Microwave ionization of Na Rydberg atoms

Electric field ionization of excited Na atoms by microwave fields between 12 and 15 GHz exhibits several interesting phenomena. The $\mathrm{}\left|m\right|=0\mathrm{} \mathrm{and} 1$ states may be ionized with a very low microwave field, $\sim \frac{1}{{3n}^5}$, corresponding to the point at which the $n$ and $n+1\mathrm{}$ Stark manifolds of principal quantum numbers $n$ and $n+1\mathrm{}$ intersect. The ionization occurs by an atom's making a succession of Landau-Zener transitions to higher-lying states, finally reaching the classical ionization limit. The $\mathrm{}\left|m\right|=2\mathrm{}$ states, on the other hand, ionize at a very high microwave field, $\sim \frac{1}{{9n}^4}$. Both these observations and the more familiar static or low-frequency result of ionization by a field of $\sim \frac{1}{{16n}^4}$ are all consistent with ionization in a completely classical fashion. The apparent differences are due to the dynamics of passing from zero field to a high ionizing field.