Filming the molecular dynamics of acetylacetone with time-resolved x-ray scattering*

Time-resolved x-ray scattering from UV photoexcited acetylacetone was measured at the LINAC Coherent Light Source in February 2020. Using a 266 nm, 42 fs, 5 uJ laser pulse, acetylacetone is excited to the ππ* state. Simulations suggest it subsequently undergoes hydrogen transfer dynamics and internal conversion to a spectroscopically dark nπ* state. The nπ* state further relaxes via intersystem crossing to a triplet ππ* state. Time-resolved x-ray scattering at 15 keV films the acetylacetone dynamics in reciprocal space up to a momentum transfer of 7 inverse Angstroms, which may then be used to recover the real-space electron distribution with 0.89 Angstroms of spatial resolution. The real-space electron distribution is sensitive to the pump-probe delay between the laser and x-ray pulses, and it generates a film strip of the real-space dynamics of acetylacetone with sub-50 fs time-resolution, which may then be compared to simulation. The fine time-resolution enables the generation of film strips in reciprocal-space and reciprocal-time through application of a temporal Fourier transform to the time-resolved x-ray scattering measurement, which provides a state-selective view of the molecular dynamics, for example, vibrations and dissociations may be segregated in this representation. Moreover, this measurement marks the finest temporal resolution (sub-50 fs) at 0.89 Angstroms of Fourier-transform limited spatial resolution in a gas-phase scattering experiment. This is thanks to the recent upgrade of the LCLS undulators, which now provide up to 1 mJ of pulse energy at photon energies around 20 keV.