  Aims: The gas-phase abundance of methanol in dark quiescent cores in the
interstellar medium cannot be explained by gas-phase chemistry. In fact, the
only possible synthesis of this species appears to be production on the
surfaces of dust grains followed by desorption into the gas. Yet, evaporation
is inefficient for heavy molecules such as methanol at the typical temperature
of 10 K. It is necessary then to consider non-thermal mechanisms for
desorption. But, if such mechanisms are considered for the production of
methanol, they must be considered for all surface species. Methods: Our
gas-grain network of reactions has been altered by the inclusion of a
non-thermal desorption mechanism in which the exothermicity of surface addition
reactions is utilized to break the bond between the product species and the
surface. Our estimated rate for this process derives from a simple version of
classical unimolecular rate theory with a variable parameter only loosely
contrained by theoretical work. Results: Our results show that the chemistry of
dark clouds is altered slightly at times up to 10^6 yr, mainly by the
enhancement in the gas-phase abundances of hydrogen-rich species such as
methanol that are formed on grain surfaces. At later times, however, there is a
rather strong change. Instead of the continuing accretion of most gas-phase
species onto dust particles, a steady-state is reached for both gas-phase and
grain-surface species, with significant abundances for the former.
Nevertheless, most of the carbon is contained in an undetermined assortment of
heavy surface hydrocarbons. Conclusions: The desorption mechanism discussed
here will be better constrained by observational data on pre-stellar cores,
where a significant accretion of species such as CO has already occurred.
