It is shown that the main part of the differences between classical and quantum predictions concerning the microwave ionization of H-atom threshold frequency dependence, commonly explained as a manifestation of quantum localization, originates from the enhanced, in classical simulations, role of the Coulomb singularity. When the Coulomb potential is softened, classical simulations reproduce quantum predictions and experimental data satisfactorily. No interference, intrinsically important for localization phenomena, is necessary.

• Received 12 May 1995

DOI:https://doi.org/10.1103/PhysRevA.52.R2523