A classical theory gives excellent agreement with the Bayfield-Koch experiment on microwave ionization of Rydberg hydrogen atoms. The time dependence of excitation and ionization is presented, and classical trajectories are divided into four significant categories. The results suggest that nonresonant laser ionization of atoms in states of low quantum number can also take place as a result of extremely high-order processes with large numbers of intermediate states of excitation.

• Received 10 July 1978

DOI:https://doi.org/10.1103/PhysRevLett.41.944