  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 angstrom). 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.
