Unraveling the Metastability of C_n²⁺ (n=2-4) Clusters

Pure carbon clusters have received considerable attention for a long time. However, fundamental questions such as what the smallest stable carbon cluster dication is remain unclear. Here, we investigated the stability and fragmentation behavior of C_n^{2+} (n=2-4) dications using state-of-the-art atom probe tomography. These small doubly charged carbon cluster ions were produced by laser-pulsed field evaporation from a tungsten carbide field emitter. Correlation analysis of the fragments detected in coincidence reveals that they only decay to C_{n-1}^+ + C^+. During C_2^{2+} => C^+ + C^+, significant kinetic energy release (\approx 5.75-7.8 eV) is evidenced. Through advanced experimental data processing combined with \textit{ab initio} calculations and simulations, we show that the field evaporated diatomic ^12C_2^{2+} dications are either in a weakly bound ^3{\Pi}_u and ^3{\Sigma}_g^- state, quickly dissociating under the intense electric field, or in a deeply bound electronic ^5{\Sigma}_u^- state with lifetimes longer than 180 ps.