Key Factors in H₃⁺ Formation from Organic Molecules: Experiments and Theory

The formation of H₃⁺ following the double ionization of small organic compounds via a roaming mechanism—where H2 is generated and proton abstraction occurs—has recently received significant attention. However, a cohesive model explaining trends in H₃⁺ yields from these unimolecular reactions is still lacking. We report yield and femtosecond time-resolved measurements after the strong-field double ionization of CH₃X molecules (where X = OD, Cl, NCS, CN, SCN, and I). These measurements, along with double-ionization-potential calculations using equation-of-motion coupled-cluster methods, help us identify the key factors influencing H₃⁺ formation in certain doubly ionized CH₃X species and its absence in others.

Additionally, we conduct ab initio molecular dynamics simulations to gain detailed insights into the mechanisms, yields, and timescales of H₃⁺ production. Our findings indicate that the excess relaxation energy released after the double ionization of CH₃X molecules, along with significant geometrical distortion favoring H₂ formation prior to proton abstraction, enhances H₃⁺ generation. This study offers valuable guidelines for exploring alternative sources of H₃⁺ in the universe.