Sub-laser-cycle electron pulses for probing molecular dynamics

Experience shows that the ability to make measurements in any new time regime opens new areas of science. Currently, experimental probes for the attosecond time regime (10(-18) 10(-15) s) are being established. The leading approach is the generation of attosecond optical pulses by ionizing atoms with intense laser pulses. This nonlinear process leads to the production of high harmonics during collisions between electrons and the ionized atoms. The underlying mechanism implies control of energetic electrons with attosecond precision. We propose that the electrons themselves can be exploited for ultrafast measurements. We use a 'molecular clock', based on a vibrational wave packet in H(2)(+) to show that distinct bunches of electrons appear during electron ion collisions with high current densities, and durations of about 1 femtosecond (10(-15) s). Furthermore, we use the molecular clock to study the dynamics of non-sequential double ionization.     

Imaging ultrafast molecular dynamics with laser-induced electron diffraction

Establishing the structure of molecules and solids has always had an essential role in physics, chemistry and biology. The methods of choice are X-ray and electron diffraction, which are routinely used to determine atomic positions with sub-?ngstr?m spatial resolution. Although both methods are currently limited to probing dynamics on timescales longer than a picosecond, the recent development of femtosecond sources of X-ray pulses and electron beams suggests that they might soon be capable of taking ultrafast snapshots of biological molecules and condensed-phase systems undergoing structural changes. The past decade has also witnessed the emergence of an alternative imaging approach based on laser-ionized bursts of coherent electron wave packets that self-interrogate the parent molecular structure. Here we show that this phenomenon can indeed be exploited for laser-induced electron diffraction (LIED), to image molecular structures with sub-?ngstr?m precision and exposure times of a few femtoseconds. We apply the method to oxygen and nitrogen molecules, which on strong-field ionization at three mid-infrared wavelengths (1.7, 2.0 and 2.3?μm) emit photoelectrons with a momentum distribution from which we extract diffraction patterns. The long wavelength is essential for achieving atomic-scale spatial resolution, and the wavelength variation is equivalent to taking snapshots at different times. We show that the method has the sensitivity to measure a 0.1?? displacement in the oxygen bond length occurring in a time interval of ～5?fs, which establishes LIED as a promising approach for the imaging of gas-phase molecules with unprecedented spatio-temporal resolution.     

Attosecond control of electronic processes by intense light fields

The amplitude and frequency of laser light can be routinely measured and controlled on a femtosecond (10(-15) s) timescale. However, in pulses comprising just a few wave cycles, the amplitude envelope and carrier frequency are not sufficient to characterize and control laser radiation, because evolution of the light field is also influenced by a shift of the carrier wave with respect to the pulse peak. This so-called carrier-envelope phase has been predicted and observed to affect strong-field phenomena, but random shot-to-shot shifts have prevented the reproducible guiding of atomic processes using the electric field of light. Here we report the generation of intense, few-cycle laser pulses with a stable carrier envelope phase that permit the triggering and steering of microscopic motion with an ultimate precision limited only by quantum mechanical uncertainty. Using these reproducible light waveforms, we create light-induced atomic currents in ionized matter; the motion of the electronic wave packets can be controlled on timescales shorter than 250 attoseconds (250 x 10(-18) s). This enables us to control the attosecond temporal structure of coherent soft X-ray emission produced by the atomic currents--these X-ray photons provide a sensitive and intuitive tool for determining the carrier-envelope phase.     

Resolving the time when an electron exits a tunnelling barrier

The tunnelling of a particle through a barrier is one of the most fundamental and ubiquitous quantum processes. When induced by an intense laser field, electron tunnelling from atoms and molecules initiates a broad range of phenomena such as the generation of attosecond pulses, laser-induced electron diffraction and holography. These processes evolve on the attosecond timescale (1?attosecond?≡?1?as = 10(-18)?seconds) and are well suited to the investigation of a general issue much debated since the early days of quantum mechanics--the link between the tunnelling of an electron through a barrier and its dynamics outside the barrier. Previous experiments have measured tunnelling rates with attosecond time resolution and tunnelling delay times. Here we study laser-induced tunnelling by using a weak probe field to steer the tunnelled electron in the lateral direction and then monitor the effect on the attosecond light bursts emitted when the liberated electron re-encounters the parent ion. We show that this approach allows us to measure the time at which the electron exits from the tunnelling barrier. We demonstrate the high sensitivity of the measurement by detecting subtle delays in ionization times from two orbitals of a carbon dioxide molecule. Measurement of the tunnelling process is essential for all attosecond experiments where strong-field ionization initiates ultrafast dynamics. Our approach provides a general tool for time-resolving multi-electron rearrangements in atoms and molecules--one of the key challenges in ultrafast science.     

飞秒激光场下NH3分子光电子能谱的理论研究

模拟了NH3分子在飞秒激光场下通过中间快速预解离态A～的共振增强多光子电离光电子能谱(REMPI-PES).应用两维的含时波包法实施动力学计算,计算过程中使用了离散变量表示(DVR)理论和劈裂算符-表象变换方法.结果表明,激光脉冲的强度、脉宽等对光电子能谱有着很大的影响,能谱中的两组峰簇是由强场对势能面作用所引起的.     

原子光诱导自电离时间特征

研究了二能级原子在光脉冲作用下的自电离时间特征。峄高斯型光脉冲，进行了薛定谔运动方程的数值求解，结果表明存在最佳光强值，在此光强下单位时间内电离数目最多。获得了指数型光脉冲作用下原子自电离几率的解析表达式。     

广义振子量子相位和经典相位的新求解

采用试探函数法求解广义振子的薛定谔方程，结果表明广义振子的波函数是中心按照经典运动方程运动的波包．与此相应，广义振子的量子几何相位由两部分组成：一部分是由波包形状的改变引起的，另一部分则是由波包中心的运动引起的．进一步通过广义振子经典运动方程的精确解，得出了广义振子的经典相位，从而也得到了广义振子量子相位和经典相位的一般关系．     

Tomographic imaging of molecular orbitals

Single-electron wavefunctions, or orbitals, are the mathematical constructs used to describe the multi-electron wavefunction of molecules. Because the highest-lying orbitals are responsible for chemical properties, they are of particular interest. To observe these orbitals change as bonds are formed and broken is to observe the essence of chemistry. Yet single orbitals are difficult to observe experimentally, and until now, this has been impossible on the timescale of chemical reactions. Here we demonstrate that the full three-dimensional structure of a single orbital can be imaged by a seemingly unlikely technique, using high harmonics generated from intense femtosecond laser pulses focused on aligned molecules. Applying this approach to a series of molecular alignments, we accomplish a tomographic reconstruction of the highest occupied molecular orbital of N2. The method also allows us to follow the attosecond dynamics of an electron wave packet.     

激光脉宽对椭偏光驱动的原子次序双电离的影响

利用经典模型,研究了激光脉宽对椭偏光驱动的Ar原子次序双电离(SDI)的影响.结果发现,二价离子动量分布强烈地依赖于激光脉宽.随激光脉宽增加,离子动量分布从二带结构变为四带结构,然后又变为六带结构.向后分析双电离轨迹既显示了椭偏光驱动的原子次序双电离中的亚周期电子出射现象,又表明带状结构随激光脉宽的演化取决于第二个电子电离时间随激光脉宽的变化.     

一次性可变概率分片标记及其压缩标记

提出了一次性可变概率分片标记方法,即对每一数据包从接入到受害主机的传输路径上的所有路由器至多对其进行一次标记,由此能够避免对任一数据包的重复标记;路由器根据数据包在网络上传输的距离d以概率1/(33-d)对其进行可变概率标记,使受害主机可等概率地收集到攻击路径中各个路由器标记的数据包.在此基础上,根据传输路径上IP信息的相似性冗余,提出了压缩一次性可变概率分片标记方法.实验结果表明,提出的方法能够消除可变概率标记方法对数据包的重复标记问题,并显著减少反向追踪攻击源所需数据包的数目,提高了对攻击源定位的准确性和实时性.     

北半球夏季的斜压波包个例分析

利用NCEP/NCAR再分析资料,使用局地能量诊断和三维准地转波动通量等工具分析了2002年北半球夏季的一次斜压波包过程。结果表明：波包发展的能量来源主要依靠斜压转换获得,频散项对波包的影响很小;波包进入衰减期以后,波包的东侧和南侧作为垂直方向的波动源,向下传播波动通量;向下的波通量进入对流层低层气旋范围,对气旋的维持起到重要作用。     

南半球夏季长生命期斜压波包的观测研究

利用Hillbert变换和NCEP/NCAR再分析资料,研究南半球夏季长生命期的斜压波包的结构和传播特征。结果表明,波包位相速度平均为11.2°/d,波包的包络传播速度（群速度）平均约为25.4°/d,群速约为相速的2.3倍。波包的位相垂直分布在波包发展阶段、成熟时期和衰减阶段是不同的。在发展阶段,波包位相在对流层低层西倾;对流层低层至高层波包位相随高度是近于垂直的;在对流层高层,其位相又开始西倾。在波包成熟阶段,在对流层低层仍然是西倾,对流层中低层以上其位相近于垂直。在波包衰减阶段,其位相在整个对流层都是近于垂直的。波包传播过程中其能量随时间变化比较复杂。有时波包呈现出一次“增长－成熟－衰减”过程,但有时却呈现出多次“增长－成熟－衰减”过程。     

可实现特殊激光微加工工艺的控制系统

以发动机气缸内表面作为激光微加工对象,基于半导体二极管泵浦YAG激光器的声光调Q技术,研制出可满足特殊激光微加工工艺要求的激光微加工设备控制系统.通过对运动控制卡MC8041A及自制的激光调Q控制卡的软件编程,实现对设备机械运动系统以及激光调Q信号输出的控制.运动控制卡控制各轴伺服电动机运转;激光调Q控制卡接收安装在旋转轴上的高精度增量式旋转编码器的反馈脉冲,对其进行计数、分频后,输出所需激光调Q信号.借助于激光调Q控制卡对激光头旋转运动和激光器单个激光脉冲输出的协调联动控制,形成单脉冲同点间隔多次激光微加工工艺,实现对发动机气缸内表面高效、高质量的微加工.     

用于非扫描3D激光成像的精密计时电路设计

针对目前带读出电路(ROIC)的APD阵列国内还缺乏产品的问题,设计采用一种高精度数字时间转换芯片(TDC-GP21)来测量激光脉冲在空中的飞行时间,从而得到目标的距离信息,实现高分辨率的距离图像. 利用FPGA控制时间数字转换芯片设计了非扫描3D激光成像的精密计时电路,进行了程序仿真与实验. 仿真结果表明设计的精密计时电路能够呈现出目标的距离图像.      

Attosecond metrology.

The generation of ultrashort pulses is a key to exploring the dynamic behaviour of matter on ever-shorter timescales. Recent developments have pushed the duration of laser pulses close to its natural limit-the wave cycle, which lasts somewhat longer than one femtosecond (1 fs = 10-15 s) in the visible spectral range. Time-resolved measurements with these pulses are able to trace dynamics of molecular structure, but fail to capture electronic processes occurring on an attosecond (1 as = 10-18 s) timescale. Here we trace electronic dynamics with a time resolution of 相似文献   

啁啾激光脉冲控制电子纵向动量关联谱研究

采用三维半经典再散射模型研究了周期量级啁啾脉冲中He原子非序列双电离电子的纵向(沿激光电场极化方向)动量关联谱,其中一、三象限分布的不对称性对啁啾参数变化很敏感.该性质是由电场分布的不对称性引起的.通过调节啁啾脉冲参数可以比调节一般周期量级脉冲的绝对相位能更灵敏地控制动量关联谱.     

自带计量标准器的二维位移工作台研究

提出以平面正交衍射光栅作为二维位移工作台计量标准器的方法，随工作台的移动，光栅能同时检测一个平面上两个方向的位移．阐述了正交衍射光栅作为位置检测元件的工作原理及其光路布置方法。二次衍射光线的两两迭加，形成两组干涉条纹信号，且这两组干涉条纹信号的相移与光栅两个正交方向的位移呈正比，通过光电转换和计数细分处理电路得到条纹信号的相移；检测光路布置在工作台下方，通过几组直角棱镜使干涉条纹信号进入光电探测器．实验分析了工作台的测量精度，有效分辨率提高到10nm．     

氢原子三重微分散射截面与能量分配的关联

氢原子单离化是原子物理中最简单的三体相互作用问题，但是由于长程的库仑作用的影响，带电粒子与原子碰撞的单离化过程仍有许多难以克服和未能解决的困难．利用跃迁矩阵元的后滞形式，同时，取末态波函数为库仑波函数和平面波函数的乘积，在初态里，波函数被分成两部分，一部分是库仑波和氢原子基态的乘积，另一部分是两电子的相对运动的库仑波和质心运动的平面波．对于快电子仍然取为平面波，如果在忽略了质心运动和两电子相对库仑波的指数因子的情况下，所得出的理论计算结果与一些实验在数值上符合得较好。     

一种报文二层预处理策略在高速NIDS上的应用

为了降低高速NIDS的丢包率和漏报率,提出一种在NIDS上应用报文二层预处理的策略.在报文二层预处理阶段,分别进行报文头匹配、报文净载部分匹配两层预处理,将可疑报文争对应的候选规则反馈给全报文匹配模块.在全报文匹配模块中只匹配候选规则,以降低NIDS的规则匹配开销.实验结果表明,应用该策略后,在高速网络环境下能降低Snort 2.8的丢包率50%以上,降低漏报率60%以上.     

1998年夏季北半球斜压波活动与长江流域洪涝灾害分析

对流层上层的斜压波、高空槽对天气的变化有重要的影响。1998年夏季我国出现了大范围的天气异常，在长江流域发生了20世纪第二大的全流域性洪水，分析了1998年夏季北半球的西风急流、风暴轴、以及对流层上的斜压波活动与气候平均情况的异同，并揭示了1998年夏季北半球斜压波活动传播的主要区域、斜压波活动的持续性、斜压波向传播的速度以及斜压波传播时所具有的明显的下游效应。1998年夏季北半球的波包 次数共有11次，与夏季气候状况相比属于正常偏多，其中5月和8月的波包活动次数稍多。但更为明显的一个特点就是在7月中旬到8月上旬大西洋－欧洲的斜压波包活动比往年集中频繁，而且持续时间偏长， 为造成我国异常梅雨天气和长江流域洪涝灾害的乌拉尔阻塞高压的发展和维持提供了能量支持，同时为长江流域以及同期我国东北的大范围降雨提供了有利的水汽输送条件。最后分析了地面气旋生成与斜压波、高空槽之间的联系，指出在大多数向东部发展的高空槽的槽前，地面上有对应气旋地生成。