Investigation of the continuum-continuum phase in attosecond time delays

长期以来，光致电离过程一直被认为是一个瞬时过程：一个足够能量的光子撞击到一个量子系统(原子、分子或表面)上，一个电子在连续统中瞬间释放。阿秒技术的发展第一次能够及时解决这一基本过程，并测量从同一系统中不同能级或不同量子系统发射的光电子之间的时间延迟。对这些延迟的测量是相关的，因为它提供了电子在离开时所经历的势的特征。所有专注于时间延迟表征的实验都是基于极端紫外线中的阿秒波形和(典型的)红外激光脉冲的组合。与后者的相互作用引入了额外的延迟(表示为连续-连续(CC)延迟)，必须对其进行校正，以便隔离量子系统对阿秒场的响应。到目前为止，对CC延迟的估计一直基于理论模型，到目前为止还没有实验测量的报道。这个项目的目标是通过利用倡导者最近在自由电子激光费米上展示的结果，首次提供CC延迟的实验表征。该项目将基于光学参数放大器的开发，与阿秒序列源相结合，将有可能产生直接编码CC延迟信息的光电子光谱。结果将与TDSE的数值解和微扰理论的预测进行比较，后者将由支持该项目的两个理论小组提供。

The process of photoionisation has been assumed for a long time to be an instantaneous process: a photon of sufficient energy impinges on a quantum system (atom, molecule or surface) and an electron is instantaneously released in the continnum. The development of attosecond technologies has allowed, for the first time, to resolve in time this fundamental process and to measure time delays between the photoelectrons emitted from different energy levels in the same system or from different quantum systems. The measurement of these delay is relevant, as it gives access to the characteristics of the potential experienced by the electrons on its way out. All experiment focused on the characterization of time delays are based on the combination of an attosecond waveform in the extreme ultraviolet and (typically) an infrared laser pulse. The interaction with the latter introduces an additional delay (indicated as continuum-continuum (CC) delay), which must be corrected for, in order to isolate the response of the quantum system to the attosecond field. The estimation of the CC delay has been based so far on theoretical models and no experimental measurement has been reported so far. The goal of this project is to provide the first experimental characterization of the CC delay, by taking advantage of the results recently demonstrated at the Free-Electron Laser FERMI by the proponent. The project will be based on the development on an optical parametric amplifier that, combined with an attosecond train source, will give the possibility to generate photoelectron spectra encoding directly the information on the CC delay. The results will be compared with numerical solution of the TDSE and predictions of perturbation theory that will be delivered by two theoretical groups supporting the project.