Population Trapping in Extremely Highly Excited States in Microwave Ionization

When a lithium atom in a Rydberg state ( $n\sim 80$) is exposed to a short, intense microwave pulse we find that substantial population is left in extremely highly excited states ( $n>\mathrm{}120\mathrm{}$), in spite of the fact that the microwave field amplitude is more than 40 times larger than required to classically ionize these states.