Time-Resolved X-ray Photoelectron Spectroscopy: Probing Bond Order Rearrangements during Ultrafast Excited-State Intramolecular Proton Transfer

Excited-state intramolecular proton transfer (ESIPT) provides a minimal yet rich framework for studying bond order rearrangements during chemical reactions. Here we combine linear-response time-dependent density functional theory (TDDFT), ab initio molecular dynamics, and neural network potentials including nuclear quantum effects, to compute time-resolved X-ray photoelectron spectroscopy (tr-XPS) during ESIPT in 10-hydroxybenzo[ h ]quinoline (HBQ) and its deuterated analog (DBQ). By tuning the photoionization probe energy, site-specific sensitivity is achieved through selective ionization of core electrons from either the donor O atom or the acceptor N atom. The tr-XPS signal exhibits a strong dependence on ESIPT through changes in photoelectron yield and binding energy. Differences in ESIPT time scales and isotope-dependent oscillatory spectral features between HBQ and DBQ enable direct connections to local bond order rearrangements near the probed X-ray chromophores. These results demonstrate that tr-XPS provides a powerful site-specific probe of ESIPT and the associated dynamics through its sensitivity to local charge redistribution that governs bond-order rearrangements, highlighting its potential for future experimental realization.