Aqueous Solutions as seen through an Electron Spectrometer : Surface Structure, Hydration Motifs and Ultrafast Charge Delocalization Dynamics

Abstract: In spite of their high abundance and importance, aqueous systems are enigmatic on the microscopic scale. In order to obtain information about their geometrical and electronic structure, simple aqueous solutions have been studied experimentally by photo- and Auger electron spectroscopy using the novel liquid micro-jet technique in conjunction with synchrotron radiation. The thesis is thematically divided into three parts. In the first part we utilize the surface sensitivity of photoelectron spectroscopy to probe the distributions of solutes near the water surface. In agreement with recent theoretical predictions we find that large polarizable anions, such as I- and ClO4-, display enhanced surface propensities compared to smaller rigid ions. Surface effects arising from ion-ion interactions at higher electrolyte concentrations and as function of pH are investigated. Studies of linear mono-carboxylic acids and benzoic acid show that the neutral molecular forms of such weak acids are better stabilized at the water surface than their respective conjugate base forms. The second part examines what type of information core-electron spectra can yield about the chemical state and hydration structure of small organic molecules in water. We demonstrate that the method is sensitive to the protonation state of titratable functional groups and that core-level lineshapes are dependent on local water hydration configurations. Using a combination of photoelectron and X-ray absorption spectroscopy we also show that the electronic re-arrangement upon hydrolysis of aldehydes yields characteristic fingerprints in core-level spectra. In the last part of this thesis we study ultrafast charge delocalization dynamics in aqueous solutions using resonant and off-resonant Auger spectroscopy. Intermolecular Coulombic decay (ICD) is found to occur in a number of core-excited solutions where excess energy is transferred between the solvent and the solute. The rate of ultrafast electron delocalization between hydrogen bonded water molecules upon oxygen 1s resonant core-excitation is found to decrease upon solvation of inorganic ions. The presented work is illustrative of how core-level photoelectron spectroscopy can be valuable in the study of fundamental phenomena in aqueous solutions.