  The diffusion of atoms and molecules in ices covering dust grains in dense
clouds in interstellar space is an important but poorly characterized step in
the formation of complex molecules in space. Here we report the measurement of
diffusion of simple molecules in amorphous solid water (ASW), an analog of
interstellar ices, which are amorphous and made mostly of water molecules. The
new approach that we used relies on measuring in situ the change in band
strength and position of mid-infrared features of OH dangling bonds as
molecules move through pores and channels of ASW. We obtained the Arrhenius
pre-exponents and activation energies for diffusion of CO, O$_2$, N$_2$,
CH$_4$, and Ar in ASW. The diffusion energy barrier of H$_2$ and D$_2$ were
also measured, but only upper limits were obtained. These values constitute the
first comprehensive set of diffusion parameters of simple molecules on the pore
surface of ASW, and can be used in simulations of the chemical evolution of ISM
environments, thus replacing unsupported estimates. We also present a set of
argon temperature programmed desorption experiments to determine the desorption
energy distribution of argon on non-porous ASW.
