Mapping Excited-State Decay Mechanisms in Acetylacetone by Sub-20 fs Time-Resolved Photoelectron Spectroscopy

Excited-State Intramolecular Hydrogen Transfer (ESIHT) is one of the fastest chemical reactions, occurring on the order of tens of femtoseconds and playing a critical role in light-driven biological processes and technological applications. Here, we investigate the early stages of coupled nuclear-electron dynamics using acetylacetone (AcAc) as a model system exhibiting ESIHT. We employ ultraviolet-extreme ultraviolet (UV-XUV) time-resolved photoelectron spectroscopy (tr-PES) with sub-20 fs resolution in combination with high-level dynamically correlated simulations (CASPT2) to map the electronic relaxation pathways and vibrational modes driving this process. Our results provide distinct spectroscopic signatures of ESIHT occurring within the first 20 fs and resolve the active vibrational modes, showing the intricate evolution of the electronic and nuclear degrees of freedom. Moreover, the analysis reveals the key role of ultrafast intersystem crossing (ISC) to triplet states in modulating the excited-state dynamics and its implications for the overall relaxation pathways. These findings refine our understanding of the photochemistry of AcAc and suggest general principles that can be applied to similar conjugated systems.