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

Abstract

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 $1300\pm 110$ K and ${10}^{7.6\pm 0.8}$ s${}^{-1}$. A comparison with prior laboratory measurements on diffusion is discussed.

GRAPHICAL ABSTRACT

Keywords:

• ISM
• diffusion
• molecules
• astrochemistry
• carbon dioxide

Acknowledgments

We dedicate this paper to the memory of Dieter Gerlich, who was a good friend and colleague of some of us.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

We acknowledge the support from the European Research Council under the Horizon 2020 Framework Program via the ERC Advanced Grant Origins 83 24 28, and from NSF Astronomy & Astrophysics Research Grant #1615897.