New Advances in Spectroscopy: Applications to Aqueous Interfaces and Liquid Carbon

This dissertation comprises two major projects. The first is the study of ions and molecules at the air/water interface using second-order nonlinear spectroscopy to measure interfacial electronic |χ(2)|2-spectra. The second is the investigation of the liquid state of carbon using time-resolved soft X-ray free electron laser spectroscopy.The nature of ions and molecules at and near aqueous interfaces is central in many chemical systems including atmospheric chemistry, electrochemistry, and biochemistry, but remains incompletely understood. Contrary to widely held earlier views, recent studies have unambiguously shown that some ions adsorb strongly to the air/water interface, with compelling evidence for enhanced surface concentrations of larger, more polarizable, weakly solvated anions. While the mechanism of selective ion adsorption has been shown to correlate strongly with ionic radii and hydration properties, considerable debate remains. In this dissertation, I describe the development and application of femtosecond broadband deep ultraviolet electronic sum frequency generation (DUV-ESFG) spectroscopy which allows quantitative analysis of peak positions, linewidths, and relative intensities that can be directly compared with theoretical calculations to yield new insights into the nature of ions and molecules at aqueous interfaces. Under the electric dipole approximation, the second-order nonlinear susceptibility vanishes in centrosymmetric media, and SFG spectroscopy is a surface-sensitive probe with a probe depth of ca. 1 nm, corresponding to a few outermost monolayers. In Chapter 2, I present the interfacial charge-transfer-to-solvent spectrum of thiocyanate (SCN−), a prototypical chaotropic anion, and discuss selection rules and salient differences between interfacial and bulk solvation that contribute to the observed spectral differences. In Chapter 3, we investigate the lowest energy charge-transfer-to-solvent bands of aqueous iodide using broadband DUV-ESFG and two-photon absorption spectroscopy and assign the observed transitions using atomic selection rules within a Rydberg transition model based on the jj-coupling limit. We find that DUV-ESFG signal at the air/water interface arises from symmetry breaking and solute-solvent vibronic coupling effects that relax the selection rules. In Chapter 4, we revisit the π→π* transition of the nitrite ion at the air/water interface and, together with MD simulations and electronic structure calculations, find no evidence of a previously reported contact ion pair induced redshift and strongly favorable bimolecular adsorption mechanism. However, the presence of a distribution of local solvation environments at the interface, including solvent separated ion pairs, solvent shared ion pairs, contact ion pairs, and small ion clusters is not ruled out. In Chapter 5, I present the |χ(2)|2-spectra of phenol at the air/water interface in the deep UV (5.4-6.3 eV) and find an apparent redshift of the electronic transitions relative to the bulk aqueous spectrum. MD simulations and electronic structure calculations suggest stabilization of the excited states due to the highly specific hydration structure effected at the interface. Additional work is ongoing to elucidate the role of selection rules and solvent environment on the observed spectral differences. In Chapter 6, I discuss my efforts to attempt heterodyne-detected DUV-ESFG spectroscopy and future directions, including incorporating a flat-jet that would enable measurements at low concentrations and liquid/liquid interfaces. The liquid state of carbon is of fundamental chemical significance and has potential practical applications, but it remains very poorly characterized. For example, studies suggest that carbon nanotubes and Q-carbon are formed through liquid intermediates. Moreover, novel carbon materials and routes for synthesizing them are of great interest, as some proposed carbon allotropes may have exciting new properties for technological applications, and better understanding of the liquid properties may suggest new routes for synthesizing them. In this dissertation, I present our recent efforts to characterize liquid carbon using pump-probe soft X-ray free electron laser spectroscopy at the carbon K-edge. In Chapter 7, I detail time-resolved resonant inelastic X-ray scattering (tr-RIXS) and X-ray emission spectra (tr-XES) of non-thermally melted carbon films measured at the Pohang Accelerator Laboratory X-ray Free Electron Laser in South Korea. An initial decrease in tr-RIXS signal intensity at short delay times was observed, corresponding to a decrease in the absorption cross-section of sp2-hybridized carbon atoms as structural reorganization occurs to form liquid carbon. Additional theoretical work is ongoing to further elucidate the dynamics of the melting and ablation processes. In Chapter 8, I present and discuss our recent attempts to demonstrate optical & soft X-ray sum frequency generation spectroscopy at the carbon K-edge, building upon soft X-ray second harmonic generation spectroscopy demonstrated by our group at FERMI in Italy.

Advisor: Saykally, Richard J.