Probing internal continua and atomic ultrafast charge transfer within size-controlled nanoparticles by post-collision interaction in core-hole clock spectroscopy.

This study investigates size-controlled, quantum-confined CdSe/ZnS core-shell quantum dots using core-hole clock spectroscopy in combination with post-collision interaction (PCI) line shape analysis, providing insights into local charge transfer dynamics and internal continuum states. We observe an acceleration of charge transfer times by almost one order of magnitude in thin-shell quantum dots, comprising only one or three double layers of ZnS, before reaching a size-independent limit. This size-dependence is governed by the existence of a faster charge transfer channel toward the CdSe core, only accessible for the inner-most shell layers, rather than a quantum confinement effect. By extending the traditional PCI model from free-electron systems to bound-state continua, we further establish a framework for interpreting line shape asymmetries and peak shifts that are frequently observed but often overlooked in resonant Auger measurements. We show that the strongly enhanced PCI in the samples with one or three double layers can be attributed to reduced collective electronic screening. This comprehensive experimental approach enables the simultaneous observation of collective electronic properties and atom-specific dynamics within in a single measurement under identical sample conditions, an advance particularly valuable for complex, sensitive materials.