Excitation of the A(3ss) Rydberg state of NO trapped in an H2 crystal leads to the electronic bubble formation around the NO impurity, resulting in an increase of 0.9 .ANG. of the ground state cage radius. Its dynamics was followed in real-time by femtosecond pump-probe spectroscopy. Bubble formation is completed in .apprx.1.5 ps without recurrences of the cage motion. A model based on classical hydrodynamics provides quant. agreement with exptl. data, suggesting that solid H can be described like a liq. on ultrashort time scales. [on SciFinder (R)]