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

Lee, Du Hyeong, Kang, Hani, Kang, Heon
Journal of physical chemistry 2019 v.123 no.6 pp. 3657-3663
ammonia, ammonium, energy, hydrochloric acid, infrared spectroscopy, physical chemistry, protons, temperature
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 ∼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.