【作者】 王春成；

【导师】 丁大军；

【作者基本信息】 吉林大学 ， 原子与分子物理， 2012， 博士

【摘要】 原子和分子与超快强激光场作用，呈现出的多光子电离，阈上电离以及双电离或多次电离等新奇的现象吸引了广泛的关注。实验上直接测量的分子的场致光电子动量谱，已经成为探索分子强场电离现象的重要工具。另外，符合测量技术在探测分子的解离双电离现象中表现出了得天独厚的优势。本论文中第一部分展示了乙烯分子（C₂H₄），乙烷分子（C₂H₆），苯分子（C₆H₆）和水分子（H2O）在超快强激光场中产生的光电子的二维动量分布，第二部分主要利用符合动量成像方法研究了甲酸分子的解离性电离现象，论文的最后部分将就反应显微成像谱仪（reaction-microscope）的原理、技术和应用做一介绍。这三部分工作的主要结果摘要如下。1.实验上利用电子飞行时间谱仪结合匀速改变激光偏振方向的方法测量了乙烯分子，乙烷分子和苯分子在强飞秒激光场中产生的重散射光电子谱（RescatteringPhotoelectron Spectroscopy, RPS）。根据Lin发展的QRS（quantitative rescattering）理论，我们从实验得到的RPS中提取了自由电子被部分准直（partially aligned）的母体离子弹性散射后的大角度离子-电子弹性微分散射截面（Differential CrossSections, DCSs）。乙烯的DCSs在返回电子动量范围0.55a.u.⁰.8a.u.内呈现平滑的分布，而在0.8a.u.¹.05a.u.的范围内则过渡为在激光偏振方向有最大值。我们应用从头算理论，结合利用弱场渐进理论（weak-field asymptotic theory）计算的角度依赖的电离速率，独立的计算了部分准直的乙烯离子-自由电子的DCSs，计算结果很好的重现了实验结果，证明了从RPS中提取的乙烯离子-自由电子DCSs是准确的，采用的方法是可信赖的。实验结果表明DCSs展现了对角度相关的分子电离速率的明显依赖，说明这种方法在未对母体分子单独进行准直操作的情况下获取了分子在强场中的电离速率的角度依赖关系。同时计算了分子坐标下的DCSs（MFDCSs），不同的电子入射角导致不同的散射电子分布，这很好的解释了电子动量从低到高变化下不同的DCSs的分布。实验上萃取了返回电子动量在0.55a.u.¹.15a.u.范围内的的部分准直的乙烷离子-电子弹性DCSs，此DCSs分布与乙烯分子不同，在整个动量范围内，DCSs均在激光偏振方向有最大值，定性的分析认为这种DCSs的分布是简并的最高占据轨道（HOMO）和HOMO-1对RPS的共同贡献导致的。我们第一次在实验上测量了环状分子苯的二维RPS分布，并从RPS中提取了部分准直的苯离子-电子的DCSs，随着返回电子动量在0.65a.u.⁰.9a.u.范围内增加，对应的DCSs的最小值分布从180°（0.65a.u.）逐渐变为110°（0.9a.u.）。此种分布可能源于苯分子HOMO的型轨道所导致的干涉相消效应。综合乙烯，乙烷及苯的离子-电子DCSs的分布，证明了我们所用的方法可以成功扩展到复杂的分子体系中，并且与HOMO（或者HOMO-1）的对称性密切相关。这些工作为利用RPS方法对复杂分子体系进行自成像进行了的探索。实验研究了乙烯分子，乙烷分子及水分子的低阶ATI电子的动量分布。我们主要分析每个ATI峰的角分布中出现的最大值和最小值交替出现的被称为jets-like结构的分布。乙烯分子的ATI电子动量角分布中呈现出清晰的jets-like结构，我们认为以高里德堡态为中间态的共振电离过程（e.g.（8+1））导致了此jets-like结构产生，同时在较高光强下出现了共振电离通道转移现象。乙烷分子的低阶ATI电子角分布呈现复杂的分布，没有清晰的jets-like结构出现，证明在同一光强下乙烷阈上电离过程中同时存在两个能量相近的共振电离通道。水分子的低阶ATI电子动量分布中出现了两系列独立的ATI能谱峰，每个能谱峰对应电子的角分布都包含清晰的jets-like结构，在实验上确定了角分布中最大值（jets）的数目。实验发现每多吸收一个光子，对应能谱峰中的jets数目就增加一个，但是两系列中的初始峰的jets数目不同。此现象可以利用微扰理论解释，定性上认为分别通过具有不同量子数的两个高里德堡态作为中间态进行的共振电离导致了这两系列ATI电子角分布的jets-like结构的分布特征。2.利用离子飞行时间谱仪结合位置灵敏探测器组成的符合测量设备，研究了甲酸分子在强激光场中的解离单电离及解离双电离现象。从离子-离子符合动量谱中，我们提取出了甲酸分子的主要的双体解离通道，在实验光强范围内，通过分析经库仑爆炸产生的离子对（OH⁺+HCO⁺）的离子产率和动能释放的激光偏振依赖和光强依赖关系，确定了甲酸分子的双电离机制是在特殊位置Rc处发生的共振增强连续双电离机制。实验上确定了在甲酸分子的强场电离过程中存在包含氢转移过程的解离双电离通道。另外，研究了来自于解离单电离过程的OH⁺和HCO⁺的离子产率的光强依赖关系，验证了OH⁺和HCO⁺来自于一价母体离子不同的激发态。文中展示了通过解离性单电离和解离性双电离过程产生的离子碎片的角分布，它们呈现出的各向异性的角分布主要源于分子电离速率的角度依赖关系。3.在本论文的第三章中详细描述了新搭建的反应显微成像谱仪的原理，技术和应用。利用此谱仪测量了惰性气体原子的离子和电子成像，说明了谱仪的各项性能能够满足实验要求。利用离子-电子符合测量方法，研究了乙烷分子在强飞秒激光场中的电离解离行为，确定了乙烷分子的主要的双体解离通道，并根据离子对的动能释放完成了离子能谱标定。更多还原

【Abstract】 The interactions between molecules and ultrafast intense laser field manifest manyinteresting phenomena, such as multiphoton ionization, above-threshold ionization（ATI）, double ionization or multiple ionization. Laser-induced photoelectronmomentum spectra provids a powerful tool for exploring various ionization processes inintense laser field. The coincidence momentum imaging method offers a great potential forinvestigating the dissociative double ionization of molecule. The first part of thisdissertation explores the two dimensional photoelectron momentum spectra of C₂H₄, C₂H₆,C₆H₆and H2O in intense femtosecond field. Tthe second part, using the ion-ioncoincidence momentum imaging method, is devoted to an investigation of the dissociativeionization process.1. Measurement of the high-order ATI photoelectron momentum spectra（so calledRescattering Photoelectron Spectroscopy, RPS） of C₂H₄, C₂H₆and C₆H₆. We measure theangle resolved RPS spectra with a264mm long linear time-of-flight spectrometer. Thepolarization direction of the light is rotated using a/2plate. Basing on the quantitativerescattering（QRS） theory, we can extract from the experimental RPS the large-angleelastic differential cross sections（DCSs） for scattering of free electrons by the partiallyaligned C₂H₄⁺, C₂H₆⁺and C₆H₆⁺. For the returning electron, the DCSs of C₂H₄⁺werefound to exhibit the rather flat distribution in the momentum range of0.55a.u⁰.8a.u.,except at the higher momentum region（0.8a.u.¹.05a.u.）, the DCSs show a clearmaximum at180degree which is parallel to the laser polarization. The theoretical DCSsare calculated by the ab initio method for free electrons. The calculation is performedusing the Schwinger configuration interaction method. The experimental and theoretical DCSs agree well which confirms the validity of the extraction procedure in the experiment.The sensitivity to the partially alignment in the signal results from the angle-dependentionization rate. The calculated Molecular Frame DCSs（MFDCSs） are used to explainthe modulation of the DCSs in the low collision momentum region. We extracted the ion-electron elastic DCSs of C₂H₆⁺of the momentum range0.55a.u.¹.15a.u.. Strikingly,the DCSs have a very clear maximum at180degree for all momentum in this range, whichis different compared to that of C₂H₄⁺. The highest occupied molecular orbitals（HOMOand HOMO-1） of C₂H₆are orbitals of degenerate type and type. Qualitatively, webelieve that both orbitals simultaneously contribute to the RPS, which explain thecharacteristic DCSs distribution of C₂H₆⁺. For first time we extended our measurement ofRPS to the cyclic molecule C₆H₆. As increasing the momentum from0.65a.u. to0.9a.u.,the DCSs of C₆H₆⁺have a minimum at180degree for0.65a.u., which changes to110degree for0.9a.u.. Since C₆H₆has type HOMO orbital, the destructive interference maylead to this minimum evolution in the DCSs. In summary, these results show that ourextraction method can be applied to polyatomic molecules and hence pave the way forapplication of RPS as a tool to probe the structure of more complex molecular systems.In addition, we also measured the low-order ATI photoelectron momentum spectra forC₂H₄, C₂H₆and H2O. We focused on the so called jets-like structures which arecharacterized by appearing the maximum and minimum alternately in the angulardistribution of each ATI peak. The significant jets-like structures were observed in firstthree ATI peaks for C₂H₄. We believe that resonance ionization（e.g.,（8+1） photonresonance ionization）, intermediating with the high Rydberg state leads this jets-likedistribution. A channel switching phenomenon turned out at a higher peak laser intensity.No clear jets-like structures have been seen for C₂H₆, this is because two resonancechannels contribute to this ATI process for same laser intenstiy and the energy differencebetween them is quite small. We found two discrete series of ATI peaks to appear in the photoelectron momentum spectra of H₂O. The clear jets-like structures were seen for eachATI peak. We extracted the number of maxima（jets number） in each jets-like ofstructure. The jets number increases one when one more photon is absorbed for both series.The jets number for first ATI peak was found to differ between the two series. We believethat these two series are born out different resonance channels intermediate with differenthigh Rydberg states. These two Rydberg states carry different quantum number.2. The dissociative single ionization and double ionization of formic acid in intenselaser field also were explored. Our results obtainded with the coincidence momentumimaging method, confirm that the enhanced sequential double ionization at Rc playsimportant roll, and securely identify that the hydrogen migration occur in the dissociativedouble ionization of formic acid. We explore the intensity dependent ion yields of OH⁺and HCO⁺originating from singly charged formic acid. The results indicate that bothfragments dissociate from electronical excited state of parent ion. The fragments born outfrom singly and doubly charged formic acid both show anisotropic angualr distributions. Aqualitative explanation for those anisotropic distributions is for the angle-dependentionization rates.3. The design and major parameter of new reaction-microscope were described. Theimages of ions and electrons producing from the ionization of rare gas were shown. Thereasonable images prove that the reaction-microscop work smoothly and satisfy therequirement of our research. The dissociative ionization of ethane was explored usingion-electron coincidence method. The main two-body Coulomb explosion channel hasbeen identified and the kinetic energy release of this channel was used to normalize the ionenergy spectrum.更多还原