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 n₀³ω up to 2.8. Classical 3D calculations which directly model this 36.021-GHz experiment agree quite well for n₀³ω<1, agree less well for 1≤n₀³ω≤2, and do not agree for n₀³ω>2. This supports theoretical predictions that as n₀³ω 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.