Structure and dynamics of ice Ih films upon HCl adsorption between 190 and 270 K. I. Neutron diffraction and quasielastic neutron scattering experiments

Neutron diffraction and quasielastic neutron experiments are performed to investigate the effect of HCl adsorption on the structure and dynamics of an ultra-thin ice Ih film (5 H2O bilayers thick) deposited on a crystalline MgO(001) substrate. Three HCl coverages have been studied 0.3, 0.6, and 1 monolayer (ML) in the temperature range 190–270 K. At 0.3 and 0.6 HCl monolayer, no mobility is measured at T⩽220 K. A translational mobility, which is a signature of a liquid phase, is observed at T=250 K. This phase occurs 15 K below the surface melting temperature of the bare ice film. The fraction of mobile molecules represents 30% (0.3 ML HCl) and 45% (0.6 ML HCl) of the film. At 1 HCl monolayer and T=220 K, HCl–dihydrate coexists with ice Ih, whereas at T=250 K the ice film becomes amorphous and only 9% of the film is mobile. The results are discussed within the context of atmospheric chemistry.