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An optical–optical double resonance (OODR) technique has been applied to study the E(0+g ) ion-pair state of Cl2 correlating with Cl−(1S)+Cl+(3P2). Observations were made on the v′=0 through v′=23 levels of the 35Cl2 isotope species. A set of Dunham molecular constants was derived by a global weighted least squares fit of 474 transitions (all in cm−1 and 3σ in parentheses): Y00=57 819.366(38), Y10=251.954(15), Y20=−1.0318(19), Y30=2.235(64)×10−3, Y01=0.116 556(62), Y11=−6.403(56)×10−4, and Y02=−4.8(37)×10−8. The emission spectra of the E(0+g )–B 3Π(0+u ) system observed in the OODR experiments were interpreted on the basis of Franck–Condon factor calculations, which established the absolute v′ numbering of the E(0+g ) ion-pair state.

The two-photon excitation spectrum of Cl2 using two tunable dye lasers has been studied in the 7 eV energy range. The f(0+g) ion-pair state correlating with Cl−(1S)+Cl+(3P0) at the dissociation limit appeared in the spectrum resulting from an optical–optical double resonance transition through the B 3Π(0+u) state. The absolute vibrational numbering of the f(0+g) state was established by direct observation of the f(0+g)–B 3Π(0+u) fluorescence. The molecular constants for the Cl2 f(0+g) state are derived (in cm−1 and 3σ in parentheses): Te=59 356.124(41), ωe=256.981(30), ωexe=1.2032(83), ωeye =0.003 69(47), Be=0.115 03(10), αe=7.58(17)×10−4, and D=1.44(68)×10−7.

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