New measurement of the diffusion of carbon dioxide on non-porous amorphous solid water

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 CO2 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 CO2 was deposited onto an np-ASW surface at 40 K, the subsequent warming of the ice activated the diffusion of CO2 molecules, and a transition from isolated CO2 to CO2 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 +/- 110 K and 10(7.6 +/- 0.8) s(-1). A comparison with prior laboratory measurements on diffusion is discussed.

Automated summary

The diffusion of molecules on interstellar grain surfaces is crucial for the molecular complexity in the interstellar medium. A new measurement of CO2 diffusion on non-porous amorphous solid water (np-ASW) surface is presented, providing values for the diffusion energy barrier and pre-exponential factor. Comparison with prior laboratory measurements is discussed.