Sticking of molecules on non-porous amorphous water ice
  Accurate modeling of physical and chemical processes in the interstellar
medium requires detailed knowledge of how atoms and molecule adsorb on dust
grains. However, the sticking coefficient, a number between 0 and 1 that
measures the first step in the interaction of a particle with a surface, is
usually assumed in simulations of ISM environments to be either 0.5 or 1. Here
we report on the determination of the sticking coefficient of H$_2$, D$_2$,
N$_2$, O$_2$, CO, CH$_4$, and CO$_2$ on non-porous amorphous solid water
(np-ASW). The sticking coefficient was measured over a wide range of surface
temperatures using a highly collimated molecular beam. We showed that the
standard way of measuring the sticking coefficient --- the King-Wells method
--- leads to the underestimation of trapping events in which there is
incomplete energy accommodation of the molecule on the surface. Surface
scattering experiments with the use of a pulsed molecular beam are used instead
to measure the sticking coefficient. Based on the values of the measured
sticking coefficient we suggest a useful general formula of the sticking
coefficient as a function of grain temperature and molecule-surface binding
energy. We use this formula in a simulation of ISM gas-grain chemistry to find
the effect of sticking on the abundance of key molecules both on grains and in
the gas-phase.
