Ultrafast photoinduced dynamics of single atoms solvated inside helium nanodroplets

Abstract

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. By variation of the photoexcitation energy, we find that the full excess excitation energy, represented by the blue-shifted in-droplet excitation band, is completely transferred to the He environment during the bubble expansion. Surprisingly, we find that variation of the droplet size has only a minor influence on the ejection time, providing insight into the spatial distribution of the ground-state atoms before photoexcitation. Simulated particle trajectories after photoexcitation are in agreement with experimental observations and suggest that the majority of ground-state atoms are located at around 16 Å below the droplet surface. Bubble expansion and oscillation are purely local effects, depending only on the ultimate dopant environment. These solvation-induced dynamics will be superimposed on intramolecular dynamics of molecular systems, and a mechanistic description is fundamental for the interpretation of future experiments.

Observation of laser-assisted electron scattering in superfluid helium

“…The following observations, in combination with predictions from 3D scattering simulations, lead us to the conclusion that the electron acceleration is due to multiple LAES processes within the He layer: (i) The energy gain strongly increases with droplet size (Figure 2). This behavior observed for strong-field ionization is in contrast to weak-field ionization inside He droplets, where the photoelectron spectrum is dropletsize independent because it is influenced only by the structure of the immediate environment of the dopant, the solvation shell 33 . In the current situation, the energy gain of the electron is related to the number of light-mediated binary electron-He-atom collisions at a distance from the remaining ion, which increases with growing droplet size.…”

Observation of laser-assisted electron scattering in superfluid helium

“…The energy gain strongly increases with droplet size (Figure 2). This behavior observed for strong-field ionization is in contrast to weak-field ionization inside He droplets, where the photoelectron spectrum is droplet-size independent because it is influenced only by the structure of the immediate environment of the dopant, the solvation shell [33].…”

Observation of laser-assisted electron scattering in superfluid helium

“…2). This behavior observed for strong-field ionization is in contrast to weak-field ionization inside He droplets, where the photoelectron spectrum is either droplet-size independent because it is influenced only by the structure of the immediate environment of the dopant, the solvation shell 41 , or develops a low-energy band revealing significant energy loss of electrons in larger droplets 42 . In the current situation, the energy gain of the electron is related to the number of light-mediated binary electron-He-atom collisions at a distance from the remaining ion, which increases with growing droplet size.…”