We report on the ultrafast dynamics of electronic bubble formation following excitation of the A 3ss Rydberg state of the NO mol. trapped in argon and hydrogen matrixes. The study is carried out by femtosecond fluorescence depletion expts. Apparently in hydrogen, relaxation occurs in .apprx.1.5 ps via a collective displacement of the cage species with no recurrencies. In argon, the cage expands by undergoing coherent recurrencies for over .apprx.3 ps, on its way to the fully relaxed bubble configuration. [on SciFinder (R)]