  The observation of chemical reactions on the time scale of the motion of
electrons and nuclei has been made possible by lasers with ever shortened pulse
lengths. Superfluid helium represents a special solvent that permits the
synthesis of novel classes of molecules that have eluded dynamical studies so
far. However, photoexcitation inside this quantum solvent triggers a pronounced
response of the solvation shell, which is not well understood. Here we present
a mechanistic description of the solvent response to photoexcitation of indium
(In) dopant atoms inside helium nanodroplets (He$_\mathrm{N}$), obtained from
femtosecond pump-probe spectroscopy and time-dependent density functional
theory simulations. For the In-He$_\mathrm{N}$ system, part of the excited
state electronic energy leads to expansion of the solvation shell within 600
fs, initiating a collective shell oscillation with a period of about 30 ps.
These coupled electronic and nuclear dynamics will be superimposed on intrinsic
photoinduced processes of molecular systems inside helium droplets.
