We report the first theoretical and experimental study of the detailed frequency dependence of the "ionization" of excited H atoms by a two-frequency ($2f$) microwave field. Our 1D quantal and 1D classical calculations, in excellent agreement with each other, predict a new $2f$ ionization peak. The peak scales classically. We present a model which shows that the location and the width of the peak are determined by classical phase-space structures. The theoretical results are supported by experimental data with 3D atoms.

• Received 28 June 1993

DOI:https://doi.org/10.1103/PhysRevLett.73.248