ION-PAIR FORMATION IN THE PHOTODISSOCIATION OF HF AND DF

The excitation functions for ion-pair formation in supersonically cool ed HF and DF have been measured using synchrotron radiation with photo n resolutions of 0.12 and 0.06 nm. The excitation functions for both m olecules are characterized by an intense, sharp peak, essentially coin cident with the thermodynamic onset for ion-pair formation, followed b y weaker, broader structure to higher energy. All of the structure is interpreted as arising from the photoexcitation of Rydberg states of t he neutral molecules that are predissociated by the V (1) Sigma(+) ion -pair state potential. Ab initio calculations using multichannel quant um defect theory to include both predissociation and autoionization en abled assignments of the observed structure to be made and the constru ction of simulated ion-pair excitation function spectra in an energy r egion encompassing similar to 0.25 eV of the lowest energy part of the experimental excitation functions. The intense first peak in the ion- pair excitation function of both molecules is assigned to a high vibra tional level of the 3s sigma Rydberg state converging to the A (2) Sig ma(+) ion state, while most of the structure immediately following the first peak is assigned to d Rydberg complexes converging to the v(+) = 1 level of both X (II)-I-2 ion substates. (C) 1995 American Institut e of Physics.