The effect of HCl adsorption on the structure and dynamics of the ice surface
Abstract
Neutron diffraction and quasi-elastic neutron scattering experiments together with molecular dynamics simulations were performed to investigate the effect of HCl adsorption in the sub-monolayer range on the structure and dynamics of a thin ice Ih film deposited on a MgO (100) substrate. We show that surface melting occurs in the supported pure-ice film down to about 8 K below the normal melting temperature of bulk ice Ih. Upon HCl adsorption, the surface melting temperature is still lowered by 15 K at HCl amounts corresponding to 0.3 and 0.6 monolayer. Results of simulations can satisfactorily interpret the lowering of the melting temperature due to the support and to HCl adsorption. Upon adsorption of one HCl monolayer, HCl dihydrate is formed at 220 K and the film becomes amorphous at 250 K. The results are discussed within the context of stratospheric and upper tropospheric/lower stratospheric chemistry.