Heteronuclear and Homonuclear High-Spin Alkali Trimers on Helium Nanodroplets

The electronic excitation spectra of all possible homo- and heteronuclear high-spin (quartet) trimers of K and Rb (${\mathrm{K}}_x{\mathrm{Rb}}_{3-x}$, $x=0\dots 3$) assembled on the surface of superfluid helium droplets, are measured in the spectral range from 10 600 to $17400{\mathrm{cm}}^{-1}$. A regular series of corresponding bands is observed, reflecting the similar electronic structure of all these trimers. For the assignment and separation of overlapping bands, we determine $x$ directly, with mass-selected beam depletion, and indirectly with a $V$-type double-resonance scheme. The assignment is confirmed by high-level ab initio calculations of the electronic structure of the bare trimers. The level structure is rationalized in terms of harmonic-oscillator states of the three valence electrons in a quantum-dot-like confining potential. We predict that three should be a magic number for high-spin alkali clusters.

Figure 1

Bottom panel: MSBD electronic-excitation spectra of ${\mathrm{K}}_x{\mathrm{Rb}}_{3-x}$ trimers, weighted to best fit the LIF spectrum above. Thick solid (red) line, ${\mathrm{KRb}}_2$; thick dashed (blue) line, ${\mathrm{K}}_2\mathrm{Rb}$; thin solid (black) line, ${\mathrm{Rb}}_3$; thin dashed (orange) line, ${\mathrm{K}}_3$, LT beam-depletion spectrum. Top panel: Comparison between a typical LIF spectrum (gray line) and the sum of the individual depletion spectra (black line).

Figure 2

Survey of the electronic-excitation bands of homo- and heteronuclear ${\mathrm{K}}_x{\mathrm{Rb}}_{3-x}$ trimers. Vertical lines mark the position of the ab initio transition energies (all quartet states).