Dynamics of Above-Threshold Ionization and Laser-Assisted Electron Scattering inside Helium Nanodroplets.

Laser-assisted electron scattering (LAES) is a fundamental three body interaction process that enables energy transfer between electrons and photons in the presence of matter. Here, we focus on the multiscattering regime of electrons generated by above-threshold ionization (ATI) of In atoms inside a high-density nanostructure, helium nanodroplets (He) of ∼40 Å radius. The stochastic nature of the multiscattering regime results in photoelectron spectra independent of laser polarization.

Observation of laser-assisted electron scattering in superfluid helium.

Laser-assisted electron scattering (LAES), a light-matter interaction process that facilitates energy transfer between strong light fields and free electrons, has so far been observed only in gas phase. Here we report on the observation of LAES at condensed phase particle densities, for which we create nano-structured systems consisting of a single atom or molecule surrounded by a superfluid He shell of variable thickness (32-340 Å). We observe that free electrons, generated by femtosecond strong-field ionization of the core particle, can gain several tens of photon energies due to multiple LAES processes within the liquid He shell.

Ultrafast photoinduced dynamics of single atoms solvated inside helium nanodroplets.

Helium nanodroplets can serve as reaction containers for photoinduced time-resolved studies of cold, isolated molecular systems that are otherwise inaccessible. Recently, three different dynamical processes, triggered by photoexcitation of a single atom inside a droplet, were observed in their natural time scale: Expansion of the He solvation shell (He bubble) within 600 fs initiates a collective bubble oscillation with a ∼30 ps oscillation period, followed by dopant ejection after ∼60 ps. Here, we present a systematic investigation of these processes by combining time-resolved photoelectron and photoion spectroscopy with time-dependent He density functional theory simulations.