OBSERVATION OF THE TRANSITION TO CHAOS IN THE LEVEL STATISTICS OF DIAMAGNETIC HELIUM

We have investigated level statistics in constant-scaled-energy spectr a of the diamagnetic helium atom. In a transversely laser-cooled beam metastable triplet atoms are excited to Rydberg states in the presence of a magnetic field, resolving all resonances. In the regime of regul ar classical motion level distributions obeying Poisson statistics are observed. Small deviations are attributed to scattering at the nonhyd rogenic He+ core as inferred from closed-orbit theory. The nearest-nei ghbor spacings in spectra in the chaotic regime demonstrate a clear sh ift towards Wigner statistics. [S0031-9007(98)07780-1].