Ultrafast dynamics at aqueous-air interfaces

Abstract

A femtosecond time-resolved surface electronic sum-frequency generation spectroscopy setup has been constructed in conjunction with a femtosecond transient absorption setup. The layouts are described, along with the principles of their use. In the case of the surface experiment, a novel combination of existing techniques has enabled nonlinear spectroscopic measurements to be performed on fluorescent samples by optical Kerr gating of the surface nonlinear response. The sum-frequency generation and transient absorption setups were used together to investigate the hydrated electron formed by photo-oxidation of the phenolate anion at the aqueous/air interface and in solution. The hydrated electron is important across a breadth of chemistry, and is relevant to areas such as atmospheric, nuclear, and biological chemistry, where it is considered an important species in reductive DNA damage. The surface electron was observed to exhibit similar dynamics to the same bulk system, albeit at an accelerated rate. The study was improved by observing dynamics across a range of wavelengths, which showed a variation in dynamics across the spectral regions probed. The variations in dynamics were attributed to a second species at the interface: the phenoxyl radical. Kinetics and associated spectra of both species were retrieved from the data, exhibiting the first optical spectrum of the hydrated electron at the aqueous/air interface. Finally, potential chemical systems and experimental developments of interest are presented.