  Experiments have shown that the microwave ionization probability of a highly
excited almost monodimensional hydrogen atom subjected to a microwave pulse
sometimes grows in steps when the peak electric field of the pulse is
increased. Classical pulsed simulations display the same steps, which have been
traced to phase-space metamorphoses. Quantum numerical calculations again
exhibit the same ionization steps. I show that the time-sequence of two level
interactions, responsible for the observed steps in the quantum picture, is
strictly related to the classical phase space structures generated by the above
mentioned metamorphoses.
