Second Harmonic Generation Studies of Ozone Depletion Reactions on Ice Surfaces under Stratospheric Conditions

Abstract

Hypochlorous acid, HOCl, an important species in the proposed heterogeneous mechanism for stratospheric ozone depletion, has been observed directly at submonolayer amounts on a single crystalline basal ice surface at 155-188 K, using the nonlinear optical method second harmonic generation. The ice is held in equilibrium with its vapor pressure. Second harmonic generation signals from 290 to 310 nm spectroscopically characterize the species and enable us to follow isothermal desorption kinetics in situ. HOCl desorbs as a single species with a ∆G des / ) 48 ( 4 kJ/mol, close to the cohesive energy of ice itself. The lifetime of HOCl on the clean ice surface at 185 K is estimated to be 4 s and the equilibrium surface coverage at 10 -11 Torr HOCl to be around 4 × 10 11 molecules/cm 2 , corresponding to about 0.1% of a monolayer. However, these same measurements performed on ice predosed with varying amounts of HNO 3 show that the HOCl lifetime is lengthened by coadsorbed HNO 3 , depending on the HNO 3 surface density.

Perspectives on the Future of Ice Nucleation Research: Research Needs and Unanswered Questions Identified from Two International Workshops

“…Surface composition and specific molecular features are likely to be important, which differ depending on physical state, surface structure, distribution of functional groups and environmental chemistry of active sites. In situ and non-invasive techniques should be used to address the role of structural parameters of ice-nucleating substrates, either to characterize a solid interface (gas-condensed phase) or the dynamic structure of a solution into which the ice nuclei are immersed [11,12].…”

“…Surface techniques mostly sensitive to interfacial chemical composition need to be explored to validate and interpret interfacial properties of ice nucleation at a molecular level. High-Energy Electron-spectroscopic and X-ray techniques based on photoelectron detection have a penetration depth of 1-2 nm [11,12,26,27]. Chemical titration may be conducted using suitable gas probes for the interrogation of the presence of several types of functional groups at the interface [13].…”

“…Probing the properties of reactive and non-reactive systems at the liquid or disordered ice interface (quasi-liquid layer forming the interface between the gas and the condensed (ice) phase) is another promising technique that has not often been applied in the last 20 years or so when you restrict your view onto those studies that are sensitive to only a few molecular layers. Two examples are the measurement of the reaction kinetics and products of ClONO 2 on pure H 2 O ice resulting in HOCl using SFG techniques [11]. The second example is the examination of the properties of HCl adsorbed on pure H 2 O ice using spectral ellipsometry [12].…”

First-principles molecular-dynamics study of surface disordering of the (0001) face of hexagonal ice

“…[67][68][69] At the unreconstructed 3m symmetric basal ͑0001͒ ice surface, the intensity of the reflected signal polarized in the surface plane varies sinusoidally with when the input light field is polarized parallel to the surface; control surfaces that are isotropic (ϱm symmetry͒, such as that of liquid water, do not show such signals. 70 While polarization studies as a function of rotational angle have not yet been obtained at the ice-water vapor interface, results for Si͑100͒ and Si͑111͒, two very-well characterized surfaces, have been obtained using second harmonic generation. 71 The distribution of angles for bilayer 1 at 190, 230, and 270 is shown in Fig.…”

Hydrogen chloride-induced surface disordering on ice

“…Surface disorder is known to exist on ice at temperatures near 273 K, but its existence has not been confirmed at stratospheric temperatures (188-203 K) (8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23). Surface disorder on hexagonal ice has been characterized by a variety of experimental and theoretical techniques, all of which agree that it does form below the bulk melting temperature, and that its thickness increases with increasing temperature.…”