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

Abstract

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 Cl2 as well as one ion-pair state for Br2 were newly determined. For I2, 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 I2 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 thef′0g+(1D2) 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.