Methanol production via interactions of low-energy CH3+ ions with ASW surface:experimental investigation of ion-surface reactions
Y. Nakai1, H. Hidaka2, and N. Watanabe2
1

RIKEN Nishina Center for Accelerator-Based Science, RIKEN, Japan2
Institute of Low Temperature Science, Hokkaido University, Japan

Various neutral and ion species have been observed in molecular clouds, despite their very
low temperatures. To understand how molecules evoluve, many chemical processes have beenproposed. In gas phase, ion-molecule reactions have been always considered to play animportant role. However, the abundances of some molecules such as water and methanol
molecules cannot be explained only by the gas-phase reactions. Consequently, reactions
between neutral radicals and neutral species on an icy grain surface have been proposed as theother essential pathways and it has been experimentally confirmed that reactions of hydrogenatoms adsorbed on an ice surface are essential for productions of hydrogen, water, methanol,and so on [1, 2]. Besides pure gas-phase and surface reactions, it is reasonably inferred thatprocesses triggered by interactions of gas-phase ions with an ice mantle play some role inmolecular evolution. In this context, the experiments of energetic ion bombardment to an ice
containing other primordial molecules have been performed for a long time. In contrast, little
is known about the interactions between low-energy ions and an ice surface, while molecular
cations have been simply considered to recombine with negative charge (electrons) on an ice
surface in theoretical models [3, 4]. However, it is not clear how electrons exist on icy grains,and thus it is also unclear if cation-electron recombination immediately occurs or not.
In fact, the reactions of very-low-energy ions with an icy surface have been theoretically
proposed as new non-energetic reaction pathways [5, 6]. Since an icy grain can be regarded asa huge molecular cluster, reactions of ions with an icy grain surface would occur without
reaction barriers as ion-molecule reactions. For example, the quantum chemical calculation
predicted that the reaction of a CH3+ ion with a water ice surface spontaneously produces amethanol molecule [5]. Despite recent theoretical progress, no experimental study for lowenergy ion reactions with ice surface have been performed due to experimental difficulties in
obtaining enough reaction products for their detection using conventional methods such as
infrared absorption spectroscopy. Accordingly, we have newly developed an experimental
setup to enable us detecting trace amounts of reaction products. Using this setup, we haveperformed the experiments for production of methanol through the reaction of low-energy
CH3+ ions with a water ice surface. The CH3+ ions with their energies as low as or lower than~10 eV impinged on an amorphous solid water (ASW) prepared on an Al substrate around 10K. Reaction products by CH3+ irradiation were monitored through pick-up reaction method byCs+ ions at several tens eV [7, 8]. After CH3+ irradiation, methanol molecules on the ASW
were observed as the theoretical study suggested.
References
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