Knowledge Bank

We use helium droplets to assemble all possible combinations of trimers of potassium and rubidium (K$3$, K$2$Rb, KRb$2$, Rb$3$) in a quartet-spin state, and we measure their electronic excitation spectra in the range 10600--17400~cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$. We use two methods to separate overlapping spectra: visible-visible V-type double-resonance and mass-selective beam depletion. We observe and assign the laser-induced-fluorescence spectra of ten new bands. Assignment is backed by high-level \textit{ab initio} calculations of the electronic levels structure of these molecules. Both experiment and calculations show highly regular patterns which we elucidate in terms of an electron-shell structure similar in spirit to that observed in Na clusters.} \textbf{52,} 2141 (1984).}