Unambiguous quantum-state assignment of Cs3 trimers on helium nanodroplets

Determine the electronic quantum state(s) of cesium trimers (Cs3) formed on helium nanodroplets under the experimental conditions of laser-induced Coulomb explosion, in particular clarifying whether the observed kinetic energy peaks at 2.36 eV and 2.57 eV correspond to the quartet 4A2' state, the doublet 2B2 state, or other states, and reconcile the droplet-size dependence of peak dominance to achieve an unambiguous state assignment.

Background

The authors measured the kinetic energy distributions of Ak+ ions resulting from laser-induced Coulomb explosion of alkali trimers formed on helium nanodroplets. For Na3, K3, and Rb3, the kinetic energy peaks and angular covariance supported assignment to the 4A2' state, allowing determination of equilibrium bond lengths. In contrast, Cs3 showed a broad structure with two peaks at 2.36 eV and 2.57 eV, and three-fold covariance confirmed these ions originate from Cs3 trimers.

The expected kinetic energy for Coulomb explosion of Cs3 in the 4A2' state at the literature equilibrium distance (5.56 Å) is 2.59 eV, which matches the 2.57 eV peak. However, measurements at different nozzle temperatures (affecting droplet size) showed that the lower-energy 2.36 eV peak dominates at higher temperature (smaller droplets), while the 2.57 eV peak dominates at lower temperature, contrary to expectations that high-spin trimers should be better preserved and dominate across droplet sizes. Energies expected for the 2B2 state (3.0 and 2.8 eV) do not match either peak. Consequently, the state assignment for Cs3 remains unresolved.