Abstract

VUV spectra involving ion-pair states for many of the halogens have been known since the beginning of quantum mechanics. To date, the use of synchrotron radiation has offered the most flexible method to record these spectra. However, since vertical transitions from the ground state probe very high ion-pair state vibrational levels, room temperature spectra tend to be severely congested from extensive rotational structure, hot bands, and naturally occurring isotopomers. We have demonstrated, for Br₂ and ICl, that combining four-wave mixing to generate tunable, coherent, and monochromatic VUV radiation, with supersonic jet cooling, allows ion-pair vibrational structures to be readily resolved. New results on the lowest ion-pair states of IBr (EO⁺) and I₂ $\left({\text{DO}}_u^{+}\right)$ are presented which include the first unambiguous vibrational analyses of their VUV spectra. Unlike I₂, IBr spectra exhibit perturbations which manifest themselves in large and erratic intensity variations in fluorescence excitation.

© 1989 Optical Society of America