Mapping ultrafast ionization of atoms and clusters with terahertz-streaking delay

We apply THz-field streaking to temporally resolve the ultrafast ionization of neutral cluster vs atomic targets exposed to intense ultrashort soft x-ray pulses from a free-electron laser (FEL). In the experiment pristine Xe and mixed Xe/Ar clusters as well as Xe atoms are ionized in the vicinity of the $4d→ɛf$ giant resonance of Xe. We compare the relative streaking delay between the center-of-mass oscillations of electrons that have final kinetic energies in the spectral region of the Xe($4d$) photoline. Our results show that clusters are ionized at the beginning of the 100 fs FEL pulse as supported by calculations of target frustration in the focal volume. We have identified a significantly larger 40 fs relative streaking delay between mixed Xe-Ar clusters vs Xe atoms compared to a 15 fs relative delay found for pristine Xe clusters. This is attributed to the high sensitivity of our spectroscopic measurement to the degree of condensation of the cluster target. Our results show that THz streaking is a powerful technique to temporally study electron emission from extended targets under intense FEL radiation on time scales that are significantly shorter than the FEL pulse duration