Direct-potential-fit analyses yield improved empirical potentials for the ground X ^1Σ _g^+ state of Be2

We have performed new direct-potential-fit (DPF) analyses of the rotationally resolved A ^1Π _u(v^' }=2,3;J^' }=1,2)rArr X ^1Σ _g^+(v^' ' }in [0, 11];J^' ' }in [0,3]) stimulated emission pumping spectra of Be₂ [J. M. Merritt, V. E. Bondybey, and M. C. Heaven, Science 324, 1548 (2009)] using two quite different analytical potential energy functions that incorporate the correct theoretically known long-range behaviour in different ways. These functions are: the damped Morse/long-range potential [R. J. Le Roy, C. C. Haugen, J. Tao, and H. Li, Mol. Phys. 109, 435 (2011)], and the Chebyshev polynomial expansion potential [L. Busevica, I. Klincare, O. Nikolayeva, M. Tamanis, R. Ferber, V. V. Meshkov, E. A. Pazyuk, and A. V. Stolyarov, J. Chem. Phys. 134, 104307 (2011)]. In contrast with the expanded Morse oscillator potential determined in the original DPF analysis of Merritt et al. [Science 324, 1548 (2009)], both of these functions unambiguously support the existence of the v″ = 11 last vibrational levels which is bound by only ∼0.5 cm^-1, and they give equivalent, essentially exact predictions for this level when using the original data set which ended at v″ = 10. These empirical potentials predict an equilibrium distance of r_e = 2.445(5) Å and a well depth of {{D}}_e=934.9(0.4) cm^-1, values which agree (within the uncertainties) with the best ab initio estimates of 2.444(10) Å and 935(10) cm^-1, respectively [J. Koput, Phys. Chem. Chem. Phys. 13, 20311 (2011)].