  Comparisons of experimental data with numerical predictions of a classical
model indicate that an excited hydrogen atom in a pulsed microwave electric
field exhibits a nonclassical increase of stability over a relatively wide
range of frequencies. I show here that this is due to selective population of
long-lived "scarred" states that are associated with the chaotic separatrix
band surrounding the principal classical resonance zone in phase space. A
quantum explanation is given in terms of adiabatic evolution of Floquet states
and the destabilizing effect of two-level quantum resonances is investigated.
The role of neighbouring classical resonance zones in defining the frequency
range of stabilization is revealed both by quasienergy curves and by Husimi
functions for the instantaneous quantum states.
