Time delay in molecular photoionization near giant resonances

用阿秒计量法测量了与束缚电子的光电离有关的时间延迟。对于单一组分的原子或固体，电离时间延迟主要在理论和实验上进行了评估。这样的分子测量本身就更具挑战性，最近有报道称，简单三原子分子的价电子发射采用了RABBITT（双光子跃迁干涉的阿秒跳动）技术。利用阿秒条纹光谱技术，我们研究了含碘原子分子的光电离时间延迟，该分子在极紫外波段具有宽带共振特征，即4d巨共振。共振被指定为形状共振，但它受多电子集体激发的强烈影响。最近关于与这种共振相关的电离时间延迟的理论工作是非常有争议的。Chakraborty和同事进行了时间依赖密度泛函理论（DFT）模拟，并预测了集体激发导致的负电离时间延迟，其中形状共振单独导致正延迟。这些结果与Kheifets使用随机相位近似（RPA）模拟的预测不一致。我们的测量跨越了碘四维共振的能量范围，我们将能够解决这一理论争议，为巨大共振中的集体激发提供详细的见解。此外，我们的实验将在分子框架内进行，利用冷靶反冲离子动量谱仪（COLTRIMS）通过发射离子检测分子取向。所得数据将允许确定电离时间延迟作为分子取向和电子发射角的函数。因此，可以以一种高度微分的方式获得时间延迟，这有助于与理论预测进行更密切的比较。我们将与Landsmann小组合作进行半经典蒙特卡罗轨迹模拟，其中包括激光场对时间延迟的影响，并且可以直接与实验数据进行比较。通过仔细考虑激光场的贡献，可以推导出固有的艾森巴德-维格纳-史密斯（EWS）延迟。另一方面，EWS延迟将直接从基于Wörner小组最近开发的框架的量子力学散射计算中获得。通过对几种分子系统的差分时滞的比较，我们期望在阿秒尺度上对分子框架光电离有更详细的了解。

The measurement of time delay associated with photoionization of a bound electron was enabled with attosecond metrology. Ionization time delays were predominantly evaluated both theoretically and experimentally for atoms or solids of single constituent species. Such measurements for molecules are inherently more challenging, and have just recently been reported for valence electron emission from simple triatomic molecules employing the RABBITT (attosecond beating by interference of two-photon transitions) technique. With this proposal, utilizing attosecond streaking spectroscopy we aim to study the photoionization time delay for molecules containing an iodine atom with a characteristic broadband resonance in the extreme ultraviolet, the 4d giant resonance. The resonance has been assigned to a shape resonance, which is, however, strongly affected by multi-electron, collective excitations. Recent theoretical work on the ionization time delays associated with such a resonance is highly controversial. Chakraborty and coworkers performed time-dependent density-functional-theory (DFT) simulations and predicted negative ionization time delays as a result of the collective excitation, where the shape resonance alone leads to positive delays. These results are at odds with predictions by Kheifets employing random-phase approximation (RPA) simulations. With our measurements spanning over the energy range of the iodine 4d resonance, we will be able to resolve this theoretical dispute, providing detailed insight into collective excitations in giant resonances. Furthermore, our experiments will be performed in the molecular frame detecting the molecular orientation via emitted ions with a cold-target recoil ion momentum spectrometer (COLTRIMS). The resulting data will permit to determine the ionization time delay as a function of molecular orientation as well as electron emission angle. The time delays can thus be obtained in a highly differential manner, which facilitates closer comparison to theoretical predictions. We will perform semi-classical Monte-Carlo trajectory simulations in collaboration with the Landsmann group, which incorporate the laser field influence on the time delay and can be directly compared to the experimental data. The intrinsic Eisenbud-Wigner-Smith (EWS) delay can be derived by careful consideration of the laser-field contributions. On the other hand, the EWS delay will be directly obtained from quantum mechanical scattering calculations based on the recently developed framework by the Wörner group. By comparison of the differential time delays for several molecular systems, we expect to gain detailed insight into molecular-frame photoionization on the attosecond scale.