In the interstellar medium, gas particles (atoms and molecules) continually collide with cold dust grain. These
collisions can lead to the sticking of gas particles on the surface and the “formation” of an ad-particle. The
solid state physical-chemistry of ad-particles is governed by three processes (and their respective probabilities):
diffusion, interaction of reactants on the surface, and desorption. The focus of this talk is the investigation of
these processes with a particular regard on oxidation chemistry.

The presented experiments have been performed with the FORMOLISM set-up, located in the Universit´e de
Cergy Pontoise, Observatoire de Paris. Via a triply differentially pumped beam, atoms and molecules are aimed
at a cold (>6 K) sample held in the UHV chamber. The products are probed using Temperature Programmed
Desorption and Reflexion Absorption Infrared Spectroscopy.

All these studies reveal that solid state chemistry is governed by a desorption-diffusion-reaction competition.
If desorption mechanism dominates, physisorbed reactive partners cannot increase the molecular complexity.
Conversely, if diffusion mechanisms are preponderant, mobile atoms will be able to scan the surface affecting
abundance and variety of the species eventually created.