Acetylacetone photodynamics at a seeded free-electron laser

Abstract

The first steps in photochemical processes, such as photosynthesis or animal vision, involve changes in electronic and geometric structure on extremely short time scales. Time-resolved photoelectron spectroscopy is a natural way to measure such changes, but has been hindered hitherto by limitations of available pulsed light sources in the vacuum-ultraviolet and soft X-ray spectral region, which have insufficient resolution in time and energy simultaneously. The unique combination of intensity, energy resolution, and femtosecond pulse duration of the FERMI-seeded free-electron laser can now provide exceptionally detailed information on photoexcitation-deexcitation and fragmentation in pump-probe experiments on the 50-femtosecond time scale. For the prototypical system acetylacetone we report here electron spectra measured as a function of time delay with enough spectral and time resolution to follow several photoexcited species through well-characterized individual steps, interpreted using state-of-the-art static and dynamics calculations. These results open the way for investigations of photochemical processes in unprecedented detail.

Fig. 1

Chemical formulas for the two tautomeric forms and possible fragmentation pathway

Fig. 2

Ratios of integrated ion intensities. Top of panel: ratio of integrated ion intensities at a given delay to the integrated ion intensity at −1 ps (FEL only) for the fragments CH_x ⁺ in the range 0–5 ps. Inset: same in the enlarged scale 0–200 ps. Bottom of panel: ratio of integrated ion intensities at a given delay to the integrated ion intensity at −1 ps (FEL only) for the fragments OH⁺ and the parent ion as a function of pump-probe delay in the range 0–5 ps. Inset: same in the extended delay range 0–200 ps

Fig. 3

Valence photoelectron spectra for a series of pump-probe delays

Fig. 4

Experimental peak areas as a function of pump-probe delay. Given for peak 1 (4.64 eV), peak 2 (6.04 eV), and peak 3 (7.14 eV)

Fig. 5

Average population of the adiabatic states. Given for the electronic ground state (S₀), the two lowest singlet states, S₂ (ππ*) and S₁ (nπ*), and two triplet states, T₂ (nπ*) and T₁ (ππ*), obtained with CASSCF-based nonadiabatic dynamics simulations

Fig. 6

A schematic overview of the relaxation mechanism of acetylacetone. The ground state S₀ (darker blue), two singlet S₂ (ππ*) (light blue) and S₁ (nπ*) (orange), and two triplet T₂ (nπ*) (light green) and T₁ (ππ*) (green) states are shown. Excited state minima and minimum energy CIs (MECI) are indicated. Relative energies with respect to the electronic ground state minimum (S_0min) are given. For details see Supplementary Tables 1–3