Attosecond-pump attosecond-probe inner-shell spectroscopy

多电子动力学和电子关联在包括原子、分子、纳米结构和固体在内的几乎所有物理系统中都具有重要的基础意义。俄歇衰变是电子相关过程的一个纯粹的例子，通常发生在几飞秒的时间尺度上。为了在时域上直接研究这些过程，需要极短的激光脉冲。虽然最简单的俄歇过程是由于两个电子之间的相关性而被很好地理解，但涉及两个以上电子的俄歇过程就不能这么说了。本课题的目标是通过开发阿秒-泵-阿秒-探针内壳光谱的新方法来研究氙中的多电子俄歇过程，该方法需要产生强烈的阿秒脉冲。为此，我们将采用最近开发的极紫外线（XUV）强度缩放方案，使用18米长的光束线进行高谐波产生（HHG）。利用几飞秒近红外脉冲在氖中产生高谐波，目标是在90 eV区域产生强烈的孤立阿秒脉冲。两个能量选择的XUV脉冲将由一个分裂和延迟单元产生。从阿秒泵浦脉冲中吸收第一个XUV光子会引起氙的内壳空位。然后用延迟的阿秒探测脉冲探测这个内壳空位的弛豫。利用不同的探针脉冲光子能量，可以研究单、双俄歇过程、俄歇级联过程以及双核孔态的弛豫。所得结果将为描述电子相关过程的理论模型提供一个基准。阿秒-泵-阿秒-探针内壳光谱技术可用于极短时间尺度下原子、分子和固体中的电子动力学研究。

Multi-electron dynamics and electron correlation are of fundamental importance in almost all physical systems including atoms, molecules, nanostructures and solids. Auger decay is a pure example of an electron correlation process and typically takes place on a time scale of a few femtoseconds. To study these processes directly in the time domain, extremely short laser pulses are required. While the simplest Auger processes as a result of the correlation between two electrons are comparably well understood, the same cannot be said for Auger processes involving more than two electrons. The goal of this project is to study multi-electron Auger processes in xenon by developing the new method of attosecond-pump attosecond-probe inner-shell spectroscopy, which requires the generation of intense attosecond pulses. To this end, we will apply a recently developed extreme-ultraviolet (XUV) intensity scaling scheme for high-harmonic generation (HHG) using an 18-meter-long beamline. High harmonics will be generated in neon using few-femtosecond near-infrared pulses with the goal to generate intense isolated attosecond pulses in the 90 eV region. Two energy-selected XUV pulses will be generated by a split-and-delay unit. An inner-shell vacancy in xenon will be induced by the absorption of a first XUV photon from the attosecond pump pulse. The relaxation of this inner-shell vacancy will then be probed by the delayed attosecond probe pulse. By using different probe pulse photon energies, it will be possible to study single and double Auger processes, Auger cascade processes as well as the relaxation of double core-hole states. The obtained results will provide a benchmark for theoretical models describing electron correlation processes. The attosecond-pump attosecond-probe inner-shell spectroscopy technique can be used in the future for the investigation of electron dynamics in atoms, molecules and solids on extremely short timescales.