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