超快强激光场中原子多光子电离的若干新效应研究
文献类型：学位论文
| 作者 | 白丽华 |
| 学位类别 | 博士 |
| 答辩日期 | 2006 |
| 授予单位 | 中国科学院上海光学精密机械研究所 |
| 导师 | 徐至展 |
| 关键词 | 多光子电离 多光子离解 光电子角分布 标度规律 位相相关现象 |
| 其他题名 | Study on several new effects of multiphoton ionization for atoms irradiated by ultra-fast laser fields |
| 中文摘要 | 超强超短激光与物质的相互作用是当今强场物理研究的一个重要前沿研究领域，新型超强超短激光为研究光与物质的相互作用提供了全新的实验手段和物理条件。超快强激光场中原子的多光子电离是强场原子物理研究中的一种重要的非线性效应，是强场物理领域中的一个研究热点，对此开展深入系统的研究对于人们了解强场原子物理的基本过程，了解原子的内部结构，发现新的物理规律都具有十分重要的科学意义。 本论文采用非微扰量子散射理论，对强激光场中原子超快多光子电离这一前沿课题展开了研究，取得了若干创新性研究成果，主要包括： 1. 研究了强激光场中原子阈上电离的光电子角分布，再现了国际上已有的实验观测，并阐明了光电子角分布的演化机制，推翻了国际上流行的将喷射结构与角动量相联系的解释。我们发现，光电子角分布的喷射结构由多光子电离的内禀特性造成，喷射结构与角动量无关。该研究不仅证实了已有的实验观测，而且还进一步发现当多吸收一个光子时，喷射结构的数目可能会增加三个、五个或其它奇数个，在考虑频率的变化时，喷射结构的数目甚至会出现减少的情况，从而证明了光电子的喷射结构与角动量无关。 2. 研究了线偏振强激光场中负离子的多光子离解现象，阐明了光电子角分布随激光强度和频率的变化规律，证实了最近实验观测中发现的“阈值效应”，确立了有质动力参数(有质动力能与光子能量的比值)在离解过程中的核心作用。我们发现激光强度和频率的变化引起了激光场的有质动力参数的变化，从而引起了光电子角分布的变化。 3. 确立了周期量级超短激光脉冲中多光子电离的标度规律，并提出了相应的实验观测方案。我们的研究表明在超短激光脉冲中，光电子角分布的主要特征由载波－包络(Carrier-envelope, CE)相位、单个脉冲包络中的光周期数、束缚参数、有质动力参数和电离过程中吸收的光子数这五个无量纲的参数决定。其中束缚参数指以激光光子能量为单位的原子束缚能。这是极端条件下的又一新的物理规律，它将周期量级超短激光脉冲中特有的位相相关现象与原子的结合能、超短激光脉冲的脉宽和强度以及光电子的能量结合起来，为不同激光脉冲中位相相关现象的实验观测指出了方向，具有重要的意义。 4. 研究了强激光场中光电子能谱的有质动力移动，再现了实验观测到的有质动力移动随光电子能量变化的现象。我们发现高能光电子主要在激光强度较高时产生，因而具有较大的有质动力移动，从而证明了激光场的强度分布是引起有质动力移动随光电子能量变化的主要原因。 5. 研究了单周期激光脉冲中H原子的多光子电离，展示了其中的位相相关现象。研究发现在圆偏振的单周期激光脉冲中，光电子角分布的最大值是一个定值，不随CE相位变化，但最大电离速率的位置随CE相位变化；光电子角分布关于与CE相位有关的轴对称。在线偏振的单周期激光脉冲中，光电子角分布是反演不对称的；最大电离速率随CE相位变化；光电子角分布关于激光偏振矢量对称。进一步研究发现单周期激光脉冲中光电子的角分布服从标度规律。 |
| 英文摘要 | The interaction of super-intense, ultrashort-short laser pulses with matters is one of the most important frontiers in modern physics. The availability of new type of ultra-short, super-intense lasers provides a new experimental tool and a physical condition in the interaction of laser with matters. Multiphoton ionization of atoms in ultra-fast, strong laser fields is an important non-linear effect and a hot topic in the domain of strong field physics. Deep and detailed study of multiphoton ionization is helpful for people to understand the basic process of atomic physics and the inner structure of atoms, and find new physical rules, which is of important scientific significance. In this dissertation, we apply a nonperturbative quantum scattering theory to study the frontier the ultra-fast multiphoton ionization of atoms. The main innovative results are as follows: 1. We study the photoelectron angular distributions (PADs) of atoms irradiated by strong laser fields. This study reproduces the related experimental observations, and gets the evolution mechanism of jets in the PADs. Our study shows that the jet-like structure of the PADs is caused by the inherent property of the ionization, and the jet-like structure is irrelevant to the photoelectron angular momentum. Our theoretical study confirms the experimental finding, and further predicts that when one-more-photon absorption, the number of jets may increase by three, five, or other odd numbers. Even with laser-frequency change, one less jet may also appear with one-more-photon absorption. 2. PADs from the multiphoton detachment of negative ions in strong linearly polarized laser fields are studied. Our studies disclose the dependence of the PADs on the laser intensity and laser frequency, confirm the “threshold effect” observed by the recent experiments, and establish the central role of the ponderomotive parameter (ponderomotive energy in units of photon energy) in the detachment process. We find that the laser intensity and laser frequency affect the ponderomotive parameter, which brings the change of the ponderomotive parameter, thus affects the PADs. 3. Establish a practial scaling law of the PADs in few-cycle ultra-short laser pulses. Possible experimental test to verify the scaling law is suggested. The scaling law denotes a fact that in ultrashort laser pulses, the main features of PADs are determined by five dimensionless numbers: the Carrier-envelope (CE) phase, i.e., the initial phase of the carrier wave with respect to the pulse envelope; the cycle number, i.e., the number of optical cycles in a single laser pulse envelope; the atomic binding number, i.e., the atomic binding energy in units of the laser photon energy; the ponderomotive number, i.e., the ponderomotive energy in units of the laser photon energy; and the number of absorbed photons. This is a new physical law under extreme conditions. It sets a relation between the phase-dependent phenomena in few-cycle laser pulses and the pulse duration, the binding energy of atoms, and the energy of photoelectrons, which is of great significance. 4. We study the pondermotive shift of energy spectrum of photoelectrons in strong laser pulses, and reproduce the experimental observation that the podermotive shift varies with the energy of photoelectrons. Our study finds that high energy photoelectrons are produced mainly at relatively higher laser intensities, with larger pondermotive shift, thus the intensity distribution of the laser field is responsible for the varying ponderomotive shifts. 5. The multiphoton ionization of H atoms in one-cycle laser pulses is studied. The phase-dependent phenomenon is investigated. Our study shows that in circularly polarized one-cycle laser pulses, the PADs are inversion asymmetric. The maximal ionization rate keeps constant for various CE phases, but position of the maximal ionization rate varies with the CE phase. The PADs are symmetric about an axis related to the CE phase. For linearly polarized one-cycle laser pulses, the PADs are inversion asymmetric; the maximal ionization rate varies with the CE phase; the PADs are always symmetric about the polarization. We further find that the PADs irradiated by one-cycle laser pulses satisfy the scaling law. |
| 语种 | 中文 |
| 源URL | [http://ir.siom.ac.cn/handle/181231/15567]  |
| 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 | 白丽华. 超快强激光场中原子多光子电离的若干新效应研究[D]. 中国科学院上海光学精密机械研究所. 2006. |
入库方式： OAI收割
来源：上海光学精密机械研究所
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