Identification of new ion-pair states of molecular chlorine

Abstract

Three new ion-pair states of molecular chlorine have been identified by the double resonance method utilizing a sequential three-photon absorption. Two independently tunable dye lasers were used to excite the chlorine molecules into a specific rovibronic level of the B 3Π0+u state, and subsequently into levels of ungerade excited states by a simultaneous two-photon transition. The method enabled us to observe transitions to the low vibrational levels of ion-pair states in the energy range of 7.3–7.7 eV, which were directly inaccessible from the ground state due to the small Franck–Condon factors. Their spectroscopic parameters were derived from the vibrational and rotational analyses of two separate isotope species, 35Cl–35Cl and 35Cl–37Cl. The two-photon transitions from the B3Π0+u state to ion-pair ones consisted of simple O, Q, and S branches. The polarization effect on their transition probability established that the symmetry of three ion-pair states was 0+u in the Hund’s case c notation. The properties of these ion-pair states are discussed in terms of the electron configuration.