Ionization of sodium and lithium Rydberg atoms by 10-MHz to 15-GHz electric fields

We have studied the ionization of Na ns and nd Rydberg atoms by {~}1-{μ}s pulses of 10-MHz, 670-MHz, 2-GHz, and 4-GHz fields, bridging the gap between the previous measurements of ionization by {~}1-{μ}s rise-time pulses and 8- and 15-GHz microwave fields. We have also studied the ionization of Li ns and nd Rydberg atoms by 4-, 8-, and 15-GHz fields, which, due to the smaller Li quantum defects, represents the high-frequency limit of nonhydrogenic microwave ionization. The Na measurements show that ionization at 10 MHz occurs without change in n at fields of 1/16n⁴ and 1/9n⁴. At 670 MHz {}m{}=0, 1, and 2 states are all ionized at fields between 1/3n⁵ and 1/9n⁴, indicating that transitions to higher-n states are occurring. As the frequency is raised further, to 2 and 4 GHz, the {}m{}=0 and 1 states exhibit an increasingly sharp threshold for ionization at E=1/3n⁵, but the {}m{}=2 states are unlikely to ionize at this field, with ionization occurring mostly at higher fields, 1/3n⁵E1/9n⁴, approaching the 15-GHz behavior of ionization at E=1/9n⁴. At 4 GHz, Li n=20 {}m{}=0 and 1 states ionize at E=1/3n⁵, but at n{}45, {}m{}=1 states ionize at E=1/9n⁴, and the {}m{}=0 states ionize at E>1/3n⁵. The {}m{}=2 states appear to ionize at E=1/9n⁴. As the frequency is raised to 8 and then to 15 GHz, the transition of {}m{}=0 and 1 states to ionization at E=1/9n⁴ occurs at lower n. When a small static field is applied, the Li ionization threshold fields drop from 1/9n⁴ to fields somewhat above 1/3n⁵. Together the Na and Li measurements show that microwave ionization depends on a rate-limiting step, which is a resonant multiphoton transition.