共查询到20条相似文献，搜索用时 203 毫秒
1.
Gilijamse JJ Hoekstra S van de Meerakker SY Groenenboom GC Meijer G 《Science (New York, N.Y.)》2006,313(5793):1617-1620
Molecular scattering behavior has generally proven difficult to study at low collision energies. We formed a molecular beam of OH radicals with a narrow velocity distribution and a tunable absolute velocity by passing the beam through a Stark decelerator. The transition probabilities for inelastic scattering of the OH radicals with Xe atoms were measured as a function of the collision energy in the range of 50 to 400 wavenumbers, with an overall energy resolution of about 13 wavenumbers. The behavior of the cross-sections for inelastic scattering near the energetic thresholds was accurately measured, and excellent agreement was obtained with cross-sections derived from coupled-channel calculations on ab initio computed potential energy surfaces. 相似文献
2.
一类单种群差分模型的周期解和混沌现象 总被引：5，自引：0，他引：5
邓小炎 《华中农业大学学报》2000,19(4):403-407
建立了一类描述有物种流动时特定区域内种群增长的差分方程模型,给出了平衡点稳定的参数区域与吸引域,获得了稳定２－周期解存在的参数区域,并研究倍周期分岔与混沌现象。 相似文献
3.
Optical control over elementary molecular motion is enhanced with timed sequences of femtosecond (10(-15) second) pulses produced by pulse-shaping techniques. Appropriately timed pulse sequences are used to repetitively drive selected vibrations of a crystal lattice, in a manner analogous to repetitively pushing a child on a swing with appropriate timing to build up a large oscillation amplitude. This process corresponds to repetitively "pushing" molecules along selected paths in the lattice. Amplification of selected vibrational modes and discrimination against other modes are demonstrated. Prospects for more extensive manipulation of molecular and collective behavior and structure are clearly indicated. 相似文献
4.
Atom transfer radical polymerization is a versatile technique for exerting precise control over polymer molecular weights, molecular weight distributions, and complex architectures. Here, we show that an externally applied electrochemical potential can reversibly activate the copper catalyst for this process by a one-electron reduction of an initially added air-stable cupric species (Cu(II)/Ligand). Modulation of polymerization kinetics is thereby tunable in real time by varying the magnitude of applied potential. Application of multistep intermittent potentials successfully triggers initiation of polymerization and subsequently toggles the polymerization between dormant and active states in a living manner. Catalyst concentrations down to 50 parts per million are demonstrated to maintain polymerization control manifested in linear first-order kinetics, a linear increase in polymer molecular weight with monomer conversion, and narrow polymer molecular weight distributions over a range of applied potentials. 相似文献
5.
Formation of glasses from liquids and biopolymers 总被引：3，自引：0，他引：3
Angell CA 《Science (New York, N.Y.)》1995,267(5206):1924-1935
Glasses can be formed by many routes. In some cases, distinct polyamorphic forms are found. The normal mode of glass formation is cooling of a viscous liquid. Liquid behavior during cooling is classified between "strong" and "fragile," and the three canonical characteristics of relaxing liquids are correlated through the fragility. Strong liquids become fragile liquids on compression. In some cases, such conversions occur during cooling by a weak first-order transition. This behavior can be related to the polymorphism in a glass state through a recent simple modification of the van der Waals model for tetrahedrally bonded liquids. The sudden loss of some liquid degrees of freedom through such first-order transitions is suggestive of the polyamorphic transition between native and denatured hydrated proteins, which can be interpreted as single-chain glass-forming polymers plasticized by water and cross-linked by hydrogen bonds. The onset of a sharp change in ddT( is the Debye-Waller factor and T is temperature) in proteins, which is controversially indentified with the glass transition in liquids, is shown to be general for glass formers and observable in computer simulations of strong and fragile ionic liquids, where it proves to be close to the experimental glass transition temperature. The latter may originate in strong anharmonicity in modes ("bosons"), which permits the system to access multiple minima of its configuration space. These modes, the Kauzmann temperature T(K), and the fragility of the liquid, may thus be connected. 相似文献
6.
Egolf DA 《Science (New York, N.Y.)》2000,287(5450):101-104
Far-from-equilibrium, spatially extended chaotic systems have generally eluded analytical solution, leading researchers to consider theories based on a statistical rather than a detailed knowledge of the microscopic length scales. Building on the recent discovery of a separation of length scales between macroscopic behavior and microscopic chaos, a simple far-from-equilibrium spatially extended chaotic system has been studied computationally at intermediate, coarse-grained scales. Equilibrium properties such as Gibbs distributions and detailed balance are recovered at these scales, which suggests that the macroscopic behavior of some far-from-equilibrium systems might be understood in terms of equilibrium statistical mechanics. 相似文献
7.
The steady-state levels of potassium and sodium of taenia coli of guinea are critically affected by varying temperature in the narrow range 12 degrees to degrees C. For the accumulation of both cations the critical temperature, T(c), is 13.8 degrees C the presence of millimolar external potassium. The value of T(c), decreases 10.0 degrees C when the external potassium is raised to 10 millimolar. Since, at a fixed Temperature, the potassium accumulation follows a cooperative mechanism, the results are compared with the quantitative predictions of this approach. The itical thermal transition behavior can be described in terms of the cooperative cumulation process. 相似文献
8.
Rind D 《Science (New York, N.Y.)》1999,284(5411):105-107
The climate that we experience results from both ordered forcing and chaotic behavior; the result is a system with characteristics of each. In forecasting prospective climate changes for the next century, the focus has been on the ordered system's responses to anthropogenic forcing. The chaotic component may be much harder to predict, but at this point it is not known how important it will be. 相似文献
9.
Patel CK 《Science (New York, N.Y.)》1978,202(4364):157-173
Spectroscopic analysis is a useful technique for identifying and quantitatively determining the presence of specific gaseous constituents. Development of high-power tunable lasers has made the spectroscopic technique for detection of trace constituents in the atmosphere very attractive for practical applications. In this article three of the currently used modes for laser detection of pollution are reviewed: (i) long-path measurements, (ii) laser Raman (differential absorption) measurements, and (iii) optoacoustic detection. Progress in the field has been extremely rapid in the last few years and very useful and reliable data on air pollution can now be obtained routinely with the techniques described. 相似文献
10.
以行为主义、建构主义学习理论及课程论为理论依据,借鉴国内外成功的职教模式,构建出“宽基础、窄模块、新专题“课程结构体系,旨在推进二年制高职物理课程体系的改革建设,体现物理学的基础性、适用性、针对性、先进性,满足高职教育的发展需要。 相似文献
11.
积极探索农村产业融合发展实践,总结典型经验,推广发展新模式,旨在进一步拓展农村产业发展空间,优化农村产业结构,建立农村产业发展新业态,实现农村产业可持续繁荣。研究基于新疆和田地区2016—2020年统计年鉴数据,运用熵值法、熵权TOPSIS综合分析法、灰色关联度分析测算研究区农村产业融合发展水平及其驱动因素。结果显示：（1）研究期内,熵权TOPSIS综合得分呈上升趋势,表明新疆和田地区农村产业融合发展正处在发展阶段,产业融合行为、产业融合效果已初见成效。（2）市场需求因素对农村产业融合发展影响最为突出;技术创新与企业合作对农村产业融合发展具有促进作用;关联产业与农村产业融合发展较为密切。（3）和田地区各县市之间农村产业融合发展水平具有一定的空间相关性,与距离和田市市区距离呈一定正相关,但并非唯一决定因素。综上,研究提出“一产为基础,二产连两头”、“一产连三产,发挥多功能”、“二产中间连,一三齐发展”等产业融合模式,为和田地区实现脱贫巩固有效衔接乡村振兴可持续发展提供参考借鉴。 相似文献
12.
Fermi resonance was observed between the OH stretch and the overtone of the OH bending modes of HDO molecules contaminated in phase VII of D(2)O ice over the pressure range from 17 to 30 gigapascals. An anharmonic coupling constant, which is related to the potential energy surface on which hydrogen-bonded protons oscillate, was found to range around 50 wave numbers through the resonant pressure range. Its experimentally obtained magnitude and pressure-insensitive behavior will be useful for theoretical studies of the potential energy surface and hence of the nature of hydrogen bonding in ice. 相似文献
13.
Weather regimes: the challenge in extended-range forecasting 总被引：1，自引：0，他引：1
Reinhold B 《Science (New York, N.Y.)》1987,235(4787):437-441
A hypothesis to explain the low-frequency (10-to 90-day) variance of the mid-latitude atmosphere is presented. In this hypothesis it is proposed that the planetary-scale waves forced by topography or other zonal inhomogeneities and the day-to-day weather disturbances (synoptic scales) influence each other to generate weather regimes. These quasi-stable flow configurations are responsible for short-range climate anomalies such as droughts, heat waves, deep freezes, and excessive precipitation, as the weather-producing disturbances are organized into storm tracks. Onset and disruption of the weather regimes may be induced by the anomalous development of perhaps a single cyclonic disturbance, which can throw the quasi equilibrium out of balance. Thus, rapid changes in flow regimes can occur almost at random. The development and transition of weather regimes may then be purely internal (that is, depending only upon the properties of the fluid motions themselves) to the atmospheric dynamics. This internal quality suggests that the chaotic, abrupt short-range climatic behavior of the mid-latitudes is a natural behavior of the system that requires no assistance from the outside. The weather regime concept presents a different view of the extended-range atmospheric behavior than the stimulus-response model, such as the atmospheric response to the El Nimo. 相似文献
14.
Synchrotron radiation sources, providing intense, tunable, polarized, and stable beams of ultraviolet and x-ray photons, are having a great impact on biology, physics, chemistry, materials science, and other areas of research. Synchrotron radiation has revolutionized solid-state photoelectron spectroscopy by enhancing its capabilities for investigating the electronic behavior of solids and solid surfaces. Several fundamental photoemission techniques that rely on synchrotron radiation are discussed in this article. These techniques have an adjustable tunable surface sensitivity and provide the first direct mapping of the electronic band structure. Recent applications of photoelectron spectroscopy for studies of chemisorption geometries and surface structures are discussed. 相似文献
15.
Meyer VG 《Science (New York, N.Y.)》1970,169(3948):886-888
A plant population from an interspecific cotton hybrid produces flowers with exposed ovules on petal margins. The variability of these abnormal flowers suggests the possibility that angiosperms may have evolved by accumulating regulator genes which narrow the range of response to fluctuations in the environment. Progressive canalization of development within a population like the "carpelloid petal" plants could have resulted in both angiospermy and speciation. 相似文献
16.
Superconductivity is related to the presence of a narrow forbidden gap in the spectrum of the possible energies for the electrons in the material. These "superconductivity gaps" have traditionally been studied with tunneling and infrared absorption experiments. A third, powerful technique has been made possible by the discovery of hightransition temperature materials: the direct observation of the gap in photoemission spectra. The data analysis requires a careful reconsideration of the standard Einstein-Fermi model of the photoelectric effect. The conclusions are surprisingly simple and offer an alternate way to measure superconductivity gaps. This approach can also be used to study the directional properties of the gap, phenomena related to the coherence length, and possible departures from Fermi-liquid behavior. 相似文献
17.
Thermoregulation in endothermic insects 总被引：2，自引：0，他引：2
B Heinrich 《Science (New York, N.Y.)》1974,185(153):747-756
On the basis of body weight, most flying insects have higher rates of metabolism, and hence heat production, than other animals. However, rapid rates of cooling because of small body size in most cases precludes appreciable endothermy. The body temperature of small flies in flight is probably close to ambient temperature, and that of flying butterflies and locusts is 5 degrees to 10 degrees C above ambient temperature. Many moths and bumblebees are insulated with scales and hair, and their metabolism during flight can cause the temperature of the flight muscles to increase 20 degrees to 30 degrees C above ambient temperature. Curiously, those insects which (because of size, insulation) retain the most heat in the thorax during flight, also require the highest muscle temperature in order to maintain sufficient power output to continue flight. The minimum muscle temperature for flight varies widely between different species, while the maximum temperature varies over the relatively narrow range of 40 degrees to 45 degrees C. As a consequence, those insects that necessarily generate high muscle temperatures during flight must maintain their thoracic temperature within a relatively narrow range during flight. Active heat loss from the thorax to the abdomen prevents overheating of the flight motor and allows some large moths to be active over a wide range of ambient temperatures. Bumblebees similarly transfer heat from the flight musculature into the abdomen while incubating their brood by abdominal contact. Many of the larger insects would remain grounded if they did not actively increase the temperature of their flight muscles prior to flight. Male tettigoniid grasshoppers elevate their thoracic temperature prior to singing. In addition, some of the social Hymenoptera activate the "flight" muscles specifically to produce heat not only prior to flight but also during nest temperature regulation. During this "shivering" the "flight" muscles are often activated in patterns different from those during flight. The muscles contract primarily against each other rather than on the wings. However, the rate of heat production during shivering and flight is primarily a function of the action potential frequency rather than of the patterns of activation. Thermoregulation is a key factor in the energetics of foraging of some of the flower-visiting insects. The higher their muscle temperature the more flowers they can visit per unit time. When food supplies are ample, bees may invest relatively large amounts of energy for thermoregulation. While shivering to maintain high body temperatures during the short intervals they are perched on flowers (as well as while in the nest), bumblebees often expend energy at rates similar to the rates of energy expenditure in flight. Unlike vertebrates, which usually regulate their body temperature at specific set points, the body temperature of insects is labile. It often appears to be maintained near the lower temperature at which the muscles are able to perform the function at hand. The insects' thermal adaptations may not differ as much from those of vertebrates as previously supposed when size, anatomy, and energy requirements are taken into account. 相似文献
18.
Conjugated polymers and indium arsenide-based nanocrystals were used to create near-infrared plastic light-emitting diodes. Emission was tunable from 1 to 1.3 micrometers--a range that effectively covers the short-wavelength telecommunications band--by means of the quantum confinement effects in the nanocrystals. The external efficiency value (photons out divided by electrons in) is approximately 0.5% (that is, >1% internal) and is mainly limited by device architecture. The near-infrared emission did not overlap the charge-induced absorption bands of the polymer. 相似文献
19.
戴林久 《安徽农业大学学报》2003,(2):52-54
危险犯的犯罪中比是指在直接故意犯罪过程中,行为人自动停比其犯罪行为,或者自动有效地防比危害结果发生的一种危险犯的犯罪形态,其时间条件是"在犯罪过程中”。对于犯罪过程”有广义和狭义的理解,对于危险犯而言,其中比形态分为三种类型,即:预备阶段的犯罪中;;实行阶段的犯罪中 ;完成实行行为、法定危险状态尚末发生阶段的 犯罪中止。不同类型的危险犯犯罪中比的时空范围不同,即时间性条件不同,应区别对待。必须区分两个界限:即犯罪中比与犯罪末遂的界限,犯罪中比与犯罪既遂的界限,从而准确地把握危险犯犯罪中比的时间性条件,正确地定罪量刑,充分发挥刑罚预防犯罪的社会作用。 相似文献
20.
1984年7月,在东北农学院院内和黑龙江省农业科学院园艺所菜豆植株上,分别采集具有花叶、疮叶、黄化、皱缩等症状的病毒标样:V-1、V-5、V-9、V-16.然后,接种到菜豆家雀蛋上,上述症状再次出现.将四个标样进行人工接种,在豆科、茄科、藜科、苋科、胡麻科、葫芦科、鸢尾科7科40种植物上测定寄主范围和寄主反应.以桃蚜(Myzus persicac)、豆蚜(Aphis medicaginis)做蚜传方式的测定.同时,测定了病毒的稳定性,并以病叶汁液观察了病毒粒体形态,通过试验证实了种子传毒,根据以上鉴定结果,这四个标样为菜豆普通花叶病毒Bean Common Mosaic Virus(BCMV). 相似文献