Algebraic Decay and Phase-Space Metamorphoses in Microwave Ionization of Hydrogen Rydberg Atoms

We study classically the microwave ionization of hydrogen atoms using the standard one-dimensional model. We find that the survival probability of an electron decays algebraically for long exposure times. Furthermore, as the microwave field strength increases, we find that the asymptotic algebraic decay exponent can decrease due to phase-space metamorphoses in which new layers of Kolmogorov-Arnold-Moser (KAM) islands are exposed when KAM surfaces are destroyed. We also find that after such phase-space metamorphoses, the survival probability of an electron as a function of time can have a crossover region with different decay exponents. We argue that this phenomenon is typical for open Hamiltonian systems that exhibit nonhyperbolic chaotic scattering.