D2 desorption kinetics on amorphous solid water: from compact to porous ice films

Abstract

The desorption kinetics of D₂ from amorphous solid water (ASW) films have been studied by the temperature-programmed desorption (TPD) technique in the 10–30 K temperature range. Compact (and nonporous) films were grown at 120 K over a copper substrate. Ultra-thin porous films were additionally grown at 10 K over the compact base. The TPD spectra from compact and from up to 20 monolayers (ML) porous films were compared. The simulation of the TPD experimental traces provides the corresponding D₂ binding-energy distributions. As compared to the compact case, the binding-energy distribution found for the 10 ML porous film clearly extends to higher energies. To study the transition from compact to porous ice, porous films of intermediate thicknesses (<10 ML), including ultra-thin films (<1 ML), were grown over the compact substrate. The thermal D₂ desorption peak was found to shift to higher temperatures as the porous ice network was progressively formed. This behavior can be explained by the formation of more energetic binding sites related to porous films. TPD spectra were also modelled by using a combination of the two energy distributions, one associated to a bare compact ice and the other associated to a 10 ML porous ice film. This analysis reveals a very fast evolution of the binding-energy distribution towards that of porous ice. Our results show that few ML of additional porous film are sufficient to produce a sample for which the D₂ adsorption can be described by the energy distribution found for the 10 ML porous film. These experiments then provide evidence that the binding energy of D₂ on ASW ice is primarily governed by the topological and morphological disorder of the surface at molecular scale.