The ground X 1Σg+ electronic state of the cesium dimer: Application of a direct potential fitting procedure

A collection of 16 544 fluorescence series spectroscopic line positions involving the A _{Σ1u +→X Σ1g+ transition in Cs2} has been analyzed by a modern direct potential fitting procedure to generate the first fully analytical potential energy function for the ground electronic state, and precise energy term values for the excited A Σ1u + state. The potential function yields an accurate representation of spectroscopic data that span 99.24% of the well depth and the number of fitted parameters is significantly less than half the number of parameters determined in conventional Dunham analyses. A novel variant of the Morse/long-range potential model has been employed in the representation of the ground state potential, and a critical comparison has been made with an extended modified Lennard-Jones potential model. Proper account has been taken of the known long-range van der Waals form of the potential, and our final potential function is determined with constrained literature values of the C₈ and C₁₀ dispersion energy coefficients, along with a fitted value of C₆=3.31(5)×10⁷ cm^-1 Å⁶=6870(100) a.u. The fitted dissociation energy (D_e) is compared with the precisely known value based on the recent analysis of data from a two-photon transfer process (STIRAP) in ultracold Cs atoms. It is concluded that hyperfine effects in the X Σ1g + state are not negligible, and that the estimate of D_e=3649.84(7) cm^-1 obtained in this work represents an effective dissociation limit lying between the two lowest hyperfine limits. Precise rotational and centrifugal distortion constants for the ground electronic state have also been calculated through conventional perturbation theory. These estimates are fully consistent with the derived potential function and the experimental spectroscopic information.