  The multiphoton ionization of hydrogen by a strong bichromatic microwave
field is a complex process prototypical for atomic control research. Periodic
orbit analysis captures this complexity: Through the stability of periodic
orbits we can match qualitatively the variation of experimental ionization
rates with a control parameter, the relative phase between the two modes of the
field. Moreover, an empirical formula reproduces quantum simulations to a high
degree of accuracy. This quantitative agreement shows how short periodic orbits
organize the dynamics in multiphoton ionization.
