| 题名 | 强激光场中双原子分子的多光子电离研究 |
| 作者 | 任向河 |
| 学位类别 | 博士 |
| 答辩日期 | 2009 |
| 授予单位 | 中国科学院上海光学精密机械研究所 |
| 导师 | 张敬涛 ; 徐至展 |
| 关键词 | 多光子电离 光电子角分布 电离抑制 半Kapitza-Dirac效应 分子取向 载波包络相位 |
| 其他题名 | Study on Multiphoton Ionization of Diatomic Molecules in Intense Laser Fields |
| 中文摘要 | 强激光场中原子、分子的多光子电离是一种重要而基本的非线性效应，在过去的几十年中得到了广泛的研究。对多光子电离的研究，丰富了人们对强场物理过程的认识，加深了人们对微观世界的了解，同时也提供了一种探索微观物质世界的重要手段。双原子分子是最简单的一类分子，其多光子电离研究具有特殊的重要性。 在本论文中，我们根据非微扰的散射理论研究了强激光场中双原子分子的多光子电离，主要包括分子轴对光电子角分布的影响、分子电离抑制现象、分子的半Kapitza-Dirac效应以及周期量级激光脉冲的载波包络相位对光电子角分布的影响。本论文的主要创新性研究成果如下: 1、 研究了 H2+分子在强激光场中的多光子电离，展示了光电子角分布随分子取向和分子核间距的变化。研究发现在线偏振激光场中，光电子角分布通常情况下只保持反演对称，只有当分子轴与偏振方向垂直或者平行时，光电子角分布是四重对称的；由于由分子多核特性所引起的干涉效应，沿分子轴方向上的电离被抑制，这种干涉效应的影响随着核间距增大，而变得更加明显；圆偏振激光场中光电子角分布唯一地由光电子的末动量和分子轴决定，因此提出了一种根据光电子角分布来确定分子的取向和核间距大小的全新方案。 2、 研究了双原子分子在强激光场中的电离抑制现象，揭示了电离抑制现象的内在物理机制，展示了电离抑制现象随分子核间距、激光强度、分子取向的变化。研究发现电离抑制现象是由分子多核特性所引起的干涉效应造成的，这种干涉效应依赖于分子价电子的初始态。对于成键态分子，电离抑制现象可以在分子核间距比较大的情况下发生；对于反键态分子，电离抑制现象会在某一激光强度范围内消失。在线偏振激光场中，干涉效应对光电子能谱有显著的调制作用，而且这种调制作用依赖于分子相对于激光偏振方向的取向。 3、研究了驻波强激光场中 H2+分子的半Kapitza-Dirac效应，展示了光电子角分布的多重分裂现象和分子结构对光电子角分布的影响。研究发现当有质动力参数2up由小变大时， 光电子角分布的分裂角会变得越来越大，当2up超过一个整数时，有一对新的峰从分裂中心长出来，原来的峰仍然存在，角分布出现多重分裂； 如果分子轴平行或者垂直激光的传播方向，光电子角分布是关于分裂中心对称的， 对其他取向的分子，光电子角分布没有任何对称性，不对称程度随着分子取向变化；对于反键态分子， 分子取向明显改变光电子角分布的对称性；对于成键态分子，只有对高能光电子，分子取向才能显著地改变光电子角分布的对称性。 4、 研究了N2，O2分子在单周期超短激光脉冲场中的多光子电离，展示了超短激光脉冲的载波包络相位对光电子角分布的影响。研究发现光电子角分布的形状随着激光脉冲载波包络相位变化，而且最大电离率以及最大电离率所在的方向也随着激光脉冲的载波包络相位变化；激光脉冲的载波包络相位影响光电子角分布的对称性，如果激光电场分布的对称轴与分子对称轴平行或者垂直，光电子角分布还有对称性的，如果激光电场分布的对称轴与分子对称轴既不平行也不垂直，则光电子角分布没有任何对称，且主要沿着一个方向出射。这表明可以选择合适的脉冲载波包络相位，控制光电子的出射方向，实现分子在强激光场中的选择电离 |
| 英文摘要 | Multiphoton ionization is an important and basic nonlinear effect in the interaction of intense laser fields with atoms and molecules,which is widely studied in the past decades. Study on multiphotonionization of atoms and molecules enriches our knowledge on strongfield physics, deepens our understanding on material constructionand provides an important tool to explore micro matters. Thediatomic molecule is the most simple kind of molecule, of which multiphoton ionization has special importance. In this dissertation, we perform a detailed study, by means of anonperturbative scattering theory, on the multiphoton ionization of diatomic molecules in intense laser fields, including the effect of molecular axis on the photoelectron angular distributions (PADs), the ionization suppression, the half Kapitza-Dirac effect and the effect of the carrier-envelope phase on PADs. The main innovative results are as follows, 1、We study the multiphoton ionization of H2+ molecules irradiated by strong laser fields and show the variation of the PADs with the molecular orientation and the internuclear distance. We find, for linear polarization, the PADs are generally two-fold symmetric, while the PADs become four-fold symmetric when the internuclear vector is vertical or parallel to the laser polarization vector. Due to the interference effect stemming from the molecular multi-core nature, the ionization along to the molecular axis is suppressed. This interference effect becomes more obvious for larger internuclear distance. The PADs irradiated by circularly polarized laser are only determined by the momentum of photoelectrons and the internuclear vector. So we put forward an approach to measure the internuclear vector by the PADs irradiated by circularly polarized laser. 2、 We study the ionization suppression of diatomic molecules in intense laser fields and reveal the reasons resulting in ionization suppression. The ionization suppression means the ionization rate of molecule is lower than that of an atom with a comparable ionization potential. We show the influence of the internuclear distance, the laser intensity and the molecular orientation on the ionization suppression. The results show the ionization suppression of molecules results from the destructive interference effect induced by the multi-core nature of the molecules. The interference effect dependents on the initial states of molecular valence electron. For the molecules prepared in the bonding shells, the ionization suppression may occur for molecule with larger internuclear distance. For the molecules prepared in the antibonding shells, the ionization suppression will disappear at a certain intensity range. The interference effect modulates the photoelectron energy spectrum and the modulation depends on the molecular orientation with respect to the laser polarization. 3、We study the half Kapitza-Dirac effect of H2+ molecule in standing wave laser fields and show the multiple splitting of PADs and the effect of the molecular structure on PADs. We find that the splitting angle becomes larger as the increasing the pondermotive parameter up. Once the pondermotive parameter 2up exceeds an integer, the PADs have a couple of new peaks appearing from the central splitting then the multiple splitting occurs. If the molecular axis is either vertical or parallel to the laser propagation vector, the PADs are symmetry; while for other orientations, the PADs have not any symmetry. For molecules prepared in the antibonding shells, the molecular orientation changes the PADs distinctively; while for molecules prepared in the bonding shells, the PADs do not change much by the molecular orientation unless the photoelectron has high enough kinetic energy. 4、We study the multiphoton ionization of O2 and N2 molecules irradiated by one-cycle ultra-short laser pulses and show the effect of the carrier-envelope (CE) phase, the molecular orientation and the molecular symmetry on PADs. We find that the shapes of PADs vary with the CE phase and the maximal ionization rate, as well as the emission direction of photoelectrons, varies with the CE phases. The CE phase affects the symmetry of PADs. Generally, the PADs do not show any symmetry, but become symmetric about an axis only when the symmetric axis of the pulse envelope is vertical or parallel to the symmetric axis of molecules. This study shows that the few-cycle laser pulses can be used to steer emitting direction of the photoelectrons and perform the selective ionization of molecules by choosing an appropriate CE phase. |
| 语种 | 中文 |
| 内容类型 | 学位论文 |
| 源URL | [http://ir.siom.ac.cn/handle/181231/15265]  |
| 专题 | 上海光学精密机械研究所_学位论文 |
| 推荐引用方式 GB/T 7714 | 任向河. 强激光场中双原子分子的多光子电离研究[D]. 中国科学院上海光学精密机械研究所. 2009. |
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