  We analyze, quantum mechanically, the dynamics of ionization with a strong,
circularly polarized, laser field. We show that the main source for
non-adiabatic effects is connected to an effective barrier lowering due to the
laser frequency. Such non-adiabatic effects manifest themselves through
ionization rates and yields that depart up to more than one order of magnitude
from a static-field configuration. Beyond circular polarization, these results
show the limits of standard instantaneous - static-field like - interpretation
of laser-matter interaction and the great need for including time dependent
electronic dynamics.
