Unraveling the Metastability of C n 2+ ( 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. 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 (∼5.75-7.8 eV) is evidenced. Through advanced experimental data processing combined with ab initio calculations and simulations, we show that the field-evaporated diatomic ¹² C ₂ ²⁺ dications are either in weakly bound ³ Π _u and ³ Σ _g ^- states, quickly dissociating under the intense electric field, or in a deeply bound electronic ⁵ Σ _u ^- state with lifetimes >180 ps.