Using the method of Doppler-free double resonance polarization spectroscopy transitions from selectively pumped levels (v′, J′) of the D1Σ+u state down to levels (v″, J″) of the X 1Σ+g state with v″= 0–140 have been measured with high accuracy. A long range analysis, based on 21 outer turning points of the IPA potential with v″=110-130 and on calculated exchange energies and second order van der Waals coefficients Cn with n=6, 8, 10, and 12 leads to a dissociation energy De =3649.5±0.8 cm−1 (0.452 49 eV). The influence of the third order C11 contribution and the hyperfine structure on the correct value of De are briefly discussed.

More than 2000 completely resolved rotational lines in the C 1Πu(1u)←X 1Σ+g(0+g) band-system and 200 lines in the D 1Σu(0+u)←X 1Σ+g system of Cs2 have been accurately measured with the techniques of Doppler-free polarization spectroscopy and optical–optical double resonance (OODR). The wave numbers of these lines were determined within 10−3 cm−1 using a traveling Michelson interferometer and a 120 cm confocal Fabray–Perot interferometer. The OODR technique allows the unambiguous assignment of even perturbed lines, and enables us to find different perturbations in the upper C 1Πu(1u) state. A thorough analysis of the measured data, including laser induced fluorescence lines from higher electronic states into high lying vibrational levels of the electronic ground state, yields molecular constants, potential energy curves, and Franck–Condon factors.

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