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Ultrafast dynamics in helium nanodroplets probed by XUV spectroscopy and x-ray imaging


Helium nanodroplets are the smallest known objects exhibiting superfluidity, and they provide a unique cryogenic matrix for high-resolution spectroscopy and ultracold chemistry applications. The relatively simple electronic structure of helium atoms and the homogeneity of the quantum fluid clusters make them excellent model systems for the study of electronic structures and dynamics in complex systems and the emergence of collective phenomena. Coupled electronic-nuclear dynamics in laser-excited helium nanodroplets are studied in the time-domain in two distinctly different excitation regimes following either single XUV-photon excitation or strong-field ionization by a near-infrared (NIR) laser pulse. Femtosecond time- resolved XUV photoelectron spectroscopy and femtosecond time-resolved X-ray coherent diffractive imaging are employed to monitor electronic relaxation dynamics in neutral droplets and the emergence and evolution of a nanoplasma in strong-field ionized droplets, respectively.

Electronically excited pure and doped He droplets are prepared using femtosecond XUV pump pulses produced by high-order harmonic generation. The excited states and subsequent relaxation dynamics are probed by ionization of transient species with a femtosecond UV probe- pulse. Pump-probe time delay-dependent photoelectron kinetic energy distributions are measured using velocity map imaging. In pure droplets excited with 23.7 eV, three dynamic pathways are identified: interband relaxation from the initially excited 1s3p,1s4p manifold to the 1s2p band, further relaxation within the 1s2p Rydberg band and rapid atomic reconfiguration involving formation of Rydberg-excited (Hen)* cores within the droplet. Ongoing efforts towards understanding energy- and charge-transfer mechanisms between the host droplets and dopant atoms are discussed. New high-harmonic generation schemes were implemented in order to directly access the lower 1s2p droplet resonances. Droplets doped with a small amounts of Kr and Ne atoms (ndopant/ndroplet < 10-4) are excited into the 1s2p Rydberg band by 21.6 eV photons. Evidence of excitation transfer to the dopant atoms via a Penning-like ionization process is reported and progress towards time-resolved experiments is described.

The dynamics of strong-field induced nanoplasmas are studied using femtosecond time- resolved x-ray coherent diffractive imaging at the Linac Coherent Light Source (LCLS). Intense 800 nm laser pulses are employed to initiate nanoplasma formation in helium droplets. Plasma formation and evolution dynamics are probed by femtosecond x-ray pulses. Anisotropic surface softening is observed within tens of femtoseconds after exposure to the NIR pulse and stabilizes within ~300fs. The saturation of the surface width is contrasted by an increase in anisotropic material loss that is twice as pronounced along the laser polarization axis compared to the perpendicular direction, resulting in significantly distorted shapes with aspect ratios of ≈1.5 and beyond on picosecond timescales. The results are interpreted within the framework of an anisotropic evaporation model that provides new insight into strong-field induced nanoplasma formation and relaxation dynamics.

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