Driven Rydberg atoms reveal quartic level repulsion

The dynamics of Rydberg states of a hydrogen atom subjected simultaneously to uniform static electric field and two microwave fields with commensurate frequencies is considered in the range of small field amplitudes. In the c ertain range of the parameters of the system the classical secular motion o f the electronic ellipse reveals chaotic behavior. Quantum mechanically, wh en the fine structure of the atom is taken into account, the energy level s tatistics obey predictions appropriate for the symplectic Gaussian random m atrix ensemble.