氩和氪气体团簇靶的制备及特性研究

 采用瑞利散射法和Mach-Zehnder（M-Z）干涉法分别测量氩、氪团簇的尺寸和密度，讨论了影响团簇尺寸和密度的因素，并比较氩团簇和氪团簇特性的异同点。通过改变诊断光和喷嘴阀门的相对延迟时间，获得了团簇尺寸和密度随时间的变化情况，发现氩、氪团簇的尺寸和密度均在3~25 ms达到最大值；在气体背景压强1.2~6.0 MPa下，测量了喷嘴轴线上的密度分布，发现离喷嘴喉部越远，气体密度越低，并且气体密度随压强呈线性上升。     

An Experimental Observation of Axial Variation of Average Size of Methane Clusters in a Gas Jet

Axial variation of average size of methane clusters in a gas jet produced by supersonic expansion of methane through a cylindrical nozzle of 0.8 mm in diameter is observed using a Rayleigh scattering method. The scattered light intensity exhibits a power scaling on the backing pressure ranging from 16 to 50bar, and the power is strongly Z dependent varying from 8.4 （Z = 3mm） to 5.4 （Z = 11mm）, which is much larger than that of the argon cluster. The scattered light intensity versus axial position shows that the position of 5mm has the maximum signal intensity. The estimation of the average cluster size on axial position Z indicates that the cluster growth process goes forward until the maximum average cluster size is reached at Z - 9 mm, and the average cluster size will decrease gradually for Z 〉 9 mm.     

超声喷流Ar团簇生长演化过程及团簇尺寸轴向分布的实验研究

针对特定形式的喷嘴产生的Ar气体团簇,利用瑞利散射法研究了团簇的生长演化过程,测量了不同背压下空间轴向上的团簇尺度,得到了团簇尺寸随轴向距离与背压的关系.实验发现,在20—60 atm（1 atm=1013 kPa）背压范围内,距离喷嘴出口5 mm处,Ar团簇尺度最大. 关键词： 超声喷流 Ar团簇 瑞利散射 团簇尺度     

Size estimates of nobel gas clusters by Rayleigh scattering experiments

Noble gases (argon, krypton, and xenon) are puffed into vacuum through a nozzle to produce clusters for studying laser-cluster interactions. Good estimates of the average size of the argon, krypton and xenon clusters are made by carrying out a series of Rayleigh scattering experiments. In the experiments, we have found that the scattered signal intensity varied greatly with the opening area of the pulsed valve. A new method is put forward to choose the appropriate scattered signal and measure the size of Kr cluster.     

瑞利散射法研究超声喷流二氧化碳团簇尺度轴向分布

针对特定尺度的小锥喷嘴,用瑞利散射方法研究了超声喷流二氧化碳团簇的瑞利散射信号S和团簇尺度N的轴向分布.实验发现,散射信号强度的对数ln(S)随着轴向距离Z的平方根的增大线性减小,团簇尺度最大位置出现在距喷嘴轴向距离4 mm附近,在背压6—17 bar范围内散射信号S与背压P满足S∝P^3.6—4.2,测量到最大的团簇约为1.6×10⁴分子/团簇. 关键词： 二氧化碳团簇 轴向分布 团簇尺度 瑞利散射     

Cluster formation processes investigated by linearly chirped spectral scattering

By employing linearly chirped spectral scattering technique, the time-resolved formation processes of argon clusters have been investigated. The redshifts of scattering spectra as a function of backing pressure as well as the time delay between the laser and the firing switch of the cluster flow have been measured. It has been found that very large-size cluster sources with very low gas density can be produced by adiabatic expansion of gases at low pressures through a conical nozzle into vacuum. It can be used as clean and important cluster target.     

低压下气流对激光沉积纳米硅晶化及尺寸的影响

纳米硅具有明显的光致发光效应和量子尺寸效应,广泛的应用在现代电子工业和太阳能光伏工业中.尺寸影响着纳米硅的实际用途,因此制备尺寸可控的纳米硅晶粒具有很重要的实际意义.本文采用脉冲激光沉积(PLD)技术,在烧蚀点水平方向、距靶2 cm处引入一束流量为5 sccm的氩(Ar)气流,在0.01-0.5 Pa的Ar气压下烧蚀高阻抗单晶硅(Si)靶.在管口正下方1 cm处水平放置衬底来沉积纳米Si薄膜;并用同一装置,在0.08 Pa的Ar气压下分别引入流量为0,2.5,5,7.5,10 sccm的Ar气流沉积纳米Si薄膜.利用原子力显微镜(AFM)、X射线衍射(XRD)、Raman散射对样品表面形貌和微观结构进行分析表征.结果表明:不引入气流时出现纳米Si晶粒的阈值气压是0.1Pa,引入气流后出现纳米Si晶粒的阈值气压为0.05 Pa.晶粒尺寸随着气流流量的增大而减小.     

飞秒强激光脉冲中氩团簇库仑爆炸特性研究

利用Bathe模型,理论模拟了氩团簇在飞秒强激光中(100 fs, 1016 W/cm2)的电离和爆炸过程.研究结果显示,在团簇尺寸较小时,离子平均能量与团簇初始半径平方成正比,爆炸机制为典型的库仑爆炸.随着团簇尺寸的增加,能量增加的速度趋缓并在一定团簇尺寸后趋于饱和.模拟结果与实验数据有较好的吻合.     

气体密度对瑞利散射法测量团簇尺寸影响的初步分析

 瑞利散射法被广泛地应用于测量团簇尺寸,通常视散射信号的强弱只正比于团簇的大小。从理论推导得到瑞利散射信号并不仅仅是团簇尺寸的函数,而与团簇尺寸、气体密度和结合率均有关系,认为采用瑞利散射法测量团簇尺寸应考虑气体密度对测量结果的影响。并用该分析方法研究了氩团簇沿喷嘴轴线的尺寸分布情况：团簇尺寸沿喷嘴轴线并不是单纯的单调变化,而是在某个位置达到最大值,并且这个位置与背景压强等因素有关。     

Growth of deuterium clusters in a gas jet and ion energy spectrum of clusters in ultra-short laser field

Large deuterium clusters are generated using a cryogenic pulse valve with a cone nozzle (21 mm long, 4° open angle). Rayleigh scattering experiment is carried out to obtain the scaling relation between scattering signal SR and backing pressure P0. A method using the Coulomb explosion model is proposed to verify that the clusters continue to grow after their leaving the nozzle. Our experiments suggest a tentatively optimized position for laser cluster interaction.     

High-Intensity Femtosecond Laser Interaction with Rare Gas Clusters

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High-Intensity Femtosecond Laser Interaction with Rare Gas Clusters

With a 45 fs multiterawatt 790 nm laser system and jets of argon and krypton atomic clusters, a study of the interaction of fs intense laser pulses with large size rare gas dusters was conducted. The maximum laser intensity of about 7 × 1016 W/cm2 and dusters composed of thousands of atoms which were determined through Rayleigh scattering measurements were involved inthe experiments. On the one hand, the results indicate that the interaction is strongly cluster size dependent. The stronger the interaction, the larger the clusters are. On the other hand, a saturation followed by a drop of the energy of ions ejected from the interaction will occur when the laser intensity exceeds a definite value for clusters of a certain size.     

Molecular dynamics simulation of Cu_n clusters scattering from a single-crystal Cu(111) surface: The influence of surface structure

By performing a molecular dynamics simulation, fragmentation of Cu_n clusters scattering from a single-crystal Cu(111) surface is studied. The interactions among copper atoms are modeled by tight-binding potential, and the positions of the copper clusters at each time step are calculated by integrating the Newton equations of motion. The percentage of unfragmented clusters depends on the incident velocities, angles of incidence, and surface structure. The influence of surface structure on the fragment distribution is discussed, and the clusters appear to be more stable under an axial channeling condition. The fragment distribution shifting toward the small fragment range for cluster scattering along a random direction is confirmed, indicating that the cluster undergoes more intensive fragmentation.     

Modeling cluster jets as targets for high-power ultrashort laser pulses

A hydrodynamic model is formulated that describes the formation of clusters in atomic gas jets expanding into vacuum, which are used as laser plasma targets. Detailed model calculations performed for an argon gas jet describe spatial distributions of the density of gas and cluster phases formed in the Laval nozzle at room temperature in a broad range of entrance gas pressures. The cluster density distribution is significantly inhomogeneous. The cluster distribution features revealed by the model calculations were qualitatively confirmed by the X-ray spectroscopic measurements of the spatial distribution of emission from the plasma created in the jet tar-gets by high-power ultrashort laser pulses.     

Hagena团簇尺度定律中锥形喷嘴的等效孔径

本文首先详细重演了锥形喷嘴的等效孔径d_eq, 并根据d_eq的定义给出了它与气体团簇喷流的径向宽度之间的依赖关系. 然后以高背压氩气团簇喷流为例, 通过成像喷流的Rayleigh 散射光的空间分布研究了不同背压下喷流的径向宽度, 并与Hagena 团簇尺度定律中直线流模型假设的喷流径向宽度进行了比较. 结果表明, Hagena 直线流模型假设的喷流径向宽度小于实际的径向宽度, 且实际宽度与气体背压有关. 进一步的研究表明, 直线流模型对喷流宽度的估计偏差导致对锥形喷嘴等效孔径的估计偏差, 这为Hagena 尺度定律估计团簇平均尺寸的偏差给出了一种可能的解释. 关键词： 气体团簇 Hagena尺寸定律 锥形喷嘴的等效孔径     

引出口的几何参数对团簇形成影响的DSMC研究

利用直接模拟蒙特卡洛方法(DSMC),模拟了气体凝聚团簇源的引出口尺寸和中心位置不同的条件下,Cu团簇的尺寸分布以及模拟了随着时间的增长,腔内团簇数目的变化.模拟结果表明:引出口的直径越大,产生大团簇的比例越小;在引出口的直径相同的情况下,引出口的中心位置偏离坐标原点比在原点产生的大团簇的比例要大;随着时间的增长,腔内团簇数目先变多后变少.     

气体凝聚团簇源中团簇形成的DSMC研究

利用直接模拟蒙特卡洛方法(DSMC) ,模拟了气体凝聚团簇源的引出口尺寸和中心位置不同的条件下,Cu团簇的尺寸分布以及模拟了随着时间的增长,腔内团簇数目的变化.模拟结果表明:引出口的直径越大,产生大团簇的比例越小;在引出口的直径相同的情况下,引出口的中心位置偏离坐标原点比在原点产生的大团簇的比例要大;随着时间的增长,腔内团簇数目先变多后变少.     

静态真空对超声喷流气体团簇制备的实验研究

研究了靶室静态真空对超声喷流气体团簇产生和制备的影响.通过瑞利散射方法测量了不同静态真空度下超声喷流氩团簇的尺度和密度参数,发现在喷嘴出口附近团簇尺度和密度受静态真空度的影响较小;在距离喷嘴较远处,氩气团簇存在同氢气团簇类似的自限制效应,验证了自限制效应在团簇制备、输运过程中的重要作用.该结果对于建造基于激光聚变原理的桌面中子源具有较大的参考意义,可据此简化真空装置以降低运行和维护成本.     

Study on the interaction of intense femtosecond laser pulses with nanometre-sized hydrogen clusters

The interaction of intense femtosecond laser pulses with hydrogen clusters has been experimentally studied. The hydrogen clusters were produced from expansion of high-pressure hydrogen gas (backed up to 8\tm10⁶Pa) into vacuum through a conical nozzle cryogenically cooled by liquid nitrogen. The average size of hydrogen clusters was estimated by Rayleigh scattering measurement and the maximum proton energy of up to 4.2keV has been obtained from the Coulomb explosion of hydrogen clusters under 2×10¹⁶ W/cm² laser irradiation. Dependence of the maximum proton energy on cluster size and laser intensity was investigated, indicating the correlation between the laser intensity and the cluster size. The maximum proton energy is found to be directly proportional to the laser intensity, which is consistent with the theoretical prediction.     

高背压超声气体团簇喷流中团簇平均尺寸沿喷流方向演化研究

本文通过对高背压(50 bar, 1 bar = 1.0×10⁵ Pa)氩气经长锥型喷嘴(长度L=30 mm)向真空绝热膨胀所形成的超声气体团簇喷流的数值模拟, 分析比较了由喷嘴喉口起沿喷流方向在喷流中心轴线上团簇平均尺寸的演化情况. 结果表明: 沿喷流方向团簇平均尺寸显示先增长后趋于饱和的变化趋势, 具有较大尺寸团簇的区域出现在距离喷嘴喉口大约20 mm. 据此本文再结合关于喷流中原子密度沿喷流方向变化的模拟结果开展了锥形喷嘴长度的优化研究. 针对由常见构型的锥形喷嘴(喉径～ 0.5 mm, 半张角～ 8.5°)在高背压下形成的团簇喷流, 20 mm左右的长度为锥形喷嘴的适宜长度.