Unexplained dissociation pathways of two-body fragmentation of methane dication

The ion-induced fragmentation of CH₄ ²⁺ into H⁺ and CH₃ ⁺ is studied using a cold target recoil ion momentum spectroscopy in coincidence with the charge state of the post-collision projectile. Using constant velocity Ar⁹⁺ and N³⁺, results from four different datasets are presented, with a selection on the final charge state of the projectile (Ar⁸⁺ or Ar⁷⁺ and N²⁺ or N⁺). Three distinct dissociation pathways (I, II, and III) are observed for each dataset, with the mean kinetic energy release values of around 4.7, 5.8, and 7.9 eV, respectively. The electronic states that are populated correspond to electronic configurations (1t₂)^-2 and (2a₁)^-1(1t₂)^-1 of the methane dication, CH₄ ²⁺. The relative branching ratios between the three pathways are discussed as a function of the charge state of the post-collision projectile, and a strong correlation with the specific nature of the ion-molecule interaction is found. The existing ab initio calculations have provided an explanation only for pathway II. In this article, we propose an explanation for pathway III, but pathway I still remains unexplained and requires further theoretical efforts. A discussion of the dependence of dissociation on the mode of excitation is presented.