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Rotational analyses are presented for selected bands of the 21 (v‘= 1−3) ← X1 (v‘‘ = 0) transition of supersonically cooled Cl2, recorded in fluorescence excitation near λ ≤ 128 nm with a tunable, coherent, and monochromatic vacuum ultraviolet laser generated by four-wave difference mixing in Kr gas. Absolute J-numbering was established in part by spectral simulation. Although the band system is perturbed, the effective rotational constants derived in this work are consistent with theoretical calculations that place the excited two-state equilibrium bond length at ∼2.1 Å.

Vacuum ultraviolet (VUV) spectra of Cl2 are highly irregular due to extensive excited state perturbations, and in the past, have been difficult to interpret1. This situation was offset substantially by the ab initio calculations of Peyerimhoff and Beunker2. They showed that the strongest VUV transition below 145 nm involves a third tier 1 ∑ u + ion-pair valence state dissociating to Cl+(1Dg) + Cl-(1Sg), which interacts with the near isoenergetic 4pπ 1 ∑ u + Rydberg state leading to the formation of a double well potential curve, labelled 1 1 ∑ u + .