Molecular beam scattering experiments have blossomed into a universal technique for understanding and controlling chemical reactions in the gas phase and on solid surfaces. Our research confronts a new frontier: the microscopic structure and reactivity of liquid surfaces. We use molecular beam techniques to probe the local geometry of liquid surfaces and to learn how molecules at the gas-liquid interface act cooperatively to promote energy transfer and chemical reactions, including proton exchange and liquid metal catalysis.

The questions we ask are simple: what does the surface of a liquid "look" like atom by atom? What does the liquid "feel" like during the picosecond time scale of a gas-liquid collision? Do liquid surfaces behave like rigid walls, simply reflecting incoming molecules back into the gas phase, or are the surfaces soft enough to momentarily trap gas molecules, allowing them to undergo solvation? Does the notion that "like dissolves like" apply to gas-liquid collisions, allowing us to predict which gases will stick and which will bounce impulsively off the liquid? And finally, how do these recoiling and solvating collisions control chemical reactions like proton exchange between water molecules and corrosive acids?

Molecular beam scattering experiments can answer these questions. Highly collimated molecules traveling at a single collision energy strike a liquid surface and stick or bounce off. Their identity and recoil direction and velocity are monitored by a mass spectrometer. We are now looking at a variety of low vapor pressure liquids, including hydrocarbons (squalane), fluorinated polyethers, hydrogen bonding liquids like glycerol, corrosive liquids like concentrated sulfuric acid, and liquid metals like gallium and indium.

Our studies bring together the most recent advances in chemical kinetics and theories of liquid structure and dynamics. By carrying out controlled collisions between a gaseous "solute" molecule and a liquid solvent, we are helping to construct an intimate picture of the chemistry of gas-liquid interfaces.