Dissociation and predissociation in the ns ¹B₁ and np¹A₂ series of H₂O

Abstract

It is shown how a carefully devised quantum defect function can be used to unify direct dissociation of the lowest member of a Rydberg series, with predissociation of the higher vibronic states. Results, for a one-dimensional model, obtained by the techniques of multichannel quantum defect theory (MQDT) are compared with those of a simpler Fano golden rule approximation. A simplified model calculation for the ns¹B₁ series of H₂O and D₂O yields results in good accord with experiment, and a dramatic isotope effect is explained. A similar application to the elusive np¹A₂ series leads to the prediction of sharp rotational structure for the np¹A₂(000) bands with n≥4, despite the evidence of a very broad Franck–Condon profile for the dissociative 3p¹A₂ state. A band at 90 051 cm⁻¹ in the 2+1 REMPI spectrum is tentatively assigned to the 4p member of this ¹A₂ series.

Keywords:

• Dissociation
• Predissociation
• MQDT
• R matrix

Acknowledgements

It is a particular pleasure to acknowledge the author's debt to Christian Jungen for help and advice on many occasions. The grant of an Emeritus Fellowship from the Leverhulme Trust and the hospitality of the Chemistry Department of ETH Zurich, where this work was initiated, are also gratefully recorded.