We present experimental relative intensity distribution and theoretical radiative probabilities for the spin-allowed $E(4){\phantom{\rule{0.16em}{0ex}}}^{1}\phantom{\rule{-0.16em}{0ex}}{\ensuremath{\Sigma}}^{+}\ensuremath{-}X{\phantom{\rule{0.16em}{0ex}}}^{1}\phantom{\rule{-0.16em}{0ex}}{\ensuremath{\Sigma}}^{+}$ and spin-forbidden $E(4){\phantom{\rule{0.16em}{0ex}}}^{1}\phantom{\rule{-0.16em}{0ex}}{\ensuremath{\Sigma}}^{+}\ensuremath{-}a{\phantom{\rule{0.16em}{0ex}}}^{3}\phantom{\rule{-0.16em}{0ex}}{\ensuremath{\Sigma}}^{+}$ rovibronic transitions studied in the KCs molecule. The absorbtion and emission Einstein coefficients were predicted in a wide range of vibrational
v and rotational
J quantum numbers for both singlet-singlet $E\ensuremath{-}X$ and singlet-triplet $E\ensuremath{-}a$ rovibronic transitions along with radiative lifetimes of the upper
E state and branching ratios of spontaneous emission into the lower-lying electronic states, including both bound-bound and bound-continuum parts of the spectra. The required spin-allowed $E(4){\phantom{\rule{0.16em}{0ex}}}^{1}\phantom{\rule{-0.16em}{0ex}}{\ensuremath{\Sigma}}^{+}\ensuremath{-}X;A;C{\phantom{\rule{0.16em}{0ex}}}^{1}\phantom{\rule{-0.16em}{0ex}}{\ensuremath{\Sigma}}^{+};B{\phantom{\rule{0.16em}{0ex}}}^{1}\phantom{\rule{-0.16em}{0ex}}\ensuremath{\Pi}$ transition dipole moments were obtained in the framework of quasirelativistic electronic structure calculations. The regular spin-orbit coupling with the nearest ${}^{3}\phantom{\rule{-0.16em}{0ex}}\ensuremath{\Pi}$ states is found to be sufficient to induce the spin-forbidden $E(4){\phantom{\rule{0.16em}{0ex}}}^{1}\phantom{\rule{-0.16em}{0ex}}{\ensuremath{\Sigma}}^{+}\ensuremath{-}a;c{\phantom{\rule{0.16em}{0ex}}}^{3}\phantom{\rule{-0.16em}{0ex}}{\ensuremath{\Sigma}}^{+};b{\phantom{\rule{0.16em}{0ex}}}^{3}\phantom{\rule{-0.16em}{0ex}}\ensuremath{\Pi}$ transitions by borrowing probabilities from the relevant $(1\ensuremath{-}5){\phantom{\rule{0.16em}{0ex}}}^{3}\phantom{\rule{-0.16em}{0ex}}\ensuremath{\Pi}\ensuremath{-}(a;c){\phantom{\rule{0.16em}{0ex}}}^{3}\phantom{\rule{-0.16em}{0ex}}{\ensuremath{\Sigma}}^{+};b{\phantom{\rule{0.16em}{0ex}}}^{3}\phantom{\rule{-0.16em}{0ex}}\ensuremath{\Pi}$ transitions. Relative intensity distributions in rotationally resolved $E\ensuremath{\rightarrow}X$ and $E\ensuremath{\rightarrow}a$ laser-induced fluorescence (LIF) progressions were determined from Fourier transform spectra. Experimental LIF intensity distributions demonstrated good agreement with their predicted counterparts. The simulations demonstrate that the translationally ultracold KCs molecules could be efficiently produced in their absolute ground state in one stimulated Raman step $X{\phantom{\rule{0.16em}{0ex}}}^{1}\phantom{\rule{-0.16em}{0ex}}{\ensuremath{\Sigma}}^{+};a{\phantom{\rule{0.16em}{0ex}}}^{3}\phantom{\rule{-0.16em}{0ex}}{\ensuremath{\Sigma}}^{+}({v}^{\ensuremath{'}\ensuremath{'}})\ensuremath{\rightarrow}E(4){\phantom{\rule{0.16em}{0ex}}}^{1}\phantom{\rule{-0.16em}{0ex}}{\ensuremath{\Sigma}}^{+}({v}_{E}^{\ensuremath{'}}=44,{J}_{E}^{\ensuremath{'}}=1)\ensuremath{\rightarrow}X{\phantom{\rule{0.16em}{0ex}}}^{1}\phantom{\rule{-0.16em}{0ex}}{\ensuremath{\Sigma}}^{+}({v}_{X}^{\ensuremath{'}\ensuremath{'}}=0,{J}_{X}^{\ensuremath{'}\ensuremath{'}}=0)$ from the weakly bound rovibronic levels ${v}^{\ensuremath{'}\ensuremath{'}}$ of both singlet
X and triplet
a states.
We present a new quantum gas apparatus for ultracold mixtures of K and Cs atoms and ultracold samples of KCs ground-state molecules. We demonstrate the apparatus’ capabilities by producing Bose–Einstein condensates of 39K and 133Cs in a manner that will eventually allow sequential condensation within one experimental cycle, precise sample overlap and magnetic association of atoms into KCs molecules. The condensates are created independently without relying on sympathetic cooling. Our approach is universal and applicable to other species combinations when the two species show dramatically different behavior in terms of loss mechanisms and post laser cooling temperatures, i.e. species combinations that make parallel generation of quantum degenerate samples challenging. We give an outlook over the next experiments involving e.g. sample mixing, molecule formation and transport into a science chamber for high-resolution spatial imaging of novel quantum-many body phases based on K–Cs.