Angle-resolved energetic-ion yield spectra have been measured in the 0 Is excitation region of N2O. Franck-Condon analysis based on ab initio two-dimensional potential energy surfaces of the core-excited Rydberg states reproduces well the observed vibrational excitations specific to the individual Rydberg states. The irregular Rydberg behavior in the Sigma-symmetry absorption spectrum is attributed the valence-Rydberg coupling in light of the second moment analysis. (c) 2006 Elsevier B.V. All rights reserved.