On the basis of exact quantum mechanical calculations we study energy-level spectra and photoabsorption cross sections for a hydrogen atom in a uniform magnetic field. In the classically chaotic region near the threshold the level distributions show the irregular pattern predicted by randommatrix theories; on the other hand, the photoabsorption spectra show not only the traditional quasi-Landau modulations but also new and unexpected modulations which may be related to classical periodic orbits embedded in a chaotic system.

• Received 14 May 1986

DOI:https://doi.org/10.1103/PhysRevLett.57.571