With this experiment we demonstrate that excitation of a two-stale sys
tem with radio-frequency fields differing in phase by 90 degrees produ
ces non intuitively different results, even for very long pulses. In a
ddition, we show bow the phase dependence of the transition probabilit
y of long pulses can he easily understood by using the single cycle ti
me propagator. Finally, we have found surprising results for real puls
es in the strong-field regime, i.e., pulses having appreciable rise an
d fall times.