Ultrafast cation-dication dynamics in ammonia borane: H-migration to roaming H2 and reduced H3+ formation under strong-field ionization

TL;DR

This study uses femtosecond spectroscopy and simulations to investigate ultrafast ionization and fragmentation in ammonia borane, revealing hydrogen migration, roaming H2, and suppressed H3+ formation relevant for hydrogen storage.

Contribution

It provides new mechanistic insights into ionization-induced hydrogen dynamics in ammonia borane, combining experimental and theoretical approaches.

Findings

Neutral H and H2 are produced from singly ionized AB.

Doubly ionized AB yields H, H2, H+, H2+, and H3+ within 1 ps.

Roaming H2 dissociates before forming H3+, suppressing H3+ formation.

Abstract

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. Mass spectrometry, combined with fragment correlation analysis and ab initio molecular dynamics simulations, is 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.