Understanding the kinetics and dynamics of radiation-induced reaction pathways in carbon monoxide ice at 10 K

Carbon monoxide is the second most abundant molecule on icy grains in the interstellar medium. It also exists on Pluto, Triton, comets, and possibly in other icy bodies of the outer solar system like Kuiper Belt objects. With the intense radiation fields that permeate virtually all unprotected regions of space, carbon monoxide ices can be processed through energetic particle bombardment (planetary magnetospheric particles, solar wind, Galactic cosmic ray particles, and UV photons). In the present study we have investigated the effects by condensing a 1 mu m layer of carbon monoxide ice on a substrate at 10 K and irradiated the sample with energetic (keV) electrons. These simulate the energetic electrons trapped in magnetospheres of planets and reproduce the irradiation effects of typical Galactic cosmic ray particles. A series of new carbon-chain (C-3, C-6) and carbon oxide species were observed including the linear isomers of C2O, C3O, C4O, C5O, C6/7O, CO2, C3O2, C4O2, and C5O2. A reaction model was proposed that outlines different reaction pathways to each of these products. Using this model, the kinetics of each route of reaction was quantified, and from this, the mechanisms and dynamics of the reactions can be understood. This work should aid in the astronomical detection of new molecular species in solar system ices as well as building up a comprehensive reaction model to describe the chemical inventory of ices on interstellar dust grains.