Microwave Ionization of H Atoms: Breakdown of Classical Dynamics for High Frequencies

We report the first measurements of microwave excitation and ionization of excited hydrogen atoms for scaled frequencies $n_0^3\omega$ up to 2.8. Classical 3D calculations which directly model this 36.021-GHz experiment agree quite well for $n_0^3\omega <1\mathrm{}$, agree less well for $1\le n_0^3\omega \le 2$, and do not agree for $n_0^3\omega >2\mathrm{}$. This supports theoretical predictions that as $n_0^3\omega$ rises above 1, quantal ionization threshold fields rise above those for the onset of classical chaos; however, the data continue to reveal local stability near certain rational frequency ratios that recalls classical behavior.