We have observed the microwave ionization of Na s and d states with 8-
GHz pulses as short as 600 ps full width at half maximum, i.e., pulses
containing five cycles. We find the ionization probability to be stro
ngly dependent on pulse length. For the shortest pulses, ionization pr
obability of \m\=0 and 1 states is approximately zero if a requirement
for the minimum number of cycles is not met, even when the microwave
amplitude exceeds the long-pulse threshold of E=1/3n(5). For the \m\=2
states, which are approximately hydrogenic, essentially net ionizatio
n occurs with the shortest pulse until the microwave amplitude equals
1/9n(4). We also find that as many as 50% of the \m\=0 and 1 atoms not
ionized by the microwaves are left trapped in higher n levels than th
e one initially excited. [S1050-2947(97)02012-X].