The 21Σg+ state of 6Li2: Observation, analysis, and examination of the isotope effect

Abstract Upon the excitation of 6 Li 2 into the B 1 Π u state by the 488.0- and 514.5-nm lines of an Ar + laser, the 2 1 Σ g + - A 2 Σ u + system has been observed by Fourier transform spectroscopy. The vibrational assignments and the rotational analysis are done in the same way as for 7 Li 2 . The following principal spectroscopic molecular constants are derived from the spectra: B e = 0.42036(5) cm −1 α e = 5.19(10) × 10 −3 cm −1 ω e = 138.920(40) cm −1 ω e X e = − 1.98(2) cm −1 R e = 3.6516(2) A . The isotopic relations are used to compare the 6 Li 2 and 7 Li 2 constants and, in nearly all cases, the agreement is satisfactory taking into account the quoted uncertainties. Both RKR and IPA potential curves are computed and their isotopic dependence is discussed. The general conclusion is that the 7 Li 2 data reproduce the 6 Li 2 potential with nearly the same accuracy as the 6 Li 2 data especially with the IPA approach. In all sections the results are compared with the ab initio calculations and the agreement is remarkable.