Dissociation and predissociation in the ns 1B1 and np 1A2 series of H2O

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 ns(1)B(1) 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 np(1)A(2) series leads to the prediction of sharp rotational structure for the np(1)A(2)( 000) bands with n >= 4, despite the evidence of a very broad Franck-Condon pro. le for the dissociative 3p(1)A(2) state. A band at 90 051 cm(-1) in the 2+1 REMPI spectrum is tentatively assigned to the 4p member of this (1)A(2) series.