Abstract—A stable symplectic scheme for calculating particle trajectories in timeperiodic force fields based on the Floquet technique and splitoperator method is described. The dynamics of a threedimensional hydrogen atom under the action of an external linearly polarized microwave electric field is studied in a numerical experiment. Under conditions of the implemented dynamical chaos, features in the evolution of angular momentum L(t) of a Rydberg electron (RE) that do not meet the assumptions of traditional theoretical approaches for describing lightinduced diffusion ionization of the RE are revealed.