Metastable hydronium ions in UV-irradiated ice

Abstract

We show that the irradiation of UV light (10-11 eV) onto an ice film produces metastable hydronium (H(3)O(+)) 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 (H∕D) isotopic exchange reaction. The methylamine adsorption experiments showed that photogenerated H(3)O(+) 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(3)O(+) species induced the H∕D 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 H(3)O(+) 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(3)O(+) 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.

Proton Transport and Related Chemical Processes of Ice

“…In the experiment performed by Lee et al, the transient mobility of excess protons in ASW was observed; a mobile proton is released from a stable hydronium structure by the external stimulus of ammonia adsorption on the ASW film surface . One might question the idling mobility of excess protons in a perfectly crystalline ice, but this would only be a hypothetical question because excess protons are almost always trapped at defects in real ice at low temperatures. ,,, There was a report about the observation of idling mobility of protons in a neutral ice I h lattice at 5 K in QENS experiments, but this claim was refuted by further investigations …”

“…Excess protons may be generated by the photolysis of ice particles under ionizing radiation or by the injection of cosmic protons into the ice. The excess protons can be stored as hydronium ion in the ice and utilized for subsequent chemical reactions. , Moon et al proposed that such hydronium ion may act as an “invisible” acid for the reactions of interstellar ice particles. The acid–base reaction between hydronium ions and ammonia molecules trapped in the ice may explain the origin of the mysteriously large abundance of ammonium ions detected in interstellar molecular clouds.…”

Effect of Coadsorbed Oxygen on the Photochemistry of Methane Embedded in Amorphous Solid Water

“…45 Caging and other embedding methods may potentially pave the way to better understand the fundamental aspects of activating short alkanes such as methane in the presence of water molecules in well-controlled, model systems. Moon and Kang 46 reported that the irradiation of deep UV light (10−11 eV) onto an ice film produced metastable hydronium (H 3 O + ) ions in the ice at low temperatures (53−140 K), which may lead to enhanced reactivity if other caged or sandwiched molecules coexist.…”

Efficient Thermal Reactions of Sulfur Dioxide on Ice Surfaces at Low Temperature: A Combined Experimental and Theoretical Study

“…This is possible if H 3 O + is very mobile in the ice lattice or exists below the surface. 25,26 The structure shown in Figure 6(e) is informative in terms of this explanation. Because H 3 O + is formed as part of an extended H-bonded water chain, and the excess proton is mobile along the water chain with a very small barrier via the Grotthuss mechanism, 25 the impact of Cs + at low energy may only facilitate the migration of H 3 O + along the water chain, making its ejection into a vacuum very unlikely to occur.…”