Fourier-transform spectroscopy and global deperturbation treatment of the $A^1Σ^+_u$ and $b^3Π_u$ states of K$_2$ in the entire bound energy range

Abstract

Rotationally-resolved Fourier-transform spectra of laser-induced fluorescence (LIF) $A^1Σ^+_u\sim b^3Π_u \rightarrow X^1Σ^+_g$ of K$_2$ molecules were recorded and analyzed yielding 4053 term values of spin-orbit (SO) coupled $A \sim b$ complex of $^{39}$K$_2$ isotopologue with ca 0.01 cm$^{-1}$ accuracy. Their compilation with 1739 term values from previously published sources allowed to cover the energy range [9955, 17436] cm$^{-1}$ from the bottom of the lower-lying $b^3Π_u$ state up to the vicinity of the atomic asymptote $4s\;^2{\rm S}_{1/2}$ + $4p\;^2{\rm P}_{1/2}$, with rotational quantum number $J\in[0,149]$. The experimental data were processed by a direct 6$\times$6 coupled-channel (CC) deperturbation treatment, which accounted explicitly for both SO and electronic-rotational interactions between six states: $A^1Σ^+_u$($0$), $b^3Π_u$(0,1,2), $c^3Σ_u$(1), and $B^1Π_u$(1). The initial parameters of the global deperturbation model have been estimated in the framework of {\it ab initio} electronic structure calculations applying multi-referenced configuration-interaction and coupled-clusters methods. The interatomic potentials analytically defined for $A$ and $b$ states, as well as SO-splitting of the triplet $b$ state and $A\sim b$ SO-coupling functions have been refined to fit 5792 term values of the $^{39}$K$_2$ isotopologue, whereas the rest parameters were fixed on their {\it ab initio} values. The resulting mass-invariant parameters of the CC model reproduced the overall rovibronic term energies of the $A \sim b$ complex of $^{39}$K$_2$ with the accuracy, which is well within the experimental errors. This deperturbation analysis provided the refined dissociation energy $T_{dis}$=17474.569(5) cm$^{-1}$ and the long-range coefficient $C_3^Σ$ = 5.501(4) $\times10^5$cm$^{-1}$ A$^3$ relevant to the non-relativistic atomic limit $4s$+$4p$.