Investigation of charging dynamics in atomic clusters on an attosecond timescale

原子团簇是令人着迷的研究对象，可以被视为复杂系统（如生物分子）的模型。强光脉冲与团簇的相互作用导致团簇的高电荷和膨胀，随后是完全解体。对低频光脉冲与星团相互作用过程的理解还远未完成。预计由激光场驱动的多光子电离和电子冲击电离是最主要的过程。该项目旨在大幅改善集群充电的情况。在第一步中，将使用光电子能谱技术研究簇电离，使用少周期近红外和中红外脉冲。这样，通过改变脉冲长度，将获得关于多光子电离和激光驱动的电子碰撞电离之间相互作用的有价值的信息。预计这两个过程的重要性取决于脉冲长度和电离波长。第二步，旨在实时研究团簇电离。在高次谐波源发展的推动下，现在可以产生脉冲持续时间为阿秒（1阿秒=10-18s）的光脉冲，这是电子的自然时间尺度。这些脉冲将作为一种探针，用于研究近红外脉冲电离过程中原子团簇中的电荷过程。由阿秒脉冲产生的光电子会经历一个能量下移，这取决于当前的簇电荷。能量下移将在近红外和阿秒脉冲之间的不同延迟进行测量，从而允许对团簇电荷进行时间分辨观察。这些迄今为止无法获得的信息将是理解簇和扩展系统中的电子动力学的巨大一步。

Atomic clusters are fascinating objects of research and can be regarded as a model for complex systems such as biomolecules. The interaction of an intense light pulse with a cluster leads to high electric charging of the cluster and its expansion, followed by a complete disintegration. The understanding of processes taking place during the interaction of a low-frequency light pulse with a cluster is far from complete. It is expected that multiphoton ionization and electron impact ionization driven by the laser field are the most dominant processes. This project aims to substantially improve the picture of cluster charging. In a first step, cluster ionization will be studied with photoelectron spectroscopy techniques using few-cycle near-infrared and mid-infrared pulses. In this way, and by varying the pulse length, valuable information will be obtained about the interplay between multiphoton ionization and laser-driven electron impact ionization. The significance of the two processes is expected to depend both on the pulse length and the ionization wavelength. In a second step, it is aimed to study cluster ionization in real time. Driven by the development of high-order harmonic sources, it is now possible to generate light pulses with attosecond (1 attosecond =10-18s) pulse duration, which is the natural time scale of electrons. These pulses will be used as a probe for the investigation of charging processes in atomic clusters during ionization with intense near-infrared pulses. Photoelectrons generated by the attosecond pulse experience an energy downshift that depends on the current cluster charge. The energy downshift will be measured at different delays between the near-infrared and the attosecond pulse, allowing the time-resolved observation of the cluster charge. This so far unaccessible information will be a huge step in the understanding of electron dynamics in clusters and extended systems in general.