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.

Sub-20-fs UV-XUV beamline for ultrafast molecular spectroscopy.

We present an ultraviolet (UV) - extreme-ultraviolet (XUV) pump-probe beamline with applications in ultrafast time-resolved photoelectron spectroscopy. The UV pump pulses, tuneable between 255 and 285 nm and with µJ-level energy, are generated by frequency up-conversion between ultrashort visible/infrared pulses and visible narrow-band pulses. Few-femtosecond XUV probe pulses are produced by a high-order harmonic generation source equipped with a state-of-the-art time-delay compensated monochromator.

Controlling Floquet states on ultrashort time scales.

The advent of ultrafast laser science offers the unique opportunity to combine Floquet engineering with extreme time resolution, further pushing the optical control of matter into the petahertz domain. However, what is the shortest driving pulse for which Floquet states can be realised remains an unsolved matter, thus limiting the application of Floquet theory to pulses composed by many optical cycles. Here we ionized Ne atoms with few-femtosecond pulses of selected time duration and show that a Floquet state can be observed already with a driving field that lasts for only 10 cycles.

Ultrashort visible energetic pulses generated by nonlinear propagation of necklace beams in capillaries.

The generation of ultrashort visible energetic pulses is investigated numerically by the nonlinear propagation of infrared necklace beams in capillaries. We have developed a (3+1)D model that solves the nonlinear propagation equation, including the complete spatio-temporal dynamics and the azimuthal dependence of these structured beams. Due to their singular nonlinear propagation, the spectrum broadening inside the capillary extends to the visible region in a controlled way, despite the high nonlinearity, avoiding self-focusing.

Influence of the spatial confinement on the self-focusing of ultrashort pulses in hollow-core fibers.

The collapse of a laser beam propagating inside a hollow-core fiber is investigated by numerically solving different nonlinear propagation models. We have identified that the fiber confinement favors the spatial collapse, especially in case of pulses with the input peak power close to the critical value. We have also observed that when using pulses in the femtosecond range, the temporal dynamics plays an important role, activating the spatial collapse even for pulses with input peak powers below the critical value.