Factors governing H3+ formation from methyl halogens and pseudohalogens

The formation of H3+ following the double ionization of small organic compounds via a roaming mechanism, which involves the generation of H₂ and subsequent proton abstraction, has recently garnered significant attention. Nonetheless, a cohesive model explaining trends in the yield of H3+ characterizing these unimolecular reactions is yet to be established. We report yield and femtosecond time-resolved measurements following the strong-field double ionization of CH₃X molecules, where X = OD, Cl, NCS, CN, SCN, and I. These measurements, combined with double-ionization-potential equation-of-motion coupled-cluster ab initio calculations used to determine the geometries and energetics of CH₃X²⁺ dications, are employed to identify the key factors governing the formation of H3+ in certain doubly ionized CH₃X species and its absence in others. We also carry out ab initio molecular dynamics simulations to obtain detailed microscopic insights into the mechanism, yields, and timescales of H3+ production. We find that the excess relaxation energy released after double ionization of CH₃X molecules combined with substantial geometrical distortion that favors H₂ formation prior to proton abstraction boost the generation of H3+. Our study provides useful guidelines for examining alternative sources of H3+ in the universe.