The ultrafast dynamics of electronic bubble formation upon excitation of the A(3ss) Rydberg state of NO trapped in solid H2 and D2 was studied by femtosecond pump-probe spectroscopy. The evolution of the spherical bubble is followed in real time by a probe pulse, which maps the transient configurations via transitions to higher-lying Rydberg states. Bubble formation is a 1-way process and no oscillations of the bubble are obsd. In addn., thermalization of the system occurs on the time scale of bubble formation. In the process, there is a net energy flow away from the excited center and 0.55-0.6 eV leave the 1st shell around the impurity. The authors directly ext. from the exptl. data the time dependence of the bubble radius, which the authors represent by a rising exponential with time consts. of 300 +- 50 fs in solid H2 and 410 +- 30 fs in solid D2 to reach a final radius of .apprx.5 .ANG.. This is confirmed by simulations of the transients. The different energy dissipation mechanisms in the expansion of the bubble are discussed and probably emission of a sound wave is the dominant one. [on SciFinder (R)]