Ionization Suppression of Heteronuclear Diatomic and Triatomic Molecules in Strong Infrared Laser Fields

Ionization is the fundamental process in interaction of atoms/molecules with femtosecond strong laser fields. Comparing to atoms, molecules exhibit peculiar behaviors in strong-field ionization because of their diverse geometric structures, molecular electronic orbitals as well as extra nuclear degrees of freedom. In this study, we investigate strong field single and double ionization of carbon monoxide （CO） and carbon dioxide （CO2） in linearly polarized 50-fs, 800-nm laser fields with peak intensity in the range of 2×10 13 W/cm2 to 2×10 14 W/cm2 using time-of-flight mass spectrometer. By comparing the ionization yields with that of the companion atom krypton （Kr）, which has similar ionization potential to the molecules, we investigate the effect of molecular electronic orbitals on the strong-field ionization. The results show that comparing to Kr, no significant suppression is observed in single ionization of both molecules and in non-sequential double ionization （NSDI） of CO, while the NSDI probability of CO2 is strongly suppressed. Based on our results and previous studies on homonuclear diatomic molecules （N2 and O2）, the mechanism of different suppression effect is discussed. It is indicated that the different structure of the highest occupied molecular orbitals of CO and CO2 leads to distinct behaviors in two-center interference by the electronic wave-packet and angular distributions of the ionized electrons, resulting in different suppression effect in strong-field ionization.

Nonadiabatic Effect on the Rescattering Trajectories of Electrons in Strong Laser Field Ionization Process

The important features of the rescattering trajectories in strong field ionization process such as the cutoff of the return energy at 3.17Up and that of the final energy at 10Up are obtained, based on the adiabatic approximation in which the initial momentum of the electron is assumed to be zero. We theoretically study the nonadiabatic effect by assuming a nonzero initial momentum on the rescattering trajectories based on the semiclassical simpleman model. We show that the nonzero initial momentum will modify both the maximal return energy at collision and the final energy after backward scattering, but in different ways for odd and even number of return trajectories. The energies are increased for even number of returns but are decreased for odd number of returns when the nonzero （positive or negative） initial momentum is applied.

Ellipticity-dependent ionization/dissociation of carbon dioxide in strong laser fields

Ionization and dissociation of linear triatomic molecules, carbon dioxide, are studied in 50-fs 800-nm strong laser fields using time-of-flight mass spectrometer. The yields of double charged ions CO2^2＋ and various fragment ions（CO^＋,O^n＋, and C^n＋（n = 1, 2）） are measured as a function of ellipticity of laser polarization in the intensity range from 5.0 ×10^13W/cm2 to 6.0 × 10^14W/cm^2. The results demonstrate that non-sequential double ionization, which is induced by laser-driven electron recollision, dominates double ionization of CO2 in the strong IR laser field with intensity lower than2.0 × 10^14W/cm^2. The electron recollision could also have contribution in strong-field multiple ionization and formation of fragments of CO2 molecules. The present study indicates that the intensity and ellipticity dependence of ions yields can be used to probe the complex dynamics of strong-field ionization/dissociation of polyatomic molecules.

Rydberg excitation of neutral nitric oxide molecules in strong UV and near-IR laser fields

Rydberg state excitations of neutral nitric oxide molecules are studied in strong ultraviolet （UV） and near-infra-red （IR） laser fields using a linear time-of-flight （TOF） mass spectrometer with the pulsed electronic field ionization method. The yield of Rydberg molecules is measured as a function of laser intensity and ellipticity, and the results in UV laser fields are compared with those in near-IR laser fields. The present study provides the first experimental evidence of neutral Rydberg molecules surviving in a strong laser field. The results indicate that a rescattering-after-tunneling process is the main contribution to the formation of Rydberg molecules in strong near-IR laser fields, while multi-photon excitation may play an important role in the strong UV laser fields.

Efficient solver for time-dependent Schrodinger equation with interaction between atoms and strong laser field

We present a parallel numerical method of simulating the interaction of atoms with a strong laser field by solving the time-depending Schr?dinger equation(TDSE) in spherical coordinates. This method is realized by combining constructing block diagonal matrices through using the real space product formula(RSPF) with splitting out diagonal sub-matrices for short iterative Lanczos(SIL) propagator. The numerical implementation of the solver guarantees efficient parallel computing for the simulation of real physical problems such as high harmonic generation(HHG) in these interaction systems.

Effect of laser polarization on strong-field ionization andfragmentation of nitrous oxide molecules

Ionization of molecules in femtosecond laser fields is the most fundamental and important step of various strong-field physical processes. In this study, we experimentally investigate strong field ionization of linear N_2O molecules using a time-of-flight mass spectrometer in 800-nm laser fields. Yields of the parent ion and different fragment ions are measured as a function of laser intensity in the range of 2.0×10^(13) W/cm^2 to 3.6×10^(14) W/cm^2. We also investigate the dependence of strong field ionization and dissociation of N_2O on laser ellipticity and polarization direction. The significant role of laser induced electron re-collision in the formation of highly charged fragment ions is proved. The physical mechanism of strong field ionization and fragmentation is discussed, based on our experimental results.

A 7 T Pulsed Magnetic Field Generator for Magnetized Laser Plasma Experiments

A pulsed magnetic field generator was developed to study the effect of a magnetic field on the evolution of a laser-generated plasma. A 40 kV pulsed power system delivered a fast （-230 ns）, 55 kA current pulse into a single-turn coil surrounding the laser target, using a capacitor bank of 200 nF, a laser-triggered switch and a low-impedance strip transmission line. A one-dimensional uniform 7 T pulsed magnetic field was created using a Helmholtz coil pair with a 6 mm diameter. The pulsed magnetic field was controlled to take effect synchronously with a nanosecond heating laser beam, a femtosecond probing laser beam and an optical Intensified Charge Coupled Device （ICCD） detector. The preliminary experiments demonstrate bifurcation and focusing of plasma expansion in a transverse magnetic field.

利用原子在圆偏振激光场中的强场电离产生自旋极化电子

圆偏振激光场中原子的非绝热强场电离为产生自旋极化电子提供了机会.我们应用这些解析电离速率公式[Ingo Barth and Olga Smirnova,Phys.Rev.A 88,013401(2013)]更系统地研究了通过Kr和Xe原子在右旋圆偏振激光脉冲中的强场电离产生自旋极化电子,并证实了不同自旋态的光电子能量分布有很大差异、电子的自旋极化敏感地依赖于光电子能量.另外,在光电子能谱的低能部分其自旋极化可以达到100%,并且通过调节激光强度和频率可以很好地控制能量积分的自旋极化.

二维强激光场中原子分子电离和谐波辐射的应用(特邀)

二维激光场具有高分辨能力,为人们在阿秒时间尺度探测和调控物质中电子的超快运动提供了可能。相较于原子,分子具有更多自由度,在强激光场中会呈现出许多新效应。本文介绍了利用强正交偏振双色激光场和椭圆偏振激光场中原子分子的电离和高次谐波辐射分辨电子超快动力学、探测分子空间结构、合成阿秒脉冲链等相关研究。分析了库仑效应、双中心干涉效应、固有偶极效应等对分子电离和谐波辐射的重要影响。通过原子在正交偏振双色场中的强场电离定义了库仑效应诱导的电离时间延迟,解释了双色场中原子光电子谱的上下不对称结构、极性分子光电子谱的各向异性、高次谐波辐射电子轨道时域性质的重要改变。对椭圆偏振激光场中原子强场电离的研究构建了描述库仑诱导电离时间延迟的基本理论模型,基于该模型分辨了椭偏场中极性分子固有偶极对电离的贡献、量化了正交双色场中原子光电子谱的特征结构位置。利用正交偏振双色场中对称分子光电子谱在特定分子取向角下呈现的不对称结构,以及椭偏场中对称分子光电子谱对分子取向角的灵敏依赖实现了对分子空间结构信息的探测。凭借正交双色场中对称分子产生的高次谐波对分子取向角的依赖特性合成了单周期只释放一次的阿秒脉冲链,以及利用小椭偏率激光场中分子产生的高椭偏谐波合成了明亮的超短极紫外脉冲。本文内容将深化人们对正交偏振双色场和椭圆偏振激光场中原子分子电离和谐波辐射机制与规律的认识,为利用二维激光场分辨和控制电子超快动力学的实验研究提供理论参考。

非均匀强激光场下正负电子对的产生

随着强激光技术领域的不断突破,使得能量转化成物质的施温格过程的实验指日可待.在这样的背景下,在接近实际强激光的模型中理论研究施温格效应至关重要.本文采用量子场论模拟方法在非均匀电场中研究施温格效应的空间特性.首先,发现在能量保持不变的情况下电场的空间尺度显著影响正负电子对的产生率、动量和空间分布.正负电子对的动量和空间分布显示,当电场的空间尺度增大时,因为电场强度变小,所以产生的正负电子对无法及时被分开,抑制正负电子对的产生.最后,用连续性方程研究正负电子对的产生位置分布,发现正负电子对的产生位置分布与电场强度分布是对应的,电子和正电子的产生位置分离一个康普顿波长.此研究结果展示了施温格效应的空间特性,同时解释了弱电场下施温格效应的抑制机制.这些结论在人们常采用的均匀电场近似研究中无法得到.我们的研究对检测施温格实验中的正负电子对具有重要的指导意义.

Method of accurately calculating mean field operator in multi-configuration time-dependent Hartree-Fock frame

The accurate theoretical expressions of the mean field operator associated with the multi-configuration time-dependent Hartree-Fock （MCTDHF） method are presented in this paper. By using a theoretical formula, derived without approxima- tion, we can study the multi-electron correlation dynamics accurately. Some illustrative calculations are carried out to check the accuracy of the expression of the mean field operator. The results of illustrative calculations indicate the reliability of the accurate expression of the mean field operator. This theoretical calculation method for the mean field operator may be of considerable help in future studies of the correlated dynamics of many-electron systems in strong laser fields.

Creation and annihilation phenomena of electron and positron pairs in an oscillating field

The combination of an oscillating and a static field is used to study the creation and annihilation phenomena during the pair creation process.The time evolution,spatial density and momentum distribution of the created particles for a fermionic system are presented,which demonstrate that with the increasing static field intensity,the number of the created particles experiences a distinguishable decrease in every period of the oscillating field,which is caused by the annihilation phenomena between the created electrons and positrons.

磁场对激光驱动的喷流演化影响的二维数值研究

喷流的触发机制、准直传输和稳定性一直是天体物理学的研究热点.近年通过观测和实验室研究发现磁场在喷流准直传输和加速中起着关键作用.本文利用开源的辐射磁流体模拟程序FLASH对强激光驱动聚苯乙烯(CH)平面靶产生的靶前喷流进行了二维的数值模拟,系统地考察和比较了Biermann自生磁场以及不同方向、不同初始强度的外加磁场对喷流演化的动力学.模拟结果表明Biermann自生磁场不会影响喷流的界面动力学,而外加磁场对等离子体出流具有重定向作用,平行于靶前等离子体出流中心流向的外加磁场有助于喷流的产生和准直传输.其形成和演化过程是等离子体热压、磁压以及冲压三者相互竞争的结果.在受力方面,在喷流演化过程中等离子体热压梯度力和磁压力起决定性作用.研究结果为后续开展和喷流相关的实验研究提供借鉴,也有助于加深对天体喷流演化的理解.

储粮品质检测评价方法研究进展

品质检测评价是粮食储藏管理工作中的关键环节,一直是研究人员高度关注的热点研究领域,其在保障粮食储藏安全以及加工制品中发挥至关重要的作用。为了今后更好地开展相关工作,本文首先回顾常规基于光学、声学、介电、理化等特性和挥发性物质的储粮品质检测评价方法的研究进展;然后阐述当前新型检测技术如强磁场、激光诱导击穿光谱、核磁共振波谱以及扫描电子显微镜在储粮品质评价中的应用,并对比分析各种技术方法的优缺点;最后展望储粮品质检测评价方法的未来研究方向。

激光参数对非理想真空激光光强极限的影响

人类在实验室可实现的激光强度极限是强场量子电动力学(QED)的重要问题。在非理想真空条件下,极端超强激光与残留的电子相互作用触发伽马光子辐射与正负电子对产生的QED级联效应,从而显著消耗激光能量,大幅降低可实现的激光峰值强度。考虑到QED级联效应与激光偏振、焦斑尺寸、脉宽长度有着密切的关系,基于囊括QED过程的粒子网格模拟方法(Particle-in-cell, PIC)对上述参数的效应进行分析,同时构建了激光场演化的自洽方程来进行解释,二者结果基本保持一致,获得的强度极限在考虑的参数范围内为10^(26)~10^(27) W/cm^(-2)。结果表明,同等情形下,圆偏振激光可激发更强的QED级联,使得激光强度上限略低于线偏振。此外,紧聚焦激光由于QED级联发生的时空间尺度更小,从而激光的吸收效应被显著抑制,进而可以实现更强的聚焦强度。对于更长脉宽的激光,由于正负电子对吸收的能量区域更加弥散,使得可实现的激光强度上限阈值有所提升。但对于超短脉宽情形(如单周期),由于QED级联的种子源电子束不能很好地被约束在激光区域,理论分析耗散的激光能量偏高。此外,在高真空度的情形下,残余电子的随机性也会对可实现激光强度产生一定的影响。研究结果可为后续开展极端强场QED实验和数100 PW级超强超短激光装置建设提供指导。

对称分子H_(2)^(+)在强短波激光场中高次谐波椭偏率性质的研究

通过数值计算,研究了强短波(400—600 nm)激光场中H_(2)^(+)分子高次谐波辐射的椭偏率性质.研究表明,H_(2)^(+)分子在不同激光强度、不同激光波长、不同核间距及不同取向角下高次谐波的椭偏率性质是不同的;特别是在两中心干涉区,激发态在高次谐波产生中起着重要作用,但在不同取向角下,激发态对谐波椭偏率的影响不同;分析表明,这些不同的影响源于沿平行和垂直于激光偏振方向辐射的高次谐波的相对产量,以及激发态对平行和垂直谐波产量的影响;此外,椭偏率的测量检验了强场近似和平面波近似在强短波激光场中是成立的,并对强短波激光场中分子动力学做了更充分的研究.

基于激光等离子体的X/γ辐射研究进展

随着激光技术的不断发展,激光功率突破10 PW量级,激光与物质相互作用进入近量子电动力学(QED)范畴。从弱相对论激光到相对论激光再到强相对论激光,激光场与物质的耦合可以产生能量从keV到MeV甚至GeV的X/γ射线。这些辐射具有通量大、亮度高、能量高和脉宽短等特点,在核物理、高能量密度物理、天体物理等基础研究以及材料科学、成像、医学等领域具有广泛应用前景。系统梳理了近年来相对论强激光与气体、近临界密度等离子体及固体靶相互作用,通过诸如同步辐射、betatron和类betatron辐射、Thomson散射和非线性Compton散射过程等产生高能X/γ射线的最新研究进展,总结了各种方案产生的X/γ射线的品质因子和潜在应用,并为下一步基于强激光大科学装置的实验研究提供理论参考。

强激光驱动线圈靶磁场产生及应用研究进展

介绍了以强激光驱动电容线圈靶的实验方法产生磁场的基本模型及其发展过程。对比了实验室中常用的三种磁场诊断方法,包含:B-dot、法拉第旋转以及质子背光,发现前两种方法在实验中仅可以获得距离靶较远处的有限个磁场值,通过结合模拟工具获得靶处的磁场值与测量点的值跨越几个数量级,容易产生误差;质子背光诊断可以在实验中获得全局磁场信息,能够较好地满足线圈靶磁场诊断的需求。由于线圈靶磁场强且可持续时间长,在时空分布上具有一定可控性,因此我们将其应用到了磁重联的研究中,并成功获得了重联出流等特征。另外线圈靶在带电粒子的约束和磁流体动力学研究等多方面也得到了应用。

激光脉冲和电子束对撞准确度的测量方法

超强激光脉冲与相对论电子束相互对撞是当前主要的强场量子电动力学(QED)实验手段。如何测量超强激光脉冲和电子束对撞的准确度,进而实现微米精度的准确对撞,是目前限制实验发展的重要因素。利用蒙特卡罗数值模拟方法,系统研究了超强激光脉冲和相对论电子束相互对撞过程,重点关注了电子和辐射光子动力学信息与激光脉冲和电子束对撞偏移量之间的对应关系。研究发现:辐射光子的空间分布信息,可以有效反映出激光脉冲和电子束的对撞偏移量。基于该研究结果,实验中可利用光子空间分布的信息,实现对激光脉冲和电子束对撞准确度的调节,从而有望促进强场QED实验技术的发展。

强场液体太赫兹波研究进展

主要综述了近几年基于液体的强场太赫兹波研究进展,讨论了不同液体结构、激发方式以及不同液体种类等对太赫兹发射的影响,最后对液体太赫兹的发展进行了总结和展望.