Microwave ionization of sodium with subnanosecond 8-GHz pulses

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 strongly dependent on pulse length. For the shortest pulses, ionization probability 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= 1/3n⁵. For the |m|=2 states, which are approximately hydrogenic, essentially no ionization occurs with the shortest pulse until the microwave amplitude equals 1/9n⁴. 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 the one initially excited.