3-fs-resolved UV/XUV photoelectron spectroscopy in the gas phase

Understanding ultrafast molecular dynamics requires experimental tools capable of capturing coupled electronic and nuclear motion on their natural timescales. Here, we introduce a novel UV-XUV pump-probe beamline that achieves a temporal resolution of approximately 3 fs, which significantly surpasses the precision available in conventional time-resolved photoelectron spectroscopy (trPES) setups. This performance is enabled by the synergistic synchronization of ultrashort tunable UV pump pulses, generated via resonant dispersive-wave emission in gas-filled hollow capillary fibers, with attosecond XUV probe pulses. We demonstrate the power of this approach by applying it to the study of photoexcited acetylacetone. Our approach distinctly resolves two sequential passages through the S₂/S₁ conical intersection (CI), revealing dynamical features previously inaccessible to trPES. By establishing a new level of temporal precision in trPES, this beamline opens a new regime for the time-domain observation of coupled electronic-nuclear wavepacket evolution in complex molecular systems, which is pivotal for advancing ultrafast photochemistry and molecular quantum control.