Evidence for Intermolecular H–H Coupling in Ionization of Dense Ammonia

. Density functional theory calculations suggest the existence of a weak intermolecular H-H interaction between ammonia. Thus, at pressure below the ionization phase transition pressure (∼135 GPa), intermolecular H-H coupling gives rise to a unique dissociation pathway leading to the formation of hydrogen in ammonia. At pressure above ∼135 GPa, intermolecular H-H coupling exists in an intermediate state in the ionization of ammonia. Together with proton transfer based on hydrogen bond, a dual-path mechanism exists in the ionization of ammonia. Our work provides new insights into the ionization mechanism of ammonia under high pressure.