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Aims: The gas-phase abundance of methanol in dark quiescent cores in theinterstellar medium cannot be explained by gas-phase chemistry. In fact, theonly possible synthesis of this species appears to be production on thesurfaces of dust grains followed by desorption into the gas. Yet, evaporationis inefficient for heavy molecules such as methanol at the typical temperatureof 10 K. It is necessary then to consider non-thermal mechanisms fordesorption. But, if such mechanisms are considered for the production ofmethanol, they must be considered for all surface species. Methods: Ourgas-grain network of reactions has been altered by the inclusion of anon-thermal desorption mechanism in which the exothermicity of surface additionreactions is utilized to break the bond between the product species and thesurface. Our estimated rate for this process derives from a simple version ofclassical unimolecular rate theory with a variable parameter only looselycontrained by theoretical work. Results: Our results show that the chemistry ofdark clouds is altered slightly at times up to 10^6 yr, mainly by theenhancement in the gas-phase abundances of hydrogen-rich species such asmethanol that are formed on grain surfaces. At later times, however, there is arather strong change. Instead of the continuing accretion of most gas-phasespecies onto dust particles, a steady-state is reached for both gas-phase andgrain-surface species, with significant abundances for the former.Nevertheless, most of the carbon is contained in an undetermined assortment ofheavy surface hydrocarbons. Conclusions: The desorption mechanism discussedhere will be better constrained by observational data on pre-stellar cores,where a significant accretion of species such as CO has already occurred.Comment: 14 pages, 9 figures. Accepted by Astronomy & Astrophysic

Non-thermal desorption from interstellar dust grains via exothermic surface reactions