A new technique uses laser sheets and laser-induced fluorescence (LIF) imaging to visualize and study gas-liquid interactions at the molecular level. Developed by researchers at Heriot-Watt University, it could be used to improve predictive atmospheric models used to study the climate. “The molecule of interest in our study, the hydroxyl (OH) radical, is an unstable fragment of a molecule that affects the whole of the understanding of atmospheric chemistry and things that genuinely affect climate,” professor Kenneth McKendrick said. “Some of these important OH reactions take place at the surface of liquid droplets, but we can’t see surface interactions directly, so we measure the characteristics of the scattered molecules from real-time movies to infer what happened during their encounter with the liquid.” Difficulties in measuring gas-liquid collisions have so far prevented the fundamental exploration of these processes. Researchers hope their new technique of enabling the visualization of gas molecules bouncing off a liquid surface will help climate scientists improve their predictive atmospheric models. This image shows a packet of hydroxyl radical molecules hitting a liquid surface and creating a broad scattered plume, which is nearly identical for the two angles of approach, vertically or at 45°. Courtesy of Kenneth McKendrick. The researchers directed a pulsed molecular beam at continually refreshed surfaces of the representative liquids perfluoropolyether, squalane, and squalene. LIF was excited by pulsed laser light shaped into a planar sheet. The LIF emission was imaged and intensified before being captured by an external camera. Sequences of images allowed the evolution of the incident packet and scattered plumes of molecules to be observed. The laser sheets were applied to scattering from a surface in a vacuum, with no other molecules present to interfere with the scattering from the molecular beam. This enabled the team to capture individual frames of molecular movement. Unlike previous methods of capturing gas-liquid interactions, all the characteristics needed to understand the interaction — speed, scatter angle, rotation, etc. — could be captured within the researchers’ films. By watching the molecular film strips, McKendrick’s team observed that molecules scattered at a broad range of angles. This simple observation led the researchers to determine that the surface of liquids was not flat. “When you get down to the molecular level, the surface of these liquids is very rough. This finding is important for understanding the chances of different molecular processes happening at the liquid surface,” McKendrick said. As they continue to improve their technique, the researchers hope to collect more refined information from liquids that interact with gases in ways that are relevant to the atmosphere. In addition to the field of atmospheric science, the technique also could be applied to uncovering the gas-solid interactions that occur in processes such as the catalytic conversion of gases in car engines. This approach has considerable potential to be applied in related gas-surface scattering experiments, the researchers believe, because it does not present the difficulties of some other imaging methods that involve charged-particle detection, and it allows a spatially extended region of the scattering plane perpendicular to the surface to be imaged. The research was published in The Journal of Chemical Physics (https://doi.org/10.1063/1.5110517).