Structural reorganization on amorphous ice films below 120 K revealed by near-thermal (~1 eV) ion scattering

Abstract

Topmost layers of amorphous ice undergo a structural transformation before the onset of crystallization and well before the premelting transitions. Ultralow-energy (~1 eV) mass-selected Ar⁺ scattering has been used to detect this structural change. The transformation is manifested in the form of drastic changes in the scattered ion intensity in the ≤2 eV collision energy range. The changes are limited to the first few monolayers as larger thicknesses produce no additional effects. The technique becomes chemically sensitive but insensitive to the morphology and structure as the energy is increased above 3 eV. A similar behavior is observed with He⁺, Kr⁺, and a polyatomic ion, CH₃⁺. Experiments revealed that the structural changes occurred only on amorphous ice and no such change occurred on crystalline ice. H₂O and D₂O ices behave similarly.