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

TL;DR

This study provides the first laboratory measurement of CO₂ diffusion on non-porous amorphous solid water, crucial for astrochemical models, revealing specific energy barriers and diffusion rates.

Contribution

It introduces new experimental data on CO₂ diffusion on np-ASW, improving the accuracy of astrochemical models by replacing assumptions with measured parameters.

Findings

Diffusion energy barrier: 1300±110 K

Pre-exponential factor: 10^{7.6±0.8} s^{-1}

Observed transition from isolated CO₂ to clusters

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 $1300\pm110$~K and $10^{7.6\pm0.8}$~s$^{-1}$. A comparison with prior laboratory measurements on diffusion is discussed.