We study the ultrafast dynamics of structural relaxation induced by the Rydberg excitation of an NO molecule in an Ar crystal. We used classical molecular dynamics simulations and normal mode analysis to describe the dynamics of the cage of Ar atoms surrounding the NO molecule. The results show a behaviour characterized by an impulsive expansion of the cage radius at short times (less than or equal to 250 fs), followed by multimodal oscillations over several picoseconds around a radius of similar to 4 Angstrom. This corresponds to a dilatation of the ground state cage radius of similar to 9%. The dynamics show a high degree of nuclear vibrational coherence. The relaxation process is described by the damping of mainly five vibrational modes. Their frequencies range from 20 to 75 cm(-1) and correspond to resonant modes of the crystal. The associated lifetimes range from 0.5 to 16 ps. The mode of highest frequency being the most anharmonic. (C) 1998 Elsevier Science B.V. All rights reserved.