Rydberg atoms in uniform magnetic fields: Uncovering the transition from regularity to irregularity in a quantum system.

We investigate the eigenvalue spectra of hydrogen Rydberg atoms in strong magnetic fields for manifestations of quantum stochasticity and find (j) a smooth transition from a Poisson.type to a Wignertype distribution of level spacings in the range of energy where classical motion becomes increasingly chaotic, (ij) the occurrence of mulliple avoided crossings, and (jji) connected with this. an extreme sensitivity of oscillator strengths, and thus of observable spectra, with respect to small va riations of an external parameter, viz., the magnetic field strength.