# Ultrafast Cation–Dication Dynamics in Ammonia Borane: H‑Migration to Roaming H2 and Reduced H3 + Formation under Strong-Field Ionization

## TL;DR

This study explores how ammonia borane breaks apart when strongly ionized, revealing ultrafast hydrogen release and formation of hydrogen ions.

## Contribution

The paper reveals new dissociative ionization pathways in ammonia borane and extends chemical principles from halogenated alkanes to hydrogen-rich molecules.

## Key findings

- Singly and doubly ionized ammonia borane produce hydrogen and hydrogen ions within 1 ps.
- Hydrogen migration and neutral H2 roaming are key mechanisms in fragment formation.
- Large adiabatic relaxation energy suppresses H3+ formation despite structural suitability.

## 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. 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.