UC Berkeley

Chemistry at interfaces is of profound importance in the natural world, but only recently have the tools been developed to adequately study such finite boundaries. Much work has gone into the study of aqueous interfaces, as they play crucial roles in atmospheric, electrochemical, and biological phenomena. The bulk of this dissertation is focused on the application of nonlinear spectroscopy to study the behavior ions at the air-water interface and their effects on interfacial chemistry. I also discuss the latest work on the elusive characterization of liquid carbon produced from diamond samples.Chapter 1: A summary of the work done in the last century surrounding interfacial ions in water is provided. Today, we know that the surface tension of salt solutions increases with added solute concentration, but at the time this observation was first made, the molecular-level understanding of ions at the air-water interface was still in its infancy. This chapter focuses on the origins of the first widely-accepted theoretical model of interfacial ions developed by Wagner, Onsager, and Samaras in the early 1930s, and on key experimental evidence that influenced the development of more sophisticated computer models. This recollection emphasizes the enormous progress made in this field, particularly in the last decade. Chapter 2: The theory of nonlinear deep ultraviolet second harmonic generation spectroscopy is reviewed. I describe the theoretical framework vital to understanding the origin of the generated second harmonic response. From this, I describe the development of a Langmuir model, used to quantify the adsorption of ions. The optical DUV-SHG setup is described in detail and data analysis methods are discussed. Chapter 3: The first experiment to identify a polyatomic cation adsorbed to the air-water interface using DUV-SHG is described. Guanidinium is found to adsorb to the interface with a similarly strong propensity as the thiocyanate anion. MD simulations identify a strong interfacial orientational preference for guanidinium ions wherein the cation lies parallel to the air-water boundary. Furthermore, the data are suggestive of ion-pairing effects, the exact nature of which remain to be established. This work highlights an important milestone in the study of interfacial specific ion effects. Chapter 4: DUV-SHG, microdroplet mass spectrometry, and kinetic modeling are employed to study atmospherically-relevant sulfur oxyanions at the air-water interface and reexamine the mechanism of thiosulfate ozonation. Previous work highlighted that the doubly- charged carbonate anion exhibits a stronger affinity for the interface than does the singly-charged bicarbonate. To determine if other multiply-charged anions are also surface active, sodium sulfate, sulfite, and thiosulfate solutions are studied. It is found that thiosulfate exhibits a strong propensity for the air-water interface, while sulfate and sulfite do not. A new kinetic model for thiosulfate ozonation is developed which agrees well with the experimental data. Chapter 5: Efforts to produce and characterize the liquid state of carbon from irradiation of diamond samples are described. Mega-electronvolt ultrafast electron diffraction is used to study the structure of crystalline diamond thin films following excitation from an ultrafast laser pulse. Additional resonant inelastic x-ray spectroscopy measurements are performed to determine the electronic structure of laser-excited diamond. It is then determined that the physical and electronic structure of diamond remains unchanged when probed at picoseconds after excitation. This work highlights the resilient nature of diamond and the experimental challenge of evidencing a clear liquid carbon signal.