An optical–optical double resonance spectroscopy of Br2: Observation of the 1g(3P2) ion-pair state

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Abstract
We report the first observation of the 1g(3P2) ion‐pair state of Br2 by the optical–optical double resonance technique. This state is accessible from the ground state stepwisely in the pump–probe excitation scheme, 1g(3P2) –A 3Π(1u) –X 1Σ+g. The double resonance transitions were interpreted on the basis of the optical selection rules. Emission spectra of the 1g(3P2) –A 3Π(1u) system were interpreted to establish the absolute v′ numbering of the ion‐pair state from the Franck–Condon factor calculations. Observations were made on the v′=0 through v′=10 levels of the 79Br2 isotope species. The molecular parameters obtained are Te =49 390.364(16), ωe =151.704(10), ωexe =0.4169(24), ωeye =4.7(16)×10−4, Be =0.042 1840(73), αe =1.5209(61)×10−4, De =1.32(10)×10−8, and q=4.91(19)×10−5 (all in cm−1 with 3σ in parentheses).