UC Berkeley

The chapters of this dissertation encompass two main projects. First is the study of ions and molecules at buried interfaces through further development of second-order nonlinear scattering spectroscopies. The goal of these studies, documented in Chapters 3-5, is to develop a more acute understanding of the role of solid-water interfaces for ocean chemistry, water purification, drug delivery, catalysis, and environmental sensors. The second project is the investigation of the liquid state of carbon using X-ray free electron laser scattering and is discussed in Chapter 6.Chapter 1 provides a brief history of ion adsorption to the air-water interface, describing how the field of nonlinear spectroscopy has aided in the development of a general theorem, viz. that highly polarizable, weakly solvated ions have a propensity for the interface. I discuss how the techniques of Second Harmonic Generation have developed to study colloidal, buried interfaces, extending our understanding of adsorption of ions and molecules to solid-water interfaces. Finally, the investigation of the liquid state of carbon is motivated. Chapter 2 discusses the methods used for the studies in this dissertation, starting with a general description of second-order nonlinear spectroscopy. Before detailing Second Harmonic Scattering (SHS), a review of Second Harmonic Generation is provided. The advancements in SHS techniques utilized in the remaining chapters, viz. competitive adsorption, angle-resolved SHS (AR-SHS), and polarization-resolved SHS are chronicled. Additionally, I include an introduction to Resonant Inelastic X-ray scattering (RIXS) for the purpose of probing the liquid carbon. Chapter 3 reports a more robust picture of the adsorption mechanism for dye molecules to charged polystyrene interfaces by investigating the temperature-dependence of the SH signal. The enthalpy and entropy contributions to the free energy were separated. Despite the small changes in ΔGads across the charged polystyrene surfaces, the sign and magnitude of ΔHads changed as a function of temperature. We find that the sign of ΔHads is affected more by the charge of the surface than by the charge of the adsorbate, and attribute the sign change to different mechanisms for adsorption to charged polystyrene surfaces. Chapter 4 extends our studies to porous colloidal interfaces, e.g. porous silica and metal organic framework (MOF) nanocrystals. We employ AR-SHS and polarization-resolved 1 measurements to elucidate the probable reorientation of malachite green dye as a function of concentration. I anticipate that the work in this chapter serves as preliminary data for the development of theories that explicitly describe a buried rough or porous interface. Chapter 5 revisits the use of a competitive adsorption model for comparing the adsorption energy of non-resonant molecules to silica nanoparticles (SNP) and polystyrene beads (PSB). I find that employing the competitive displacement model with our SHS measurements permits resolving the difference in ΔGads for caffeine on SNP and PSB, which amounts to ~1 kcal/mol. Chapter 6 details time-resolved RIXS and XES measurements of ultrananocrystalline diamond (UNCD) and amorphous carbon (a-C) to directly probe changes in the electronic structure of the samples following laser irradiation. However, we find no evidence of changes to the electronic structure and attribute decreases in the time-resolved intensities for a-C and UNCD to transition blocking, subsequent relaxation, and eventual ablation of the samples. A rich comparison of experimental parameters used in a multitude of X-ray spectroscopy techniques is discussed.