by using IR and ¹³C and ²⁹Si solid state NMR spectroscopy. The influence of the amount of silica and/or coupling agent on their structure and thermal properties was described.     

CONSERVATION OF ENDANGERED ENDEMIC SEABIRDS WITHIN A MULTI‐PREDATOR CONTEXT: THE BARAU'S PETREL IN RÉUNION ISLAND

Abstract Seabirds breeding on islands are vulnerable to introduced predators, such as rats and cats, and the removal of such predators is generally viewed as a priority for seabird conservation and restoration. However, multiple invasive mammal species interacting may generate unexpected outcomes following the removal (eradication) of one species. Generally these indirect interactions are not well understood or demonstrated. We propose and study a prey (seabird)‐mesopredator (rat)‐superpredator (cat) model, taking into account the juvenile stages in the prey population, in order to direct conservation management for seabird conservation. We give a more biologically realistic differential system than those studied before (Courchamp et al. [1999] ; Fan et al. [2005] ), in particular for long‐lived seabird species. We present a theoretical study and show existence and uniqueness of a positive solution as well as a qualitative study of the equilibria that may appear. Because standard numerical methods, usually implemented in scientific softwares, can fail to give the right biological approximations ( Anguelov et al. [2009] ), we propose a reliable algorithm that preserves most of the qualitative properties of the continuous system, using the theory of nonstandard finite difference methods. Finally, we use biologically realistic parameters available for the representative Barau's petrel (Pinet et al. [2008] ), an endemic species from Réunion island, to present numerical simulations that support the theoretical study. Cats play the major role in seabird prey population dynamics. Seasonality in seabird breeding delays but does not prevent extinction. In all scenarios, cat control (or preferably eradication) is imperative to prevent extinction of vulnerable long‐lived seabirds, like the Barau's petrel.     

Unraveling the "pressure effect" in nucleation

The influence of the pressure of a chemically inert carrier gas on the nucleation rate is one of the biggest puzzles in the research of gas-liquid nucleation. Experiments can show a positive effect, a negative effect, or no effect at all. The same experiment may show both trends for the same substance depending on temperature, or for different substances at the same temperature. We show how this ambiguous effect naturally arises from the competition of two contributions: nonisothermal effects and pressure-volume work. Our model clarifies seemingly contradictory experimental results and quantifies the variation of the nucleation ability of a substance in the presence of an ambient gas. Our findings are corroborated by molecular dynamics simulations and might have important implications since nucleation in experiments, technical applications, and nature practically always occurs in the presence of an ambient gas.     

星载海洋激光雷达最佳工作波长分析

星载激光雷达是实现海洋垂直剖面探测的有效工具,也是目前迫切需求的海洋光学遥感手段。对星载海洋激光雷达的波长参数进行评估对保证探测有效性具有重要意义。本文从探测深度和信噪比两方面分析了星载海洋激光雷达探测全球海洋的最佳波长。利用MODIS 10个波段的水体光学特性数据,估算全球海水探测深度及相应的最优波长;并根据太阳夫琅禾费暗线特性,对信号信噪比进行优化。结果表明:在探测深度方面,最优探测波长在488 nm波段的海洋占全球海洋面积的70%左右,并且全球95%以上的海域在488 nm波段的探测深度优于0.8倍的真光层深度;在信噪比方面,相对于488 nm波段,486.134 nm夫琅禾费暗线处采用0.1 nm带宽的滤光片可以将背景光强度降低70%,相应地回波信噪比整体提升了约5.0%。就全球海洋探测来说,使用486.134 nm作为探测波长可以提高探测深度,有效抑制太阳背景光,提高信噪比,因此,486.134 nm是星载海洋激光雷达的最佳工作波长。     

Evaluation of calcium carbonate in eggshells using thermal analysis

The eggshell of the hen is an important structure which provides protection for the developing chick, and also a container for the egg in the food industry. Egg breakage can reach up to 10% of total egg production, causing considerable economic losses. The eggshell consists of membranes, composed mainly of proteins, and the mineral shell composed mainly of the calcite polymorph of CaCO₃. The average CaCO₃ content of a chicken eggshell is between 93 and 97%, depending on animal genotype, age, housing system of laying hens and mineral nutrition. In the present study, eggs of the same breed and approximately same age were collected from four different production systems: organic farming, free-range production, deep litter system and battery cage system. The CaCO₃ content was determined by the standard titration method and by atomic absorption spectroscopy. Thermal properties of dried eggshell powder were measured by thermogravimetric measurements between 30 and 900 °C in air, showing a significant mass loss of?≈?43% between 600 and 850 °C corresponding to the decomposition of CaCO₃ to CaO and CO₂. The relations between the thermal mass loss and CaCO₃ content determined by the titration method/atomic absorption spectroscopy were studied using predictive models fitted by the linear regression method. A good prediction ability with an average prediction error of 0.01% was obtained between CaCO₃ determined by titration and the thermal mass loss, indicating that TG could provide a reliable method for evaluation of CaCO₃ content in eggshells.