Transport and trapping properties of protons in amorphous ice films

Abstract

In this thesis, we studied the transport and trapping behaviors of protons in amorphous ice films. Experimental methods of Cs+ reactive ion scattering, low energy sputtering technique and reflection absorption IR spectroscopy were used for this study. The trapping time and migration distance of protons were measured for photo-produced H3O+ ions in ice. Also, we estimated the diffusion length of H3O+ and OH− ions released at the surface and the proton transfer distance associated with these species. The fundamental information about protons and photo-produced H3O+ ions in ice may be important for understanding the reactions taking place in polar stratospheric clouds, interstellar media, and etc. Chapter I introduces the structure of ice and the current understanding of proton transfer reactions in condensed phase. This gives brief background information to understand the proton transfer phenomena in ice and to analyze the experimental result. In chapter II, the basic principle of experimental methods-reflection absorption IR spectroscopy, reactive ion scattering and low energy sputtering methods-are introduced. These spectroscopic and ion scattering methods are used for analyzing the sample films in this study. Instrumentation of the methods is also described. In chapter III, we show that UV irradiation of an ice film produces hydronium ions, and measure the lifetime and migration distance of the photo-produced hydronium ions at low temperature (53−140 K). In order to observe the hydronium ions, we added methylamine molecules on the UV-irradiated ice film. Adsorbed methylamine reacted with hydronium ions, and was converted into methylammonium ions. By monitoring the changes in the population of methylammonium ions, the decay of hydronium population in ice was traced at the various temperatures. The half-life of hydronium species was longer than one hour at ~53 K, and it decreased to ~5 min at 140 K. A small portion of hydronium species could survive for extraordinarily long times even at 140 K. In the other experiment, the migrations of photo-produced hydronium ions were monitored. The migration dynamics was observed for H/D exchange reaction occurring in H2O-D2O mixed ice samples under the irradiation of UV light. The model analysis was carried out to estimate migration distance of protons. The results indicated that the proton hopping distance is ~4-5 water molecules in the UV-photolyzed ice at ~54 K and ~32 molecules at 100 K. In chapter IV, the transfer lengths of protons mediated by H3O+ and OH− ions in amorphous ice were measured. H3O+ or OH− species were externally provided at the surface of a thin ice film containing the probe molecules, such as NH3 or NH4+ species, which can accept or donate a proton to H3O+ or OH− species, respectively. The proton transfer efficiency between the donor-acceptor pair was monitored as a function of the donor-acceptor separation distance. It was found that the migration distance of H3O+ species was 11 BL and that of OH− was 3.5 BL. The asymmetric migration distance indicates that the traditional mirror-image concept is inadequate for the proton transfers of H3O+ and OH−, and more advanced model is needed.