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

Under cold compression, hydrogen bonding (N-H···N) was considered to dominate the intermolecular interaction during the ionization of ammonia. Here, we provide experimental evidence of intermolecular H-H (N-H···H-N) coupling in dense ammonia. We observed a previously unreported phenomenon. Sporadic molecular hydrogen was observed during cold compression, accompanied by the absence of molecular nitrogen. Two fingerprint Raman modes (L₁ and ν_b) previously assigned to the ionic phase (β) disappear upon further compression or heating. By comparing the soft vibrational modes associated with intermolecular interactions in solid hydrogen, nitrogen, and ammonia, we find the slope of the phonon frequency versus pressure curve (k) and atomic number (Z) can be well described by an empirical correlation of the form $\sqrt{Z_H/Z_N}=k_{N-N}/k_{H-H}$. We note that k_H-H = k_N-H. 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.