Effect of closed classical orbits on quantum spectra: ionization of atoms in a magnetic field

Measurements of the absorption spectrum near the ionization threshold for an atom in a strong magnetic field showed that the spectrum is a superposition of many oscillatory terms (''quasi-Landau oscillations''). We have developed a quantitative theory which shows that each classical closed electron orbit which begins and ends near the nucleus contributes an oscillatory term to the average oscillator strength. The theory gives new understanding of the behavior under combined Coulomb and Lorentz forces, and it elucidates the roles of isolated closed orbits in chaotic systems. The first results of this theory are shown to be in good agreement with experimental results.