Ionization dynamics of helium clusters and droplets in intense short-wavelength light pulses

短波长辐射的强脉冲和超短脉冲与物质的相互作用开启了从原子和分子到等离子体物理的令人兴奋的研究机会。随着自由电子激光(FEL)和强高次谐波(HHG)激光光源在极紫外区(XUV)的出现，这一领域得到了又一次强有力的推动，因为这些光源为高空间分辨率的超快动力学成像提供了巨大的前景。这项研究的目的是详细了解纳米粒子在从阿秒到纳秒的时间尺度上的强XUV光脉冲中复杂的超快电离动力学。弹性光散射将是关键的实验方法，因为它允许拍摄粒子中的激发和动力学的准瞬时快照。由于其简单的电子结构，氦团簇、纳米和微滴以及液体喷射将作为拟议工作的模型系统。位于的里雅斯特的种子费米探测器，由于其窄带宽和高可调谐性，为详细研究He团簇中的共振和非共振电离过程提供了独特的可能性。在第一个资助期成功的费米实验的基础上，我们将在那里继续我们的计划，并将我们在Max-Born-Institute的研究扩展到阿秒区域，以跟踪激发期间的超快动力学。我们计划研究电离过程、纳米等离子体的形成以及激发态和电荷态的空间分布。飞秒到纳秒范围内的时间分辨测量将在单次激发下对单个星团进行，从而克服了由于平均不同星团大小和功率密度而造成的共同限制。在费米，我们将使用我们最近开发和测试的双色泵浦探测衍射方法来成像团簇中的光诱导过程。通过选择激发(即共振激发到束缚态或连续态)，表面或体原子可以被具体地处理，以发展对团簇中的共振/非共振衍射的理解。光谱宽的阿秒脉冲的衍射成像将会带来新的突破。为了进入阿秒状态，我们将开发一种氦微滴和液体射流的来源，即使在最短的脉冲下也能提供足够的信号。实验数据的分析和建模将一如既往地与理论小组密切合作，提供纳米颗粒中超快激发、电离和电子动力学的非常详细的图景。

The interaction of intense and ultrashort pulses of short-wavelength radiation with matter has opened exciting research opportunities ranging from atomic and molecular to plasma physics. With the advent of free-electron lasers (FELs) and intense high harmonic (HHG) laser-based sources in the extreme ultraviolet regime (XUV), the field got another strong push since these sources offer great prospects for imaging of ultrafast dynamics with high spatial resolution. The aim of the proposed study is to obtain a detailed understanding of the complex ultrafast ionization dynamics of nanoparticles in intense XUV light pulses on timescales from attoseconds to nanoseconds. Elastic light scattering will be the key experimental method as it allows for taking quasi-instantaneous snapshots of the excitation and dynamics in the particles. Thanks to their simple electronic structure helium clusters, nano- and microdroplets as well as liquid jets will serve as model systems for the proposed work. The seeded FEL FERMI in Trieste, with its small bandwidth and high tunability, offers unique possibilities to perform detailed studies of resonant and non-resonant ionisation processes in He clusters. Based on the successful FERMI experiments of the first funding period, we will continue our program there and extend our studies at the Max-Born-Institute into the attosecond regime, in order to follow the ultrafast dynamics during excitation. We plan to address the ionization processes, nanoplasma formation and the spatial distribution of excitation and charge states. The time-resolved measurements in the femtosecond to nanosecond regime will be performed on single clusters in single shots, thereby overcoming the common limitation due to averaging over different cluster sizes and power densities. At FERMI, we will use our recently developed and tested approach of two-color pump probe diffraction to image the light-induced processes in the clusters. Via selective excitation (i.e. resonant excitation into bound or continuum states) the surface or bulk atoms can be specifically addressed to develop an understanding of resonant/non-resonant diffraction in the cluster. Diffraction imaging with spectrally broad attosecond pulses is going to break new ground. To enter the attosecond regime, we will develop a source for helium microdroplets and liquid jets which will provide sufficient signal even with the shortest pulses. The analysis and modeling of the experimental data will be carried out as before in close collaboration with theory groups, delivering a very detailed picture of the ultrafast excitation, ionization, and electron dynamics in nanoparticles.