Spectroscopic analysis of the $\left( 2 \right)^3\Sigma ^+$ state of ${ }^{\mbox{41}}\mbox{K}{ }^{\mbox{87}}\mbox{Rb}$ towards ultracold rovibronic ground-state molecules

Optical transitions from loosely bound ${ }^{\mbox{41}}\mbox{K}{ }^{\mbox{87}}\mbox{Rb}$ molecules to the $\left( 2 \right)^3\Sigma ^+$ state were investigated to search for an optimal intermediate state in the STIRAP transfer into the rovibrational ground state. The loosely bound molecules were produced by photoassociation of laser-cooled ${ }^{\mbox{41}}\mbox{K}$ and ${ }^{\mbox{87}}\mbox{Rb}$ atoms and detected by resonance-enhanced multi-photon ionization. High-resolution depletion spectra were obtained by scanning Ti: Sapphire laser in the wavelength range 880-920nm where no information on the $\left( 2 \right)^3\Sigma ^+$ state was available. Vibrational progressions over 17 levels and rotational progressions of J = 0-5 ($\Omega =0)$ and J = 1-5 ($\Omega =1)$ were observed. Rotational constants extracted from the spectra show a wide variation among vibrational levels which indicates spin-orbit mixing of the $\left( 1 \right)^3\Pi $ state into the $\left( 2 \right)^3\Sigma ^+$ state. The observed small splitting in each rotational line was understood in terms of hyperfine interaction between the nuclear spin of ${ }^{\mbox{87}}\mbox{Rb}$and electronic spin.