Motivation: The excited state dynamics of molecules have been extensively studied in the gas-phase and condensed phases. But what about at the interface between these? Probing dynamics there is difficult because one requires both species and surface selectivity and consequently, very little is know about how dynamics change here, even though one might anticipate large changes because of, for example, interfacial electric fields and rapidly changing densities. One specific species we have had interest in is the hydrated electron – and electron solvated in a cavity by a number of water molecules – has been studied in the bulk for many decades. However, charge-transfer processes typically occur at interface, but little is known about the hydrated electron at interfaces. For example, can a hydrated electron even exist at the water/air interface?

Philosophy: We use time-resolved second-harmonic generation (SHG) and/or sum-frequency generation (SFG) to look for the hydrated electron or molecular dynamics at interfaces because SHG/SFG is surface sensitive and can be resonantly enhanced to have chemical specificity too.

Hydrated electron at water/air interface: We have shown that the electron can indeed exist at this interface for several 100s ps. Hence, as a highly reactive radical, it could also be reactive to gas-phase species at the interface. What remains unknown and is a puzzle we are aiming to resolve is whether the free energy of bringing the hydrated electron to the water/air interface has a negative or positive free energy.

Charge-transfer to solvent at the water/air interface: Using iodide as a source of electrons, we have used time- and phase-sensitive SHG to probe the charge-transfer-to-solvent dynamics at the water/air interface. These reveal that the initial charge is injected parallel to the surface rather than into the bulk, presumably because of the more porous water structure at the interface.

Beyond hydrated electrons: Our current research is also aiming at exploiting our methodology to probe atmospherically relevant reactions. After all, much of atmospheric chemistry takes place at the interface of aerosols, while in reality, very little research is aimed at such interfaces. To this end, we have developed new methodologies to enable us to probe such dynamics.