Observation of the state by infrared–infrared double resonance

The Cs 2 3 3 Σ g + state, which was observed previously by two-photon excitation [D. Li, F. Xie, Li Li, S. Magnier, V.B. Sovkov, V.S. Ivanov, Chem. Phys. Lett. 441 (2007) 39], has been observed by infrared–infrared (IR–IR) double resonance spectroscopy. One hundred and seventy IR–IR double resonance lines have been assigned to transitions into the 3 3 Σ g + v = 2–12 levels. Hyperfine structure of this state has been resolved in the excitation spectra. Molecular constants and the potential energy curve of this state are reported in this Letter.