Fathoming ice Using non-linear ultrafast spectroscopy to look at interfacial properties

Water in its condensed states in the atmosphere has catalytic properties and influences relevant reaction mechanisms, the most infamous example being the catalysis of ozone hole formation by ice crystals. Using sum frequency generation spectroscopy in the infrared frequency range, the interfacial vibrational properties of water in its solid and liquid states were probed, to answer fundamental questions on the microscopic differences we observe for the two states. It was found that the interfacial vibrational bend mode of ice, centered at ~1600 cm-1, is primarily homogeneously broadened, is much wider in comparison to water, and has negligible temperature dependence. The vibrational stretch mode of ice is in contrast much narrower than of water, and is strongly temperature dependent. Using the pump-probe scheme of the SFG spectroscopy, the relaxation dynamics of the hydrogen-bonded OH-stretch oscillators for ice were elucidated to be much faster compared to water, while surprisingly a rather slow time constant in the free-OH region was observed. Experimental measurements of interfacial ice in the OH-stretch region were correlated to calculated SFG spectra in an attempt to unravel the nature and mechanism of proton ordering on the surface of ice. Estimations of the enthalpic and entropic contributions of differently proton-ordered surfaces were performed to complement the simulation models and experimental data. Finally, experiments and simulations on the water-cholesterol interface reveal that the structural order of an ice-nucleating agent outweighs the molecular structure for its ice-nucleating efficiency.