飞秒强激光脉冲与氢团簇相互作用产生的最大质子能量与团簇尺寸的相关性

报道了飞秒强激光脉冲(60 fs,790 nm,2×1016W/cm2)与纳米尺寸的氢团簇(半径rc约为1~3 nm)相互作用,产生的最大质子能量Emax对于团簇半径rc相关性的实验研究结果。从激光-氢团簇相互作用产生的质子的飞行时间谱测量以及团簇尺寸的瑞利散射诊断结果,得到Emax与r2c成线性正比关系,比例系数为0.75,与报道的理论模拟结果一致,表明氢团簇发生纯库仑爆炸。实验结果同时提示,进一步的理论模拟应考虑气体喷流中团簇的尺寸分布。     

激光光谱学的应用:Ⅰ 分子团簇研究

1引言材料的结构、电子和化学特性主要决定于凝聚物所处的状态和它本身的尺寸。团簇是包含2～104个粒子（原子、分子或离子）的有限的凝聚物。它靠物理的或化学的结合力组合在一起，已显示出奇特的物理和化学现象。团簇研究为探索物质从分子到凝聚态物质的逐渐过渡或演变提供了方法和手段，并且由于团簇具有大的表面体积比，因而有利于人们研究微观表面现象。团簇在分子、表面和凝聚态化学物理之间架起了一座桥梁。团簇科学是一门新型的交叉学科，是当代科学基础研究的前沿课题之一。团簇研究除涉及到许多未知的过程和新奇的现象之外，也是…     

导数荧光法研究异丙醇-水配合物的团簇性质

实验获得了异丙醇-水配合物的激发光谱和发射光谱,分析了其荧光发射机理,并用波长为220和232 nm的紫外光激发不同体积百分比的配合溶液得到了荧光光谱,在FFT低通滤波和平滑处理基础上运用导数荧光分析法对其进行优化,进而研究了异丙醇-水配合物的分子团簇性质。结果表明,配合物中异丙醇分子与水分子在H键作用下,以不同方式结合可以形成7种能发荧光的团簇分子,其发射荧光的中心波长分别为277、284、293、309、328、345和362 nm,并根据相对峰位强度得到了各团簇分子数量随溶液浓度变化的规律。     

全同周期排列的纳米团簇阵列的自发生长

我们提出了利用分子束外延的自组织生长过程大面积制备二维周期性纳米金属团簇阵列的一种方法.该方法的普适性通过研究 Si(111)-7×7衬底上生长的Ⅲ族元素、贵金属、磁性金属以及它们的合金团簇得到证实.通过对In团族点阵的原位扫描隧道显微镜分析和第一性原理总能量计算,我们确定了这些团簇独特的原子结构,阐明了这些结构的形成机理.我们发现团簇和表面之间的强相互作用是获得团簇的特定尺寸和其有序排列的关键所在.     

醇/水混合团簇的多光子电离质谱及从头算研究

利用多光子电离技术和飞行时间质谱仪实验研究了醇/水混合团簇的光电离质谱。在脉冲激光波长为355nm条件下,观测到以质子化(ROH)n(H2O)H+ 混合团簇离子和(ROH)nH+团簇离子为主的电离产物。醇水混合团簇电离后团簇离子发生内部质子化转移反应是形成质子化团簇离子的主要原因。应用量化计算,构造了质量数较小的几个团簇离子的可能的空间几何构型,发现二元团簇离子(CH3OH)n(H2O)H+是以(CH3OH)H+作为内核离子,再通过氢键与其它分子组合而构成团簇离子。     

基于多光子电离的氨/醇混合团簇的飞行时间质谱研究

用多光子电离技术结合飞行时间质谱仪对氨/醇混合团簇进行了研究。在脉冲激光波长为355nm条件下观测到团簇离子。主要的电离产物为质子化的（ROH）n（NH3）mH＋（n=1~6,m=0~4）混合团簇离子,且各个序列的离子强度随m的增大而减小。经分析,氨与醇混合团簇电离后团簇离子发生内部质子化转移反应是形成质子化团簇离子的主要原因。比较分析质谱图可知,当团簇离子比较小的时候二元团簇解离以失去醇类分子为主,随着团簇离子的增大,解离由失去醇分子为主逐渐变为失去氨分子为主。     

Si_xN~±_y团簇离子的形成和稳定性研究

结合ＤＶ－Ｘα方法的理论计算结果，对激光蒸发方法产生的ＳｉｘＮ团簇离子的形成和稳定性进行了细致的研究。发现上述四簇离子可能以ＳｉＮ３或ＳｉＮ４作为初始单元，较大的团簇离子可由某种单元与另一质谱上较稳定的复合分子组成。质谱强度变化的规律表明：若团簇离子质量是Ｓｉ原子量的倍数时，呈现极大值，此时，团簇离子包含偶数个Ｎ原子。其奇偶性是由初始单元强度差异引起的。     

红外线揭示气相金属碳化物

研究人员已经获得气相金属碳化物Ti8C12和Ti14C13团簇的首次直接红外光谱。反映形成分子结构的原子力振动光谱支持团簇的早期设想,可能推进它们在催化中的应用。 荷兰内伊梅根大学等离子体物理研究所和雅典乔治亚大学的一个研究组应用红外共振增强多光子电离光谱术。该技术比传统的红外吸收光谱术灵敏几个量级,能测量仅含有几千个分子的样品的光谱。 该组详细描述了该研究。在掺有氩气的CH4膨胀过程中使钛蒸发。研究人员使分子束电离以萃取所需的团簇粒子,并用自由电子激光器输出的20～30 mJ、6～25 μm、5 μs“宏脉冲”激光(间隔为1 ns的1 ps脉冲列)激发这些团簇。以飞行时间质谱仪测量分子的响应。 该研究所的Gert von Helden说：“对于我们已做的实验,自由电子激光器Felix很重要。该激光器具有其他激光器不可能有的几种特色。”特别是可调谐激光宏脉冲结构允许样品吸收、再分布和吸收更多的光子,并与激光源保持共振。 避免样品振动模的非谐振特别重要。研究钛碳化物之类新奇团簇的困难是在烧蚀过程中很少产生这种团簇：只有1万～100万个分子能进入样品束。von Helden说：“所以必须吸收许多光子,这只是推论,是否可能还不清楚。” 光谱表明：Ti8C12显示碳-碳键,而Ti14C13却没有。这暗示前者为四面体结构,后者为立方体结构,仅带有金属-碳键。 对金属团簇结构的更好理解可能与催化和化学材料科学的其他方面有关。团簇与块状材料或原子的化学反应完全不同。这能用于化学工业。 研究人员计划继续他们的金属碳化物团簇研究工作,并将它扩展到金属氧化物和氮化物。其中一些可能与天体物理有关,特别是与宇宙尘埃的形成有关。他们已用已获得的数据去辨认几个星球周围的TiC纳米晶体。     

笼形团簇材料

近年来,在原子、分子和凝聚态两层次之间出现了原子团簇这一重要的学科领域。本文介绍了笼形团簇材料的国内外最新研究进展,分析了笼形团簇材料的研究现状,并对其未来的发展方向进行了展望。     

乙醇/水混合团簇的多光子电离质谱研究

利用激光器(输出波长355nm)激发乙醇和水二元混合团簇中的离子-分子反应,运用飞行时间质谱技术检测反应后生成的团簇离子.实验观测到多个序列的质子化团簇离子:(C2H5OH)nH 和(C2H5OH)n(H2O)mH (n=1-12,m=1-4),其产生是通过团簇内的质子转移反应.通过对团簇离子的峰强度同乙醇数目n和水分子数目m的关系的研究发现,团簇离子(C2H5OH)8(H2O)H 是幻数结构.     

近场热辐射的最新研究进展

主要从理论数值模拟和近场辐射实验研究的角度介绍了近几年在近场热辐射传热方面的最新研究成果。理论研究的焦点主要集中在石墨烯复合材料、人工加工或合成超材料等方面的传热研究。实验研究的焦点是实验室基于纳米尺度近场热辐射测量的设备制造与方法创新。目前实验上已经实现了最小距离仅为2 nm的极近场热辐射测量。近场热辐射的进一步研究可为热光伏、辐射制冷以及高效能源收集应用提供理论基础。     

飞秒强激光脉冲与H原子团簇相互作用的库仑爆炸过程模拟

运用经典粒子动力学模拟方法，研究了飞秒强激光脉冲(10^15～10^16W／cm^2)与正二十面体构型氢原子团簇H13,H55，H147和H309的相互作用。通过模拟分析团簇的膨胀过程，发现团簇的膨胀是各向同性的，团簇的膨胀尺度R(t)／R(0)随团簇尺寸的增大而减小，即团簇尺寸愈大，与激光相互作用后膨胀碎解过程愈慢。研究结果表明，随着团簇中原子数目的增多，团簇库仑爆炸后所产生的离子的动能相应增大。由于正二十面体的对称壳层结构，离子动能分布具有尖峰结构。团簇库仑爆炸后离子的最大动能Emax与团簇库仑爆炸前的尺寸的平方成正比。且Emax随激光光强I增加而增大。但是当I增大到一定值Is时，Emax将出现饱和，这是因为I的增强已经不再改变团簇内原子的电离状态。随着团簇尺寸的增大，激光光强饱和值和离子能量将会继续提高。     

信息检索中的聚类分析技术

信息检索/搜索引擎技术的快速发展使得信息的查全率有较大提高,而查准率以及人们获取信息的效率改善却不明显。文本聚类和多文档关键词的自动生成技术将有助于解决这一问题。其基本思想是对检索到的部分文档进行聚类处理,并对每类文档自动生成关键词,从而帮助用户判断各个类别的文档和检索需求是否相关。该文提出文档相关度和类别相关度的概念,并利用词频信息以及知网(HOWNET)中词的概念计算模型计算类别相关度,将其作为聚类合并的依据。信息获取的仿真实验表明文档检索效率有较大提高。     

无线传感网络中基于蚁群的路由算法

In the wireless sensor networks, high efficient data routing for the limited energy resource networks is an important issue. By introducing Ant-colony algorithm, this paper proposes the wireless sensor network routing algorithm based on LEACH. During the construction of sensor network clusters, to avoid the node premature death because of the energy consumption, only the nodes whose residual energy is higher than the average energy can be chosen as the cluster heads. The method of repeated division is used to divide the clusters in sensor networks so that the numbers of the nodes in each cluster are balanced. The basic thought of ant-colony algorithm is adopted to realize the data routing between the cluster heads and sink nodes, and the maintenance of routing. The analysis and simulation showed that the proposed routing protocol not only can reduce the energy consumption, balance the energy consumption between nodes, but also prolong the network lifetime.     

集群计算中并行文件系统的设计研究

文中首先阐述了基于集群的并行文件系统和并行I/O的研究现状、并行文件访问特点及其研究必要性,然后介绍了所研究、设计的一种应用于集群中的新型并行文件系统CLUPFS,重点介绍了CLUPFS中的各组成部分的功能和实现,最后指出了CLUPFS并行文件系统进一步研究与实现的方向.     

Cluster Space Specification and Control of Mobile Multirobot Systems

The cluster space state representation of mobile multirobot systems is introduced as a means of enabling enhanced control of mobile multirobot systems. A conceptual framework is proposed for the selection of appropriate cluster space state variables for an n-robot system, the development of formal kinematics that associate the cluster space state variables with robot-specific variables, and the implementation of a cluster space control system architecture. The cluster space approach is then demonstrated for examples of two- and three-robot clusters consisting of differential drive robots operating in a plane. In these examples, we demonstrate cluster space variable selection, review the critical kinematic relationships, and present experimental results that demonstrate the ability of the systems to meet control specifications while allowing a single operator to easily specify and supervise the motion of the clusters.     

Energy and Trust Management Framework for MANET using Clustering Algorithm

In General, Mobile Ad-Hoc Network (MANET) has limited energy resources, and it cannot recharge itself. This research goal focuses on building a power management scheme that saves energy in the MANET. Due to power instability, there is a chance that cluster heads fail and function incorrectly in cluster-based routing. As a result, instability occurs with the cluster heads while collecting data and communicating with others effectively. This work focuses on detecting the unstable cluster heads, which are replaced by other nodes implementing the envisaged self-configurable cluster mechanism. A self-configurable cluster mechanism with a k-means protocol approach is proposed to designate cluster heads effectively. The proposed k-means procedure is based on periodic irregular cluster head rotations or altering the number of clusters. We also propose a trust management mechanism in this research to detect and avoid MANET vulnerabilities. Because of the continuously changing topology and limited resources (power, bandwidth, computing), the trust management algorithm should only use local data. Consequently, compared to traditional protocols, the proposed approach with the k-means procedure and its experimental results show lower power usage and provide an optimal system for trust management.

     

An unequal cluster-based routing scheme for multi-level heterogeneous wireless sensor networks

In wireless sensor networks (WSNs), clustering can significantly reduce energy dissipation of nodes, and also increase communication load of cluster heads. When multi-hop communication model is adopted in clustering, “energy hole” problem may occur due to unbalanced energy consumption among cluster heads. Recently, many multi-hop clustering protocols have been proposed to solve this problem. And the main way is using unequal clustering to control the size of clusters. However, many of these protocols are about homogeneous networks and few are about heterogeneous networks. In this paper, we present an unequal cluster-based routing scheme for WSNs with multi-level energy heterogeneity called UCR-H. The sensor field is partitioned into a number of equal-size rectangular units. We first calculate the number of clusters in each unit by balancing energy consumption among the cluster heads in different units. And then we find the optimal number of units by minimizing the total energy consumption of inter-cluster forwarding. Finally, the size of clusters in each unit is elaborately designed based on node’s energy level and the number of clusters in this unit. And a threshold is also designed to avoid excessive punishment to the nodes with higher energy level. Simulation results show that our scheme effectively mitigates the “energy hole” problem and achieves an obvious improvement on the network lifetime.     

MH-TRACE: multihop time reservation using adaptive control for energy efficiency

In this paper, we propose multihop time reservation using adaptive control for energy efficiency (MH-TRACE), which is a medium access control (MAC) protocol that combines advantageous features of fully centralized and fully distributed networks for energy-efficient real-time packet broadcasting in a multihop radio network. We introduce a novel clustering algorithm that dynamically organizes the network into two-hop clusters. MH-TRACE clusters are just for coordinating channel access and minimizing interference; thus, ordinary nodes are not static members of any cluster. Time is organized into cyclic superframes, which consist of several time frames, to support reservation-based periodic channel access for real-time traffic. Each clusterhead chooses the frame with least interference based on its own measurements for the operation of its cluster. Energy dissipation for receiving unwanted or collided data packets or for waiting in idle mode is avoided through the use of information summarization packets sent prior to the data transmissions by the source nodes. Through the use of transmission schedules within each cluster, managed by the clusterheads, intracluster data collisions are completely eliminated and intercluster collisions are minimized. We investigated MH-TRACE through extensive simulations and theoretical analysis. Our results show that MH-TRACE outperforms existing distributed MAC protocols like IEEE 802.11 and sensor MAC, in terms of energy efficiency and throughput, approaching the theoretical maximum throughput and theoretical minimum energy dissipation.