Ultrafast Cation-Dication Dynamics in Ammonia Borane: H-Migration to Roaming H₂ and Reduced H₃⁺ Formation under...

We report a femtosecond time-resolved strong-field study of ammonia borane (AB, BH₃NH₃) following both single and double ionization, revealing ultrafast fragmentation dynamics and hydrogen release. Time-resolved mass spectrometry and ab initio molecular dynamics simulations are used to identify the molecular origin of the neutral and ionic products. Singly ionized AB produces neutral H and H₂, while doubly ionized AB produces neutral H and H₂ along with H⁺, H₂⁺, and H₃⁺, all within 1 ps. Electronic-structure calculations show that H, H⁺, H₂, H₂⁺, and H₃⁺ originate predominantly from hydrogen atoms bound to the boron center and that their formation proceeds through hydrogen migration and, in some channels, neutral H₂ roaming. The calculations further indicate that the dication meets the structural and energetic requirements for neutral H₂ release, a prerequisite for forming astrochemically relevant H₃⁺. However, the large adiabatic relaxation energy causes most roaming H₂ to dissociate before proton abstraction, suppressing H₃⁺ formation. These results provide new insight into dissociative ionization pathways in hydrogen-rich molecules, extend mechanistic principles developed for halogenated alkanes to ammonia borane, and suggest implications for hydrogen-release chemistry in ammonia-borane-based storage materials.