Ultrafast Cation-Dication Dynamics in Ammonia Borane under Strong-Field Ionization

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-of-flight mass spectrometry, ab initio molecular dynamics, and high-level electronic-structure calculations are used to identify the molecular origin of the neutral and ionic products. Singly ionized AB releases only neutral H and H₂, whereas doubly ionized AB releases neutral H and H₂ along with H⁺, H₂⁺, and H₃⁺, all within 1 ps. Computations indicate that these hydrogen fragments originate predominantly from the boron-bound hydrogens and are formed through rapid H migration; in key channels a neutral H₂ intermediate roams before subsequent reaction. The dication satisfies the structural and energetic requirements for neutral H₂ loss, a prerequisite for generating astrochemically relevant H₃⁺. However, its large adiabatic relaxation energy favors H₂ escape over efficient proton abstraction, thereby suppressing H₃⁺ formation. Kinetic energy release distributions extracted from the ionic channels further constrain the potential energy surfaces. Together, the measurements and simulations clarify dissociative ionization pathways in a hydrogen-rich molecule, extend mechanistic ideas developed for halogenated alkanes to AB, and inform hydrogen-release chemistry relevant to ammonia-borane-based storage materials. These findings benchmark AB as a compact model for hydrogen-rich fragmentation.