Dimer photofragmentation and cation ejection dynamics in helium nanodroplets.

We present femtosecond pump-probe photoionization experiments with indium dimers (In2) solvated in helium nanodroplets (HeN). At short pump-probe time delays, where the excited In2* is still located inside the droplet, we surprisingly observe detachment of InHen+ ions with n = 1 to ∼30 from the droplet. These ions indicate that fragmentation of In2 occurs and that the kinetic energy release enables In+ to overcome the attractive HeN potential, which typically prevents ion ejection from the droplet. We find that the transient InHen+ signal reveals vibrational wave packet motion in neutral In2*. By correlating the InHen+ signal with the corresponding photoelectrons through covariance detection, we unequivocally identify the ionization pathway leading to InHen+: pump-excitation from the ground-state In2 creates a vibrational wave packet in In2*, followed by probe-ionization to the cationic ground state In2+. Subsequently, a further probe photon promotes the molecule to an excited ionic state In2+* of nonbonding character, leading to fragmentation and kinetic energy release. This interpretation is additionally supported by probe power- and droplet-size dependencies, as well as energetic considerations. Unambiguous assignment of the ionization path to absorption-ionization-dissociation (fragmentation of the ion) in contrast to absorption-dissociation-ionization (fragmentation of the neutral) is enabled by ion ejection and electron-ion correlation. This complementary observable for ultrafast photochemical processes inside HeN will be particularly valuable for more complex systems.

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. Supported by Monte Carlo 3D LAES and elastic scattering simulations, these results provide the first insight into the interplay of LAES energy gain/loss and dissipative electron movement in a liquid. Condensed-phase LAES creates new possibilities for space-time studies of solids and for real-time tracing of free electrons in liquids.