Fully differential studies for two-photon non-sequential double ionization of helium using a reaction microscope at FLASH

氦的双光子诱导双电离（TP-DI）对推进非线性理论和探索电子-电子相关性，特别是对非顺序电离具有决定性和基础性的重要性。利用汉堡的自由电子激光器（FLASH），世界上唯一运行的第四代辐射源，在超短脉冲持续时间（~30秒或更短）下提供前所未有的强度（~1013-1016 W/cm2），结合最先进的多粒子检测系统-海德堡反应显微镜，我们首次研究了光子能量为44 eV（先前提议的目标之一）的He2+反射电离子动力分布（RIMD）。结果表明，两个电子的优先背靠背发射具有几乎相等的能量共享，这与我们的全微分测量在44 eV下观察到的Ne的顺序电离有明确的区别。然而，目前的RIMD不能最终对不同最近先进计算预测之间的微分和总横截面的显著偏差进行基准测试，因此，高度要求对He的直接TP-DI进行完全微分测量，这是本提案的最终目标。这些研究对于理解更复杂的情况是必不可少的，当fel辐射在极紫外状态下与许多电子原子，分子，簇，表面或大块物质相互作用时。

Two-photon induced double ionization (TP-DI) of helium is of decisive and fundamental importance to advance non-linear theories and explore electron-electron correlations, particularly for non-sequential ionization. Using the free electron laser (FEL) at Hamburg (FLASH), the only fourth generation radiation source operational world wide, delivering unprecedented intensities (~1013-1016 W/cm2) at ultra-short pulse durations (~30 fs and less) in combination with the most advanced multi-particle detection system - the Heidelberg reaction microscope - we studied for the first time He2+ recoil-ion momentum distributions (RIMD) at a photon energy of 44 eV, one of the goals of the previous proposal. The results represent evidence for preferential back-to-back emission of two electrons with almost equal energy sharing, which unambiguously distinguishes it from sequential ionization of Ne observed at 44 eV by our fully differential measurements. However, the present RIMD can not finally benchmark significant deviations in differential as well as total cross sections between the predictions of different recent advanced calculations and, therefore, fully differential measurements for direct TP-DI of He are highly demanded, an ultimate aim of this proposal. These studies are indispensable for the understanding of more complex situations when FEL-radiation interacts with many-electron atoms, molecules, clusters, surfaces or bulk-matter in the extreme ultraviolet regime.