Time delay in molecular photoionization near giant resonances

用阿秒计量学能够测量与束缚电子的光致电离有关的时间延迟。电离时间延迟主要是从理论上和实验上对单一组成物种的原子或固体进行评估的。这种对分子的测量本身就更具挑战性，最近才报道了使用Rabbitt(双光子干涉阿秒拍频)技术的简单三原子分子的价电子发射。在此基础上，我们利用阿秒扫描光谱研究了碘原子分子在极紫外四维巨共振中的光致电离时延。这种共振被认为是一种形状共振，然而，它受到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和他的同事进行了依赖于时间的密度泛函理论(DFT)模拟，并预测了集体激发导致的负电离时间延迟，其中仅形状共振就会导致正延迟。这些结果与Kheifets使用随机相位近似(RPA)模拟的预测不一致。由于我们的测量覆盖了碘4d共振的能量范围，我们将能够解决这一理论争议，为巨型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下进行，通过冷靶反冲离子动量谱仪(COLTRIMS)发射的离子来检测分子取向。由此得到的数据将允许确定作为分子取向和电子发射角的函数的电离时间延迟。因此，可以以高度不同的方式获得时间延迟，这便于与理论预测进行更接近的比较。我们将与Landsmann小组合作进行半经典的蒙特卡罗轨迹模拟，其中包含了激光场对时间延迟的影响，并可以直接与实验数据进行比较。本征Eisenbud-Wigner-Smith(EWS)延迟可以通过仔细考虑激光场的贡献来推导。另一方面，基于沃尔纳小组最近发展的框架，EWS延迟将直接从量子力学散射计算中获得。通过比较几个分子系统的时延差，我们期望对阿秒尺度上的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.