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tokens: with elastic collision with the surface, reproduces the observations well. Although it is an empirical approach, it allows sorting among the products that should undergo the chemical desorption. The basic idea underlying our proposition is inspired by dynamic calculations in specific systems such as H+H on graphite ( Morisset et al. 2004 ; Bachellerie et al. 2007 ) or O+H on graphite ( Bergeron et al. 2008 ). We have illustrated the mechanism of this scenario in Fig. 4 . The first step consists of a collinear approach parallel to the surface. When the molecule is formed, the trajectories of both atoms adopt a more disordered motion symptomatic of the high internal energy in vibration and rotation of the newly formed molecules, up to a point where the molecule interacts with the surface and that a part of the motion is transferred perpendicularly to the surface that causes the desorption. Fig. 4 Illustration of the chemical desorption process. After an initial meeting (1), the molecule is highly excited; (2) and converts a fraction of its energy into vertical motion through interaction with the surface (3). We point out that the chemical desorption process seems to favor small molecules with low binding energies. Therefore, this process may be part of the reason COMS are present in dark clouds. We note that our prediction of about one percent of chemical desorption for methanol would be sufficient to sustain the observed value, as modeled by Vasyunin Herbst (2013) . The second part of our paper will examine the effect of the deriving our formulae in an astrophysical context. That the chemical desorption efficiency on water ice substrates is reduced is somehow to be expected. A similar behavior has been observed in photodesorption of CO. Pure CO ices exhibit an efficient wavelength-dependent photodesorption of up to one percent per incident photon ( Bertin et al. 2013 ) or possibly lower by a factor 3–11 ( van Hemert et al. 2015 ). Some authors have estimated the CO photodesorption on water substrate to be about 10 ( Öberg et al. 2009 ) or one order of magnitude lower ( DeSimone et al. 2013 ). However, even if the CO photodesorption process is poorly constrained, the water substrate clearly prevents or dramatically reduces its efficiency. This probably is due to the very fast transportation of energy in the water network and to the reduction of the indirect photodesorption. For chemical desorption on water substrate, the weakness of our coarse model probably lies in the assumption of elastic collisions, which are not adapted to the case of water. Many more experimental investigations and detailed computations would help to solve this problem. Finally, we note that even if water ice substrate seems to be a good trapping environment and can dramatically reduce the chemical desorption efficiency, CO layers in the central part of dark clouds are expected to cover an inner water layer around the solid seeds of dust. Therefore, another set of