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

Abstract

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.

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Tr-PES of acetylacetone. (a) Diagram of the experiment: a UV pump pulse excites the system in the S₂ level and an XUV probe ionizes it; the resulting electron Binding Energy (eBE), or equivalently the electron Kinetic Energy (eKE), tracks the induced dynamics. Experimental (b) and simulated (c) evolution of the eBE as a function of the pump and probe delay. Both traces show a rapid increase of the eBE in the first tens of femtoseconds, followed by a higher binding energy signal exhibiting a periodic chemical shift. In white, superimposed to the traces, is the evolution of the center of mass.

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Analysis of the ultrafast dynamics. (a) Spectral and (b) temporal amplitudes of the three components resulting from the global fit analysis, on the experimental (continuous lines) and simulated traces (dotted lines). The relevant fitting parameters of the experimental trace are displayed (further details in Section S2 of the Supporting Information). The IRF of the experiment resulting from the global fit, which matches the results of the independent two-color Ar photoionization measurement (Section S1 of Supporting Information), is marked in yellow. (c) Population dynamics from the simulation; (d) evolution of the center of mass (CM) in the experimental (blue) and simulated maps and its zoom in the inset (red: full simulation; yellow: simulation with singlet states only). In (a), (b), and (d), the shaded area represents the standard deviation of the experimental results.

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Vibrational coherences. Experimental (red) and simulated (blue) background-subtracted CM calculated between 5.3 and 6.5 eV (a) and its static Fourier Transform (FT) (b). In (c), the Gabor analysis of the experimental data was performed with a 300 fs-wide moving window. The vibrational modes associated with the S₂ and S₁ surfaces are shown in (d) and (e) and are highlighted in panel (c) in green and orange, respectively.

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Acetylacetone deactivation mechanism. (a) Potential energy surfaces of S₁ and S₂ in the space of ESIHT + BLA coordinates (ϕ) and ring opening (φ): after S₂ → S₁ decay, ESIHT is interrupted in most of the cases (67%), and trajectories deviate along φ to reach the S₁ minimum; (b) potential energy profiles for AcAc along geodesic interpolations between key geometries (S₀ min → S₂/S₁ MECI → S₁ min → S₁/S₀ MECI, via S₁ TS and T₁ min). The inset magnifies the vicinity of S₁ min. The predicted PES signals (binding energies, in eV) from critical points are shown as vertical arrows. The most activated vibrations in each step of the deactivation path are shown as horizontal arrows. Geometrical parameters: Tw = O10–C9–C5–O6 dihedral; C7–C9 conjugation = cosine of the angle between the p orbitals of C7 and C9 (1 = complete conjugation, 0 = orthogonal orbitals, no conjugation).