Knowledge Bank

Like their homonuclear brethren, the heteronuclear diatomic halogens display intense emission in the UV-visible region following optical or electrical excitation in the presence of inter buffer gases. These spectra are dominated by the charge-transfer transition labeled $D\prime \to A\prime$, but they also display features attributable to transitions originating from other ion-pair states. For reasons not fully understood, the ion-pair states of a given cluster (correlating with a given ionic asymptote) tend to bunch more closely energetically in the heteronuclear than in the homonuclear halogens. This means that perturbations (from rotational coupling) are prominent in the heteronuclear halogen spectra, even for the low $v\prime$ levels which dominate the emission spectra. For example, in ICI these perturbations offer a beautiful ``textbook” illustration of an accidental $perturbation.1$ For a proper characterization of such transitions a full deperturbation analysis is essential. Using tesla discharge sources containing single halogen isotopomers, we have recorded and analyzed perturbed emission spectra of ICI, BrCl, and IBr. This talk will review recent progress on the deperturbation analyses of these systems, which are now being done in conjunction with complementary work on these same transitions recorded in absorption in a free-jet $expansion.2$