Heavy Rydberg behaviour in high vibrational levels of some ion-pair states of the halogens and inter-halogens

We report the identification of heavy Rydberg resonances in the ion-pair spectra of I-2, Cl-2, ICl, and IBr. Extensive vibrational progressions are analysed in terms of the energy dependence of the quantum defect delta(E-b) rather than as Dunham expansions. This is shown to define the heavy Rydberg region, providing a more revealing fit to the data with fewer coefficients and leads just as easily to numbering data sets separated by gaps in the observed vibrational progressions. Interaction of heavy Rydberg states with electronic Rydberg states at avoided crossings on the inner wall of the ion-pair potential is shown to produce distinctive changes in the energy dependence of delta(E-b), with weak and strong interactions readily distinguished. Heavy Rydberg behaviour is found to extend well below near-dissociation states, down to vibrational levels similar to 18 000-20 000 cm(-1) below dissociation. The rapid semi-classical calculation of delta(E-b) for heavy Rydberg states is emphasised and shows their absolute magnitude to be essentially the volume of phase space excluded from the vibrational motion by avoiding core-core penetration of the ions. (C) 2015 AIP Publishing LLC.