We have examined the elementary molecular processes responsible for proton transfer and HD exchange in thin ice films for the temperature range of 100-140 K. The ice films are made to have a structure of a bottom D(2)O layer and an upper H(2)O layer, with excess protons generated from HCl ionization trapped at the D(2)OH(2)O interface. The transport behavior of excess protons from the interfacial layer to the ice film surface and the progress of the HD exchange reaction in water molecules are examined with the techniques of low energy sputtering and Cs(+) reactive ion scattering. Three major processes are identified: the proton hopping relay, the hop-and-turn process, and molecular diffusion. The proton hopping relay can occur even at low temperatures (<120 K), and it transports a specific portion of embedded protons to the surface. The hop-and-turn mechanism, which involves the coupling of proton hopping and molecule reorientation, increases the proton transfer rate and causes the HD exchange of water molecules. The hop-and-turn mechanism is activated at temperatures above 125 K in the surface region. Diffusional mixing of H(2)O and D(2)O molecules additionally contributes to the HD exchange reaction at temperatures above 130 K. The hop-and-turn and molecular diffusion processes are activated at higher temperatures in the deeper region of ice films. The relative speeds of these processes are in the following order: hopping relay>hop and turn>molecule diffusion.

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Lieber obenauf: Untersuchungen zum Protonentransfer und H/D‐Austausch in Eisfilmen zeigen, dass sich das Proton vorrangig auf deren Oberfläche befindet. Es kann kurze Strecken mithilfe eines Staffel‐Hopping‐Mechanismus zurücklegen (siehe Bild), selbst wenn die Molekülbewegung weitgehend eingefroren ist. Diese Eigenschaften legen die Möglichkeit für protonenkatalysierte Reaktionen an kalten Eisoberflächen, z. B. auf interstellaren Staubpartikeln, nahe.

The effect of the adsorbate mass on an Eley-Rideal (ER) reaction is examined for the reactive ion scattering (RIS) of Cs+ from physisorbed adsorbates (Ar, Kr, Xe and N2) on a Ru(0001) surface at hyperthermal energy (10-60 eV). The measurement of the RIS cross-section, or the efficiency for ER-type abstraction of surface adsorbates, reveals an inverse proportionality between the square root of the cross-section and the mass of the noble gases. This relationship is explained by the inertia effect which can determine the probability of successful abstraction events of the adsorbates. The result confirms a theoretical prediction that the inertia effect is important for the dynamics of an ER reaction in the hyperthermal energy regime.

We studied the initial-stage mechanism of the electrophilic addition reaction of ethene with HCl by examining the interactions between ethene and HCl on water-ice and frozen molecular films at temperatures of 80-140 K. Cs(+) reactive ion scattering (RIS) and low-energy sputtering (LES) techniques were used to probe the reaction intermediates that were kinetically trapped on the surface, in conjunction with temperature-programmed desorption (TPD) mass spectrometry to monitor the desorbing species. The reaction initially produced the π complex of HCl and ethene at temperatures below about 93 K and an "ethyl cationic species" at temperatures below about 100 K. The ethyl cationic species was formed via direct proton transfer from the HCl molecule to ethene with the assistance of water solvation, rather than via the interaction of hydronium ions and ethene. At high temperatures, this species dissociated into ethene and hydronium and chloride ions. The reaction did not, however, complete the final transition state on the ice surface to produce ethyl chloride. The observation gives evidence that the electrophilic addition reaction of ethene occurs through an ethyl-like intermediate with an ionic character.