Ionization-Induced Fragmentation of Argon Clusters: A Journey from Micro-Explosion to Sequential Evaporation

Abstract aThe ionization-induced fragmentation dynamics of neutral argon clusters ($\text{Ar}_N$, ($N=4-50$)) are investigated using molecular dynamics simulations based on high-precision ab initio pairwise potentials. The interaction curves are constructed at the CCSD/cc-pVQZ level for the neutral state and the MRCI/aug-cc-pVQZ level for the cationic state to accurately describe the energetic driving forces. Following ionization, the cluster undergoes rapid contraction, leading to an explosive multi-body dissociation that promptly ejects several neutral atoms and leaves a smaller, vibrationally hot cluster ion, which subsequently cools via slow, sequential evaporation of one or a few additional atoms until a stable cationic core is formed. Statistical analysis reveals a highly sub-linear scaling relationship between the initial neutral cluster size $N_0$ and the final stable cationic core size $N_f$. For instance, a substantial increase in parent size from $N_0=13$ to $N_0=50$ results in only a marginal increase in the surviving product. This disproportionate mass loss is attributed to a mechanism: atoms farther from Ar$^+$ release more energy during cooling while requiring less energy to evaporate. As a result, the relaxation energy released from cooling a single outer atom can drive the evaporation of many others. Our results suggest that the ionization process acts as a "size-focusing" filter, implying that stable cationic clusters observed in mass spectrometry likely originate from significantly larger neutral precursors.