Two techniques for rotational analysis of the nd←1b1 Rydberg spectra of H2O and D2O are discussed—a
Coriolis coupled asymmetric top model for the room temperature 3d←1b1 cluster, and an extension of
multichannel quantum defect theory to asymmetric tops, which is applied to recent jet cooled photoionization
spectra of the nd←1b1 series converging to the (100) vibrational levels of H2O+ and D2O+. The
quantum defects obtained by the two methods are in good agreement and new ionization limits to the
(100) series are derived: IP(100) = 104985(2) cm−1 for H2O and 104262(1) cm−1 for D2O.