Elucidating how trace gases interact with ice surfaces utilizing sum frequency generation spectroscopy.

The interaction between ice surfaces and trace gases plays a significant role in atmospheric chemistry, such as chemical and photochemical reactions contributing to ozone depletion and secondary aerosol formation. The study of molecular-level properties of the ice surface and small organic molecule adsorption, are essential to understand the impact of hosting these molecules and further chemical reactions. To capture a molecular understanding of the interface, the use of a surface selective technique, such as sum frequency generation (SFG) spectroscopy, is crucial to probe ice surfaces and observe the adsorption of molecules on ice surfaces. We observe significant differences in the structure of the water molecules for ice and water surfaces upon the addition of acetone and methanol. At the methanol-ice interface, a blue shift of ∼80 cm-1 is observed, indicating a weakening of the hydrogen bond. This is an opposing shift to the acetone-ice interface, which red shifted by ∼10 cm-1. These changes in the fundamental frequencies could impact atmospheric models and in particular impact overtone pumping reactions. The distinct behavior of water molecules and small oxygenated organic compounds is linked to differences in reactivity and rates of photochemical reactions via overtone pumping on ice and water surfaces.