The dynamics of non-polar electronic solvation has been studied on a model system consisting of NO-doped solid Ar. Excitation of the lowest Rydberg state A(3s sigma) of NO leads to formation of an electronic 'bubble', which has been studied by femtosecond pump-probe spectroscopy and classical Molecular Dynamics simulations. The early time behaviour of the transients is identified as the inertial component of the medium response. This first response is followed by a recurrent low-frequency oscillatory motion of the cage for up to similar to 2 ps, indicative of coherence in the bath modes.