Direct-potential fit of the K2 ground state in the entire bound, quasi-bound, and continuum energy range

Based on extensive experimental (spectroscopic) and theoretical (ab initio) information, the semi-empirical adiabatic potential of interatomic interaction has been constructed in analytical form for the electronic ground state of potassium dimer molecule K2. As the main massive of experimental input data, more than 15 000 lines of laser-induced fluorescence (LIF) A1Σu+-b3Πu→X1Σg+ progressions of 39K2 recorded by a high-resolution Fourier transform spectrometer with the accuracy of 0.001-0.002 cm-1 were used, which cover the entire bound and quasi-bound energy range up to the ground state dissociation threshold, in the range of vibrational quantum numbers vX = 0-82 and rotational quantum numbers JX = 1-149. More than 760 detected LIF lines have been referred to minor 39K41K and 41K2 isotopologues and used for an independent proof of mass-invariant properties of the constructed semi-empirical potential. In order to clarify the behavior of potential at long internuclear distances, the quasi-bound levels lying above the dissociation threshold were included, as well as the scattering lengths based on the positions of Feshbach magnetic resonances obtained in Tiemann et al. [Phys. Rev. Res. 2, 013366 (2020)]. The inner limb of the semi-empirical curve has been extrapolated above the experimental region by means of non-empirical interatomic potential calculated by a single-reference coupled-cluster CCSD(T)/CBS method. Two alternative continuous functions (CPE and MLR) were used for analytical approximation of the interatomic potential with a physically consistent extension into the continuum region. Both potentials appeared to be applicable in the whole bound, quasi-bound, and continuum range, allowing to avoid, at least within +/-0.002 cm-1 limit, the non-physical oscillation-like behavior, which appeared at reproduction of experimental LIF frequencies at using the analytical potential in Tiemann et al. [Phys. Rev. Res. 2, 013366 (2020)] and the molecular constants in the form of the Dunham polynomial in Amiot et al. [J. Chem. Phys. 103, 3350 (1995)].