Radiative lifetimes and self-quenching rate constants of the ion-pair states of halogen molecules

A series of charge-transfer excited states of halogen molecules is known as ion-pair states. The radiative lifetime and self-quenching rate constant for a certain excited state are among the most important physical properties representing the concerned ion-pair state. Thus, the determination of these parameters is indispensable for discussing the radiative and nonradiative dynamics in the excited manifolds under collision conditions. In the present study, the optical-optical double resonance spectroscopy technique was used to measure the fluorescence lifetimes and self-quenching rate constants of the ion-pair states of halogen molecules. The fluorescence lifetimes and self-quenching constants of five ion-pair states for Cl-2 as well as one ion-pair state for Br-2 were newly determined. For I-2 , fluorescence lifetimes of five ion-pair states and self-quenching rate constants of thirteen ion-pair states were evaluated for the first time. The lifetimes and self-quenching constants of all 18 ion-pair states of I-2 so far identified spectroscopically were determined. The collision-free lifetimes of the ion-pair states range from several nanoseconds to several tens of nanoseconds, except for the f' 0(g)(+)(D-1(2)) state, which are shorter than those of lower-valence electronic states. All ion-pair states of halogen molecules have very large self-quenching cross-sections, which can be explained by the harpoon mechanism. (C) 2021 Elsevier Ltd. All rights reserved.