Ab initio analysis of Coulomb explosion in ionized molecules

The ultrafast dynamics of Coulomb explosion is fundamental to understanding molecular behavior under intense ionization. This work presents a comprehensive ab initio study of these processes in alkali metal dimers (Li₂ to Cs₂) and trimers (Li₃ to Cs₃) following sudden ionization. Using high-level multireference configuration interaction (MRCI+Q) and coupled-cluster [CCSD(T)] wavefunctions, we calculated relevant molecular properties of the X¹Σ+g and a³Σ+u electronic states of dimers and the high-spin quartet (⁴A') state of trimers. In our model, the reflection principle was employed to reconstruct kinetic energy release (KER) spectra and velocity map imaging (VMI) directly from the calculated nuclear probability densities. The present simulated spectra show agreement with experimental benchmarks, validating the theoretical framework. Furthermore, we also include a kinematic model with an effective ionization time of 4 fs to quantify the mass-dependent breakdown of the frozen-nuclei approximation. The results reveal that for lithium trimers, ZPE-induced nuclear displacement becomes sufficiently large to produce an energy spread ΔE that exceeds the experimental resolution threshold, leading to spatial delocalization that may obscure the VMI signal. This work provides a computationally efficient methodology for predicting fragmentation patterns and establishes a quantitative link between initial quantum states and experimental observables in ultrafast Coulomb explosion imaging.