超快强光场中的分子隧穿电离及其诱导产生的相干激射研究
文献类型：学位论文
| 作者 | 李贵花 |
| 学位类别 | 博士 |
| 答辩日期 | 2014 |
| 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 院士，程亚 研究员 |
| 关键词 | 分子隧穿电离 高次谐波 电离抑制 相干激射 时空聚焦 |
| 其他题名 | Tunnel Ionization of Molecules in Ultrafast Intense Laser Fields and Generation of Laser by Tunnel Ionization of Molecular Nitrogen |
| 中文摘要 | 在超快强光场条件下，光与物质相互作用可导致一系列高度非线性、非微扰的物理现象的产生，成为当代物理学研究的重要前沿。在足够强的光强下，激光场的电场强度与原子分子内部的库仑场强度已可比拟，此时电子有一定的概率通过隧穿过程被电离。隧穿电离是强场与物质相互作用过程中最重要的量子过程之一，也是众多强场物理新现象的基础，如阈上电离、高次谐波产生、分子电离解离等。因此，长期以来强场隧穿电离一直是国际强场物理领域的热点之一。 近年来，随着研究的不断深入，强场隧穿电离的关注对象已逐渐从最初的单电子原子体系，逐渐推广到多电子原子分子体系，为强场领域注入了新的活力。然而，分子隧穿电离研究还有许多亟需解决的问题，例如：面向多原子、多电子体系的隧穿电离理论模型仍未建立、分子电离中的多轨道干涉效应有待完善、隧穿电离诱导产生的相干激射机制仍未辨明。因此，深入研究分子隧穿电离不仅具有重要的科学意义，也促进分子成像、超快化学动力学操控等重要应用的发展。 基于上述背景，本论文着眼于超短强光场中分子隧穿电离以及基于分子隧穿电离的若干新现象、新效应，揭示分子隧穿电离过程的量子相干效应的物理图像，分子隧穿电离诱导相干激射的产生机制以及微观超快动力学操控过程。主要工作和创新成果如下： 1. 利用中红外可调谐飞秒光源研究双原子分子隧穿电离过程中量子干涉效应对高次谐波的影响，发现O2高次谐波谱截止能量高于Xe，并具有奇特的波长依赖关系。研究发现这种截止能量拓展现象来源于O2的电离抑制，理论上澄清了双原子分子O2的电离抑制的物理机制源于O2分子的不同氧原子核电离的电子波包的量子相消干涉。 2. 利用取向依赖的分子荧光信号来探测分子低能占据轨道隧穿电离信息，并成功获取CO2分子低能占据轨道（HOMO-1，HOMO-2）的空间结构信息，为分子低能占据轨道成像提供了一种有效的全光学方法。 3. 基于双色场泵浦-探测装置，在低气压下（~25mbar）研究氮气隧穿电离诱导的相干激射时间包络特性随等离子通道长度的变化趋势，观察到相干激射脉冲中存在时间包络振荡的新现象。该研究结果表明，上述氮分子离子中的相干激射可能源于种子触发产生的超辐射。 4. 利用时空聚焦方式产生的新型光场，通过在中心对称的原子或分子气体中诱导隧穿电离，获得高信噪比的二次谐波。通过进一步理论分析，证实了二次谐波的产生来源于时空聚焦光场产生的陡峭等离子体密度梯度。 |
| 英文摘要 | Interaction of ultrafast intense laser fields with matter has led to a series of interesting highly nonlinear, non-perturbative physical phenomena and making it one of the most significant frontiers in modern physics. Particularly, when the strength of electric field of laser becomes comparable to the Coulomb field of atom/molecule, the bound electron can have a chance to escape from the distorted Coulomb potential, resulting in the well known tunnel ionization. Tunnel ionization is not only an important and fundamental quantum process in the strong field laser physics, but also the initiator of many interesting highly nonlinear phenomena, such as above-threshold ionization, high-order harmonic generation, molecules dissociation, and so on. Therefore, invesitigation of tunnel ionization of atoms and molecules is vital for better understanding of strong field physics, and opens new possibilities for many practical applications. Recently, tunnel ionization gradually shifts from atoms governed by single-electron behaviors to more complex atomic and molecular systems which involve multielectron, multi-center, or even multi-channel interaction effects. Along this direction, many remarkable achievements, such as molecular orbital imaging, quantum interference effects in molecules, probing of ultrafast molecular dynamics, have been made during the past decade. Meanwhile, there are still many problems to be solved. For example, the theoretical model of tunnel ionization in multi-electron systems is to be established, and a consensus on the underlying mechanism of a free-space laser generated by strong-field ionization of molecular nitrogen has not been reached yet. Therefore, further investigations on tunnel ionization and explorations of its applications will remain to be anactive subject of strong field physics in the future. Based on these considerations, in this thesis we focus on several new phenomena and effects of the tunnel ionization of molecules, including indentification of ionization suppression in HHG in different kinds of molecular gases, mapping of underlying orbitals in molecules with tunnel ionization induced alignment-dependent fluorescence emission, second harmonic generation in centro-symmetric gases with spatiotemporally focused laser, and generation of free-space laser in tunnel ionized molecular nitrogen. The main results and innovations of this thesis are listed as follows: 1. We experimentally investigate the influence of quantum interference effects on high-order harmonic generation in diatomic molecules with mid-infrared femtosecond laser pulses and find out that the cutoff extension of O2 as compared to that of Xe shows a peculiar dependence on driving wavelengths, which origins from the ionization suppression. Besides, our theoretical analysis clarifies that two-center interference model should be responsible for such ionization suppression. 2. The ionization probabilities of the lower-lying orbitals of molecules are measured by experimentally detecting alignment-dependence of fluorescence emission, and then the structure imformation of lower-lying orbitals (HOMO-1, HOMO-2) for CO2 is obtained. This all-optical approach provides a potential of retrieval of geometrical information of lower-lying orbitals in a simple manner. 3. Based on the two-color pump-probe scheme, we systematically investigate of temporal characteristics of coherent lasing by experimentally examining its evolution with the increase of the plasma channel induced by the tunneling ionization of nitrogen molecules at a pressure of 25 mbar. We observe unexpected temporal oscillations in the 391-nm laser. This finding suggests that the free-space laser in tunnel ionized molecular nitrogen may originate from the triggered superradiance. 4. Unexpectedly strong second harmonics is generated in tunnel ionized atomic/molecular gas by use of spatiotemporally focused femtosecond laser pulses. Further theoretical calculations demonstrate that second harmonics mainly originates from the steep gradient of nonuniform plasma dynamically controlled by the spatiotemporally focused laser field. |
| 语种 | 中文 |
| 源URL | [http://ir.siom.ac.cn/handle/181231/15886]  |
| 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 | 李贵花. 超快强光场中的分子隧穿电离及其诱导产生的相干激射研究[D]. 中国科学院上海光学精密机械研究所. 2014. |
入库方式： OAI收割
来源：上海光学精密机械研究所
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