Metastable hydronium ions in UV-irradiated ice

摘要

We show that the irradiation of UV light (10-11 eV) onto an ice film produces metastable hydronium (H_3O~+) ions in the ice at low temperatures (53-140 K). Evidence of the presence of metastable hydronium ions was obtained by experiments involving adsorption of methylamine onto UV-irradiated ice films and hydrogen-deuterium (HD) isotopic exchange reaction. The methylamine adsorption experiments showed that photogenerated H _3O~+ species transferred a proton to the methylamine arriving at the ice surface, thus producing the methyl ammonium ion, which was detected by low energy sputtering method. The H_3O~+ species induced the HD exchange of water, which was monitored through the detection of water isotopomers on the surface by using the Cs~+ reactive ion scattering method. Thermal and temporal stabilities of H3O + and its proton migration activity were examined. The lifetime of the hydronium ions in the amorphized ice was greater than 1 h at ～53 K and decreased to ～5 min at 140 K. Interestingly, a small portion of hydronium ions survived for an extraordinarily long time in the ice, even at 140 K. The average migration distance of protons released from H_3O~+ in the ice was estimated to be about two water molecules at ～54 K and about six molecules at 100 K. These results indicate that UV-generated hydronium ions can be efficiently stabilized in low-temperature ice. Such metastable hydronium ions may play a significant role in the acid-base chemistry of ice particles in interstellar clouds.