Recent developments in laser technology, in particular the advances in high-harmonic generation, enable the generation of ultrashort extreme ultraviolet (XUV) pulses with attosecond (1 as = 10-18 s ) duration. Such tools open the opportunity to study electron dynamics in atoms and molecules on its intrinsic time scale. As an example, the attosecond streaking technique was recently applied to time resolve the photoionization process in atomic and solid systems. In this technique, an isolated attosecond XUV pulse, that ionizes the electron in the target system, is superimposed with a few-cycle streaking pulse (usually of near-infrared wavelengths). The streaking pulse modulates the final momentum (or energy) of the photoelectron. The measured streaking trace, i.e., the final momentum (or energy) as a function of the relative delay between these two pulses, contains time information of the photoionization process. By comparing two streaking traces measured for photoionization from the 2s and 2p orbitals in a neon atom, Schultze et al. [Science 328, 1658 (2010)] found a temporal offset of 21+/-5 as between them and interpreted this value as the time delay between photoionization from the 2s and 2p orbitals. This experiment has initiated a debate among theoreticians, in particular about the origin of the measured time delay. A correct interpretation of the delay is extremely important for our understanding of the attosecond streaking technique and an exact analysis of time resolved measurements of this and other ultrafast processes.;In this thesis we systematically study the attosecond time delays in photoionization using numerical simulations. We first propose a new method, based on the fundamental definition of a time delay, to theoretically study the photoionization process induced by an XUV pulse from a time-dependent perspective. We then turn to analyze the time delays measured in streaking experiments. Our results show that for single-photon ionization the observed streaking time delay arises from the finite-range propagation of the photoelectron in the coupled field of the ionic potential and the streaking pulse. Consequently, we conclude that the photon absorption occurs instantaneously at the center of the XUV pulse, i.e., with no time delay. Our analysis further reveals that the streaking time delay can be interpreted as a finite sum of piecewise field-free time delays weighted by the relative instantaneous streaking field strength and provides itself as a useful tool for imaging the presence of an additional potential located at a distance from the ionic core. We finally extend our time delay studies to the two-photon ionization process and show that the absorption time delay is significantly different for nonresonant and resonant two-photon ionization. Our results imply that the absorption of two photons in the nonresonant case occurs instantaneously, without time delay, at the center of the XUV pulse. However, in the resonant scenario we find a substantial absorption time delay that changes linearly with the duration of the XUV pulse. Our further theoretical analysis shows that this absorption time delay can be related to the phase acquired by the electron during its transition from the initial ground state to the continuum.
展开▼
机译：激光技术的最新发展，特别是高谐波产生的发展，使得能够产生持续时间为A秒（1 as = 10-18 s）的超短极紫外（XUV）脉冲。这样的工具为研究原子和分子在其固有时间尺度上的电子动力学提供了机会。例如，最近应用了阿秒条纹技术来解决原子和固体系统中的光电离过程。在这种技术中，一个隔离的，在目标系统中使电子离子化的，以秒为单位的XUV脉冲与几个周期的条纹脉冲（通常是近红外波长）叠加在一起。条纹脉冲调制光电子的最终动量（或能量）。测得的条纹痕迹，即作为这两个脉冲之间的相对延迟的函数的最终动量（或能量），包含光电离过程的时间信息。通过比较从氖原子的2s和2p轨道测得的两个条带痕迹，了解它们的光电离，Schultze等人。 [Science 328，1658（2010）]发现它们之间存在21 +/- 5的时间偏移，并将此值解释为从2s和2p轨道进行光电离之间的时间延迟。这项实验引起了理论家的争论，尤其是关于所测量的时延的起源。正确地解释延迟对我们理解阿秒条纹技术和准确解析此和其他超快过程的时间分辨测量非常重要。在本文中，我们使用数值模拟系统地研究了电离中的阿秒时间延迟。我们首先提出一种基于时间延迟的基本定义的新方法，从时间相关的角度理论上研究XUV脉冲引起的光电离过程。然后，我们转向分析在裸奔实验中测得的时间延迟。我们的结果表明，对于单光子电离，观察到的条纹时间延迟是由光电子在离子电势和条纹脉冲耦合场中的有限范围传播引起的。因此，我们得出结论，光子吸收在XUV脉冲的中心即刻发生，即没有时间延迟。我们的分析进一步表明，条纹时间延迟可以解释为由相对瞬时条纹场强度加权的分段无场时间延迟的有限总和，并提供了自身作为对位于远处的附加电势进行成像的有用工具来自离子核最后，我们将时延研究扩展到了双光子电离过程，并显示了非共振和共振双光子电离的吸收时间延迟显着不同。我们的结果表明，在非共振情况下，两个光子的吸收在XUV脉冲的中心瞬间发生，没有时间延迟。然而，在共振情况下，我们发现吸收时间的延迟会随着XUV脉冲的持续时间线性变化。我们进一步的理论分析表明，这种吸收时间延迟可能与电子从初始基态到连续态跃迁期间获得的相位有关。
展开▼
