Quasi-Landau Structure of Diamagnetic Helium Rydberg Atoms

Diamagnetism in helium Rydberg atoms is studied near the ionisation threshold using constant scaled-energy laser spectroscopy. Quasi-Landau resonances in the Fourier transform of the energy spectrum are explained using the classical periodic-orbit theory. Long laser scans combined with a high-frequency resolution allow for a detailed comparison between experiment and theory. Resonances with a scaled action /π up to 20 are identified and reproduced with an accuracy of 0.015. It is demonstrated that interference effects between resonances should be taken into account.