Tunneling Diffusion of Excess Protons in Amorphous Solid Water at 10 and 80 K

Abstract

This work explores the plausibility of quantum tunneling of excess protons in amorphous solid water (ASW) at temperatures as low as 10 K. The transport of protons from a proton donor (HCl) to an acceptor (NH3) species located at a controlled separation distance in an ASW film was investigated at 10 and 80 K. The proton transfer efficiency was measured by quantitation of the yield of ammonium ions using reflection-absorption infrared spectroscopy. The average transport distance of excess protons in the ASW films was similar to 10 water molecules in the temperature range 10-80 K and showed no noticeable dependence on the temperature. On the other hand, the transport of protons along the surface of the ASW film was restricted to within a range of two water molecules at 10 K because of an energy barrier. The mobility of excess protons in the interior of ASW at temperatures close to 10 K indicated quantum tunneling of protons, most likely in the microscopic crystalline regions of the ASW samples.