We use Kepler maps to study the classical ionization of the hydrogen atom d
riven by electric fields; we compare the ionization rate (IR) due to chaoti
c motion (constant field amplitude) with the IR due to randomness (random f
ield amplitude). We find that for weak fields the ionization due to random
amplitude is more effective, while for strong fields the ionization due to
chaos is more effective; so, there is a cross of such ionization curves. A
physical explanation of this phenomenon, based on the trajectory behavior,
is given.