  The diffusion of molecules on interstellar grain surfaces is one of the most
important driving forces for the molecular complexity in the interstellar
medium. Due to the lack of laboratory measurements, astrochemical modeling of
grain surface processes usually assumes a constant ratio between the diffusion
energy barrier and the desorption energy. This over-simplification inevitably
causes large uncertainty in model predictions. We present a new measurement of
the diffusion of CO$_2$ molecules on the surface of non-porous amorphous solid
water (np-ASW), an analog of the ice mantle that covers cosmic dust grains. A
small coverage of CO$_2$ was deposited onto an np-ASW surface at 40~K, the
subsequent warming of the ice activated the diffusion of CO$_2$ molecules, and
a transition from isolated CO$_2$ to CO$_2$ clusters was seen in the infrared
spectra. To obtain the diffusion energy barrier and pre-exponential factor
simultaneously, a set of isothermal experiments were carried out. The values
for the diffusion energy barrier and pre-exponential factor were found to be
$1300\pm110$~K and $10^{7.6\pm0.8}$~s$^{-1}$. A comparison with prior
laboratory measurements on diffusion is discussed.
