  Many interstellar complex organic molecules (COMs) are believed to be
produced on the surfaces of icy grains at low temperatures. Atomic carbon is
considered responsible for the skeletal evolution processes, such as C-C bond
formation, via insertion or addition reactions. Before reactions, C atoms must
diffuse on the surface to encounter reaction partners; therefore, information
on their diffusion process is critically important for evaluating the role of C
atoms in the formation of COMs. In situ detection of C atoms on ice was
achieved by a combination of photostimulated desorption and resonance enhanced
multiphoton ionization methods. We found that C atoms weakly bound to the ice
surface diffused approximately above 30 K and produced C2 molecules. The
activation energy for C-atom surface diffusion was experimentally determined to
be 88 meV (1,020 K), indicating that the diffusive reaction of C atoms is
activated at approximately 22 K on interstellar ice. The facile diffusion of C
at T > 22 K atoms on interstellar ice opens a previously overlooked chemical
regime where the increase in complexity of COMs as driven by C atoms. Carbon
addition chemistry can be an alternative source of chemical complexity in
translucent clouds and protoplanetary disks with crucial implications in our
current understanding on the origin and evolution of organic chemistry in our
Universe.
