Protonation-state-dependent charge transfer to solvent in aqueous L-cysteine probed by resonant Auger–Meitner spectroscopy

Charge-transfer-to-solvent (CTTS) couples electronic, nuclear, and solvent dynamics on femtosecond timescales. In biologically relevant L-cysteine, resolving how protonation and local hydrogen bonding influence CTTS upon resonant excitation remains challenging. Sulfur KLL resonant Auger–Meitner spectroscopy qualitatively resolves different final states separated in kinetic energy. These include localized excited states, delocalized CTTS states, and the charge-transfer continuum, enabling direct identification of different decay channels in L-cysteine. At pH 12, the dianion exhibits a pronounced charge-transfer channel extending below the ionization threshold, negligible at pH 5. Transition-potential density functional theory and Born–Oppenheimer molecular dynamics link this to enhanced thiolate hydrogen-bond coordination, revealing a delocalized core-excited state nearly degenerate with the bright localized resonance. A Fano-based core-hole-clock approach, accounting for coherent population of the localized state and charge-transfer continuum, yields a charge-transfer time of 43± 8 fs, compared with 15± 2 fs from conventional core-hole-clock analysis, highlighting the importance of accounting for direct excitation into the chargetransfer continuum and its coherent coupling to the localized core-excited state when extracting charge-transfer times. Together, these results establish resonant Auger–Meitner spectroscopy with Fano-based analysis as a general strategy for probing protonation-and solvation-dependent ultrafast electronic delocalization in aqueous biomolecules.