Time delay in molecular photoionization near giant resonances

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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.