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The activation energy for desorption (Edes) and that for surface diffusion(Esd) of adsorbed molecules on dust grains are two of the most importantparameters for the chemistry in the interstellar medium. Although Edes is oftenmeasured by laboratory experiments, the measurement of Esd is sparse. Due tothe lack of data, astrochemical models usually assume a simple scalingrelation, Esd = fEdes, where f is a constant, irrespective of adsorbed species.Here, we experimentally measure Esd for CH4, H2S, OCS, CH3OH, and CH3CN onwater-ice surfaces using an ultra-high-vacuum transmission electron microscope(UHV-TEM). Compiling the measured Esd values and Edes values from theliterature, we find that the value of f ranges from ~0.2 to ~0.7, depending onthe species. Unless f (or Esd) for the majority of species is available, anatural alternative approach for astrochemical models is running multiplesimulations, varying f for each species randomly. In this approach, ranges ofmolecular abundances predicted by multiple simulations, rather than abundancespredicted by each simulation, are important. We here run 10,000 simulations ofastrochemical models of molecular clouds and protostellar envelopes, randomlyassigning a value of f for each species. In the former case, we identifyseveral key species whose Esd most strongly affects the uncertainties of themodel predictions; Esd for those species should be investigated in futurelaboratory and quantum chemical studies. In the latter case, uncertainties inthe Esd of many species contribute to the uncertainties in the modelpredictions.

Diffusion activation energy and desorption activation energy for astrochemically relevant species on water ice show no clear relation