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 approximately 1-mu-s pulses of 10-MHz, 670-MHz, 2-GHz, and 4-GHz fields, bridging the gap between the previous measurements of ionization by approximately 1-mu-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, 1/3n5 < E < 1/9n4, 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 congruent-to 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.