In-Plane Structure of the Liquid-Vapor Interface of an Alloy: A Grazing Incidence X-ray Diffraction Study of Bismuth:Gallium

The liquid-vapor interface of a bismuth-gallium mixture (0.2 percent bismuth and 99.8 percent gallium) at 36 degrees C has been studied by grazing incidence x-ray diffraction. The data show, in agreement with thermodynamic arguments, that bismuth is heavily concentrated in the liquid-vapor interface. The x-ray diffraction data are interpreted with the assistance of a simple model that represents the interface as a partial monolayer of bismuth. This analysis leads to the conclusion that the bismuth concentration in the interface is about 80 percent, that there is no significant mixing of gallium and bismuth in the interface, and that the structure function of the interfacial bismuth is like that of supercooled bulk liquid bismuth.     

Measuring the surface dynamics of glassy polymers

The motion of polymer chain segments cooled below the glass transition temperature slows markedly; with sufficient cooling, segmental motion becomes completely arrested. There is debate as to whether the chain segments near the free surface, or in thin films, are affected in the same way as the bulk material. By partially embedding and then removing gold nanospheres, we produced a high surface coverage of well-defined nanodeformations on a polystyrene surface; to probe the surface dynamics, we measured the time-dependent relaxation of these surface deformations as a function of temperature from 277 to 369 kelvin. Surface relaxation was observed at all temperatures, providing strong direct evidence for enhanced surface mobility relative to the bulk. The deviation from bulk alpha relaxation became more pronounced as the temperature was decreased below the bulk glass transition temperature. The temperature dependence of the relaxation time was much weaker than that of the bulk alpha relaxation of polystyrene, and the process exhibited no discernible temperature dependence between 277 and 307 kelvin.     

中密板和刨花板的生产与甲醛含量动态

[目的]研究刨花板和中密度纤维板生产过程中甲醛含量动态及影响因素,有针对性地提出应对措施。[方法]在贵州省内随机抽取中密板和刨花板生产厂家各1个,历时3年跟踪监测中密板和刨花板的生产与甲醛含量动态。[结果]2家产品的甲醛平均含量差异显著,分别为289.4和97.7 mg/kg。与《室内装饰装修材料人造板及其制品中甲醛释放限量》(GB18580-2001)相比,69.70%的刨花板产品可直接投放市场、30.30%的产品经饰面后可投放市场,54.90%的中密板产品经饰面后可进入市场、45.10%的产品超标。刨花板的半数月份产品可直接进入市场、半数月份产品经饰面后可进入市场,中密板的58.33%月份产品经饰面后进入市场、41.67%月份产品超标。2种产品不同规格间的甲醛含量有异、部分规格间的差异显著,工班间所生产中密板的甲醛含量有异、部分工班间产品的差异显著。中密板和刨花板不同年度产品的平均甲醛含量呈降低趋势,中密板和刨花板的甲醛含量同板材含水率呈正相关。[结论]2种人造板产品的平均甲醛含量差异显著,这种差异受规格、含水率与工班的影响,呈现出明显的季节动态。     

Femtosecond dynamics of electron localization at interfaces

The dynamics of two-dimensional small-polaron formation at ultrathin alkane layers on a silver(111) surface have been studied with femtosecond time- and angle-resolved two-photon photoemission spectroscopy. Optical excitation creates interfacial electrons in quasi-free states for motion parallel to the interface. These initially delocalized electrons self-trap as small polarons in a localized state within a few hundred femtoseconds. The localized electrons then decay back to the metal within picoseconds by tunneling through the adlayer potential barrier. The energy dependence of the self-trapping rate has been measured and modeled with a theory analogous to electron transfer theory. This analysis determines the inter- and intramolecular vibrational modes of the overlayer responsible for self-trapping as well as the relaxation energy of the overlayer molecular lattice. These results for a model interface contribute to the fundamental picture of electron behavior in weakly bonded solids and can lead to better understanding of carrier dynamics in many different systems, including organic light-emitting diodes.     

Quantum confinement and host/guest chemistry: probing a new dimension

Nanoparticulate metals and semiconductors that have atomic arrangements at the interface of molecular clusters and "infinite" solid-state arrays of atoms have distinctive properties determined by the extent of confinement of highly delocalized valence electrons. At this interface, the total number of atoms and the geometrical disposition of each atom can be used to significantly modify the electronic and photonic response of the medium. In addition to teh novel inherent physical properties of the quantum-confined moieties, their "packaging" into nanocomposite bulk materials can be used to define the confinement surface states and environment, intercluster interactions, the quantum-confinement geometry, and the effective charge-carrier density of the bulk. Current approaches for generating nanostructures of conducting materials are briefly reviewed, especially the use of three-dimensional crystalline superlattices as hosts for quantum-confined semiconductor atom arrays (such as quantum wires and dots) with controlled inter-quantum-structure tunneling.     

Hydrophobic collapse in multidomain protein folding

We performed molecular dynamics simulations of the collapse of a two-domain protein, the BphC enzyme, into a globular structure to examine how water molecules mediate hydrophobic collapse of proteins. In the interdomain region, liquid water persists with a density 10 to 15% lower than in the bulk, even at small domain separations. Water depletion and hydrophobic collapse occur on a nanosecond time scale, which is two orders of magnitude slower than that found in the collapse of idealized paraffin-like plates. When the electrostatic protein-water forces are turned off, a dewetting transition occurs in the interdomain region and the collapse speeds up by more than an order of magnitude. When attractive van der Waals forces are turned off as well, the dewetting in the interdomain region is more profound, and the collapse is even faster.     

Ultrafast vibrational dynamics at water interfaces

Time-resolved sum-frequency vibrational spectroscopy permits the study of hitherto neglected ultrafast vibrational dynamics of neat water interfaces. Measurements on interfacial bonded OH stretch modes revealed relaxation behavior on sub-picosecond time scales in close resemblance to that of bulk water. Vibrational excitation is followed by spectral diffusion, vibrational relaxation, and thermalization in the hydrogen-bonding network. Dephasing of the excitation occurs in 相似文献   

Electrons in finite-sized water cavities: hydration dynamics observed in real time

We directly observed the hydration dynamics of an excess electron in the finite-sized water clusters of (H2O)n- with n = 15, 20, 25, 30, and 35. We initiated the solvent motion by exciting the hydrated electron in the cluster. By resolving the binding energy of the excess electron in real time with femtosecond resolution, we captured the ultrafast dynamics of the electron in the presolvated ("wet") and hydrated states and obtained, as a function of cluster size, the subsequent relaxation times. The solvation time (300 femtoseconds) after the internal conversion [140 femtoseconds for (H2O)35-] was similar to that of bulk water, indicating the dominant role of the local water structure in the dynamics of hydration. In contrast, the relaxation in other nuclear coordinates was on a much longer time scale (2 to 10 picoseconds) and depended critically on cluster size.     

Structural relaxation of polymer glasses at surfaces, interfaces, and in between

We analyzed the glassy-state structural relaxation of polymers near surfaces and interfaces by monitoring fluorescence in multilayer films. Relative to that of bulk, the rate of structural relaxation of poly(methyl methacrylate) is reduced by a factor of 2 at a free surface and by a factor of 15 at a silica substrate interface; the latter exhibits a nearly complete arresting of relaxation. The distribution in relaxation rates extends more than 100 nanometers into the film interior, a distance greater than that over which surfaces and interfaces affect the glass transition temperature.     

Mechanism and dynamics of ion transfer across a liquid-liquid interface

A detailed molecular model for ion transfer across the interface between water and 1,2-dichloroethane provides insight into the mechanism of the transfer across the interface between two immiscible liquids. Extensive molecular dynamics computer simulations underscore the roles of surface roughness and capillary distortions in this process, demonstrate that ion transfer is an activated rather than a simple diffusive process, and provide a test for the applicability of theoretical models.     

Electron spectroscopy of aqueous solution interfaces reveals surface enhancement of halides

It has been suggested that enhanced anion concentrations at the liquid/vapor interface of airborne saline droplets are important to aerosol reactions in the atmosphere. We report ionic concentrations in the surface of such solutions. Using x-ray photoelectron spectroscopy operating at near ambient pressure, we have measured the composition of the liquid/vapor interface for deliquesced samples of potassium bromide and potassium iodide. In both cases, the surface composition of the saturated solution is enhanced in the halide anion compared with the bulk of the solution. The enhancement of anion concentration is more dramatic for the larger, more polarizable iodide anion. By varying photoelectron kinetic energies, we have obtained depth profiles of the liquid/vapor interface. Our results are in good qualitative agreement with classical molecular dynamics simulations. Quantitative comparison between the experiments and the simulations indicates that the experimental results exhibit more interface enhancement than predicted theoretically.     

The structure of the first coordination shell in liquid water

X-ray absorption spectroscopy and x-ray Raman scattering were used to probe the molecular arrangement in the first coordination shell of liquid water. The local structure is characterized by comparison with bulk and surface of ordinary hexagonal ice Ih and with calculated spectra. Most molecules in liquid water are in two hydrogen-bonded configurations with one strong donor and one strong acceptor hydrogen bond in contrast to the four hydrogen-bonded tetrahedral structure in ice. Upon heating from 25 degrees C to 90 degrees C, 5 to 10% of the molecules change from tetrahedral environments to two hydrogen-bonded configurations. Our findings are consistent with neutron and x-ray diffraction data, and combining the results sets a strong limit for possible local structure distributions in liquid water. Serious discrepancies with structures based on current molecular dynamics simulations are observed.     

Insights into phases of liquid water from study of its unusual glass-forming properties

The vitrification of pure water is compared with that of molecular solutions rich in water, and gross differences are noted. Thermodynamic reasoning and direct observations on noncrystallizing nanoconfined water indicate that the glass transition in ambient-pressure water is qualitatively distinct from that found in the usual molecular liquids. It belongs instead to the order-disorder class of transition seen in molecular and ionic crystalline materials. The distinctive "folding funnel" energy landscape for this type of system explains the extreme weakness of the glass transition of water as well as the consequent confusion that has characterized its scientific history; it also explains the very small excess entropy at the glass transition temperature. The relation of confined water behavior to that of bulk is discussed, and the "fragile-to-strong" transition for supercooled water is interpreted by adding a "critical point-free" scenario to the two competing scenarios for understanding supercooled bulk water.     

分布式水文模型在徒骇马颊河流域灌溉管理中的应用Ⅰ.参数率定和模拟验证

【目的】建立徒骇马颊河流域的分布式水文模型,为进行该地区灌溉制度优化提供支持。【方法】首先运用洗牌复形演化（SCE-UA）算法在禹城试验站和洪门试验站对分布式水文模型（SWAT）中的作物参数进行率定,接着以遥感监测的蒸散数据为目标,应用拉丁超立方-单次单因素（LH-OAT）方法对蒸散发（ET）相关的参数进行了敏感性分析,用序贯不确定性（SUFI-2）算法优化出一套参数范围并进行不确定性分析。在此基础上,在考虑模型不确定性的情况下对冬小麦－夏玉米轮作体系产量的长期模拟结果进行验证。【结果】作物参数率定结果表明,禹城试验站上冬小麦和夏玉米产量的相对误差分别为7.02%和16.60%,洪门试验站上冬小麦和夏玉米产量的相对误差分别为0.09%和0.10%。ET参数率定和统计年鉴产量验证结果：除了NS系数,P_factor、R_factor以及R2的模拟精度较高。【结论】依据徒骇马颊河流域的特点进行SWAT模型相关参数的率定,得到较好的模拟结果,为进一步深入研究该地区水分生产函数和优化灌溉制度打下了基础。     

青海省三江源黄河源区地表水水质状况

为了解三江源黄河源区地表水水质状况,于2005-2009年在黄河源设7～13个断面,对黄河源地表水中18项水质指标进行了连续监测和分析。结果表明,高锰酸盐指数、氨氮和总磷含量偏高,略超过GB3838—2002《地表水环境质量标准》中Ⅰ类标准,超标主要是由黄河源特殊的自然条件引起,不属于外源人为污染;5日生化需氧量(BOD5)、化学需氧量(COD)、氟化物、铜、锌、六价铬、镉、汞、硫化物、挥发酚、氰化物、阴离子表面活性剂、石油类和粪大肠菌群符合GB3838—2002《地表水环境质量标准》中Ⅰ类标准浓度限值。总体而言,三江源黄河源区地表水水质良好,未受到人类活动的明显影响。     

北京市经济林生态系统服务功能评估

依照《森林生态系统服务功能评估规范》（LY/T 1721-2008）,以北京市13个区县、14个经济林树种为研究对象,定量评估北京市经济林生态系统服务功能。结果表明,北京市年经济林生态系统涵养水源和保育土壤等7项评估指标总价值为78.41亿元,以涵养水源（30.15亿元·a^-1）、固碳释氧（17.42亿元·a^-1）、净化大气环境（11.59亿元·a^-1）为主;北京市各区经济林生态系统中密云区、平谷区和怀柔区的服务价值相对高于其他区县;以板栗林生态服务价值量较高（29.90%）;北京市各项生态服务功能空间分布格局由大到小基本呈现为:东北部、西北部、西南部、东南部、中部。     

基于凸壳模型的福建省水土流失核心区划分

以福建省为研究区域,结合"3S"技术与野外调查,提取土地利用、植被覆盖度和坡度分级等信息,生成2007年水土流失现状图,并采用凸壳模型进行水土流失核心区划分。结果表明:福建省水土流失核心区共有"四大片"(汀江片、晋江片、九龙江片与赛江片)和"六小片"(涉及邵武、顺昌、南平、古田、罗源、大田、德化和永泰等县、市、区)。将水土流失核心区与前人划定的严重水土流失区和重点治理区相比较,"四大片"的主体范围较为一致,但是严重水土流失区和重点治理区以行政区域为单元,划分指标为各类水土流失面积占土地总面积的比例,而水土流失核心区直接确定水体流失严重的地域,更加详细具体。     

三江源黄河地表水水环境现状评价

[目的]监测与评价三江源黄河地表水质量状况,为三江源的保护与开发提供理论依据。[方法]以三江源黄河地表水为研究对象,通过水质监测,对三江源黄河地表水进行水环境评价,并对该地区的生态及环境保护提出几点建议。[结果]监测与评价结果表明,三江源黄河地表水的质量状况如下：水温10．7℃,pH8．05,高锰酸盐指数1．54mg／L,化学需氧量COD8．19mg／L,生化需氧量BOD5 2mg／L,氨氯0．139mg／L,氟化物0．163mg／L,硫化物0．02mg／L,总磷0．03mg／L,铬（六价）0．004mg／L,挥发酚0．002mg／L,总汞0．00001mg／L,石油类0．02mg／L,粪大肠恩斯特群〈20个／L。[结论]三江源黄河地表水水质均未超过于《地表水环境质量标准》（GB3838-2002）的Ⅰ类标准．水体未受到污染,水质良好。     

Single-polymer dynamics in steady shear flow

The conformational dynamics of individual, flexible polymers in steady shear flow were directly observed by the use of video fluorescence microscopy. The probability distribution for the molecular extension was determined as a function of shear rate, gamma;, for two different polymer relaxation times, tau. In contrast to the behavior in pure elongational flow, the average polymer extension in shear flow does not display a sharp coil-stretch transition. Large, aperiodic temporal fluctuations were observed, consistent with end-over-end tumbling of the molecule. The rate of these fluctuations (relative to the relaxation rate) increased as the Weissenberg number, gamma;tau, was increased.     

Reconciling the magnitude of the microscopic and macroscopic hydrophobic effects

The magnitude of the hydrophobic effect, as measured from the surface area dependence of the solubilities of hydrocarbons in water, is generally thought to be about 25 calories per mole per square angstrom (cal mol-1 A-2). However, the surface tension at a hydrocarbon-water interface, which is a "macroscopic" measure of the hydrophobic effect, is approximately 72 cal mol-1 A-2. In an attempt to reconcile these values, alkane solubility data have been reevaluated to account for solute-solvent size differences, leading to a revised "microscopic" hydrophobic effect of 47 cal mol-1 A-2. This value, when used in a simple geometric model for the curvature dependence of the hydrophobic effect, predicts a macroscopic alkane-water surface tension that is close to the macroscopic value.