The photoionization spectrum of helium shows considerable complexity close to the double-ionization threshold. By analyzing the results from both our recent experiments and ab initio three- and one-dimensional calculations, we show that the statistical properties of the spacings between neighboring energy levels clearly display a transition towards quantum chaos.

• Received 2 June 2000

DOI:https://doi.org/10.1103/PhysRevLett.86.3747