Quantum defect theory for asymmetric tops: Application to the Rydberg spectrum of H2O

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Abstract
The multichannel quantum defect theory of rotational–electronic channel interactions and spectroscopic absorption intensities for rigid asymmetric tops is described. The symmetry selection rules are given in terms of the overall rotational–electronic parity and the even or odd character of K+a=Ka−λ. The angular momentum recoupling from Hund’s case (b) to (d) is shown to be accompanied by increasingly strong propensity rules K+a=‖K‘a−λ‘‖ and ‖N+−J‘‖≤l‘, which explain the simplicity of the rotationally resolved photoionization spectra of H2O and D2O without recourse to any predissociation mechanism. The main features of this spectrum are well reproduced, and ionization limits of 101 772(2) and 101 920(5) cm−1 are derived for H2O and D2O.