The thickness of the quasi-liquid layer on ice and its interaction with atmospheric gases as seen by X-ray absorption and photoelectron spectroscopies

Abstract

Ice is ubiquitous in the environment and plays a role in atmospheric chemical cycles, which impacts the environment and human health. Therefore, it is important to understand the mechanism by which ice interacts with the air. The commonly accepted picture is the presence of a quasi-liquid layer (QLL) on the surface of the ice that increases in thickness with increasing temperature or with increasing concentrations of soluble species. X-ray excited electron spectroscopy, X-ray photoelectron spectroscopy (XPS), and partial electron yield X-ray absorption spectroscopy (PEY-XAS) are well-suited surface science techniques to investigate the QLL at the interface between the ice and the air of the atmosphere. While XPS enables a quantitative surface sensitive elemental and chemical composition analysis, PEY-XAS provides insights into the structure of the hydrogen bonding (HB) network in the ice lattice close to the surface. In the frame of this thesis, I investigated the thickness of the QLL on ice as a function of temperature. I developed a careful analysis of the different uncertainties related to PEY-XAS, with a special focus on the assessment of the probing depth in PEY-XAS, using a Monte Carlo (MC) simulation of electron scattering. The results indicate that, at the probing kinetic energy (KE) window typically used to detect electrons in PEY-XAS, the escape depth is about ∼3 times larger compared to that of unscattered electrons and that gas phase scattering decreases this factor down to ∼1.5, at 5 mbar. Consideration of electron scattering in PEY-XAS is novel and the improved analysis of the PEY-XAS data reveals that the QLL is of the order of ∼2nm close to the melting point. After the QLL investigation on pure ice, I studied the adsorption of hexylamine (HA) on ice to complement the large literature that is available about the adsorption of acids on ice. I measured the surface coverage of HA as a function of gas-phase partial pressure, and for two different temperatures. I found that a saturated monolayer is present already at a partial pressure of 2 × 10−5mbar, and that more than half of the HA molecules are protonated. The PEY-XAS data revealed that the adsorbed HA on ice at -20°C increases the fraction of water molecules in disordered configurations, and give a PEY-XAS signal similar to that of pure ice at -1°C. This is expected and has been observed in the literature with different acids such as HCl. Finally, I studied the temperature dependence of the surface disorder in liquid water and found that the latter increases with temperature, similar to the QLL on ice. As a logical continuation of the MC simulation results, I established the proof-of-principle of PEY-XAS depth profiling ability on an ice sample, by probing the HB network and the carbon contamination as a function of depth. Last but not least, I investigated the direct impact of gas phase pressure (in this experiment, N2) on the surface disorder of the ice and found no impact at -120°C.