Ultrafast Cation–Dication Dynamics in Ammonia Borane: H-Migration to Roaming H 2 and Reduced H 3 + Formation under Strong-Field Ionization

High Resolution Image Download MS PowerPoint Slide We report a femtosecond time-resolved strong-field study of ammonia borane (AB, BH 3 NH 3 ) 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 H 2, while doubly ionized AB produces neutral H and H 2 along with H +, H 2 +, and H 3 +, all within 1 ps. Electronic-structure calculations show that H, H +, H 2, H 2 +, and H 3 + originate predominantly from hydrogen atoms bound to the boron center and that their formation proceeds through hydrogen migration and, in some channels, neutral H 2 roaming. The calculations further indicate that the dication meets the structural and energetic requirements for neutral H 2 release, a prerequisite for forming astrochemically relevant H 3 + . However, the large adiabatic relaxation energy causes most roaming H 2 to dissociate before proton abstraction, suppressing H 3 + 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.