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This study investigates the real-time dynamics of charge-transfer-to-solvent (CTTS) states during core-level photoexcitation of aqueous metal ions, challenging the common assumption that the formation of these excited states is instantaneous. By employing time-dependent configuration interaction, the authors reveal how electronic charge flows from localized sites to solvent-supported states, transitioning from oscillatory behavior to irreversible delocalization. Their findings provide a detailed microscopic mechanism for the emergence of CTTS character and suggest experimental signatures for probing these dynamics using advanced spectroscopic techniques.
The dynamics of charge transfer in solution evolves from oscillatory to irreversible delocalization, revealing a nuanced understanding of CTTS state formation.
Charge-transfer-to-solvent (CTTS) excitations provide a chemically central route to generating hydrated electrons and initiating redox chemistry in solution, yet the earliest stage of CTTS---the formation of the excited state itself---is usually treated as instantaneous. Here we present a time-domain perspective of how CTTS character builds up during core-level photoexcitation of an aqueous metal ion. Using time-dependent configuration interaction, we simulate the coherent evolution of a dense manifold of core-excited states and track the ultrafast flow of electronic charge from the initially localized site into solvent-supported final states. We find that the dynamics evolves from a few-state, oscillatory behavior to effectively irreversible delocalization, as the charge disperses among many coupled configurations, providing a microscopic mechanism for the early-time emergence of CTTS character. Our results offer a transparent real-time interpretation of what core-level spectroscopies and core-hole-clock-type measurements can probe in solutions, outlining experimental signatures for probing the build-up of CTTS states on the core-hole-lifetime timescale. Looking ahead, attosecond and sub-femtosecond X-ray pump--probe approaches at X-ray free-electron lasers provide a realistic route to directly time-resolve the core-excited CTTS wave packets in solution.