Effects of core scattering on level statistics in helium atoms in scaled external fields

We recorded constant scaled-energy spectra of helium Rydberg atoms in both electric and magnetic fields in a regime where the hydrogen atom behaves re gularly. Jonckheere, Gremaud, and Delande [Phys. Rev. Lett. 81, 2442 (1998) ] recently showed that the nearest-neighbor statistics of sufficiently high ly excited states follows a delta-Poisson distribution, which depends on th e quantum defect delta. We find global deviations from this delta-Poisson m odel, which we attribute to the size of the effective Planck constant h(eff ). Significant deviations at small level spacings indicate that the descrip tion of level statistics with the delta-Poisson model is incomplete. [S1050 -2947(99)50409-5].