RESONANCE STRUCTURE IN THE VACUUM-ULTRAVIOLET FLUORESCENCE EXCITATIONAND MULTIPHOTON IONIZATION SPECTRA OF IBR

Vacuum-ultraviolet fluorescence excitation and resonance enhanced mult iphoton ionization (REMPI) spectra of IBr, cooled in a free-jet expans ion, are reported. Resonance structure, observed in the region lambda > 179 nm, is greatly reduced on cooling and is ascribed to heterogeneo us coupling between the E(0+) ion-pair state and the a6 Rydberg state. Observation of a new Rydberg series b6'[2PI1/2]c6s(0+) <-- X(0+) conf irms the origin of the dip-resonance structure in the fluorescence exc itation spectrum at wavelengths < 178 nm to be via homogeneous couplin g of this Rydberg state with the E(0+) ion-pair state of IBr. Both the a6 and b6' Rydberg states are predissociated by the repulsive limb(s) of a valence state(s), and thus the observed dip resonances result fr om interactions between three states, i.e., valence, Rydberg, and ion- pair states.