Production of extremely-high-lying states in 17-GHz microwave ionization of Li and Na

In the ionization of Li and Na by a 16.91-GHz microwave field, roughly 20% of the atoms are not actually ionized but trapped in highly excited states within one or, at most, two microwave photons of the ionization limit. The highly excited atoms appear at the same field as the onset of microwave ionization, irrespective of whether ionization occurs at the field $E=1/3{n}^{5}$ or $E=1/16{n}^{4}$. The highly excited atoms are only detected in static fields less than 35 mV/cm, a field which roughly corresponds to field ionization of an atom bound by twice the microwave frequency. We also show that Na $|m|=1$ atoms require a microwave field of $E=1/9{n}^{4}$ for ionization in zero static field, but the required field for ionization is reduced to $E=1/3{n}^{5}$ if a static field of 1 V/cm is present.