Local Proton Disorder Induced Intermolecular H-H Coupling in Ionization of Dense Ammonia

TL;DR

This study reveals that proton disorder and intermolecular H-H coupling significantly influence ammonia's ionization process under high pressure, challenging the traditional view that hydrogen bonding dominates in such conditions.

Contribution

It provides experimental and theoretical evidence of H-H coupling and proton disorder effects in dense ammonia, highlighting a new ionization pathway independent of hydrogen bonding.

Findings

Proton disorder increases with pressure, promoting H-H coupling.

Intermolecular H-H coupling leads to a unique dissociation pathway below 135 GPa.

A dual path ionization mechanism exists, involving proton transfer and H-H coupling.

Abstract

Under cold compression, hydrogen bonding was considered to dominate intermolecular interaction during the ionization of ammonia. Here, we provide experimental and theoretical evidence of intermolecular HH coupling in dense ammonia. Ab initio molecular dynamics simulations (AIMD) reveal an increasing degree of proton disorder in ammonia with increasing pressure, which promotes intermolecular H-H coupling. At pressure below the ionization phase transition pressure (135 GPa), intermolecular HH coupling gives rise to a unique dissociation pathway in ammonia. Sporadic molecular hydrogen was observed without laser heating, accompanied by the absence of molecular nitrogen. At pressure above 135 GPa, intermolecular HH coupling serves as an intermediate state in the ionization of ammonia. Two fingerprint Raman modes previously assigned to the ionic phase disappear upon further compression or heating. Together with proton transfer based on hydrogen bond, a dual path mechanism exists in the ionization of ammonia. Our results demonstrate a case of hydrides phase transition pathway that occurs independently of hydrogen bonding under high pressure.