Surface of ice as viewed from combined spectroscopic and computer modeling studies

Abstract

The structure of the ice surface and its interaction with adsorbates are investigated by several experimental tools, combined with computer modeling. Spectroscopic features characteristic of icy surfaces were identified and assigned. Adsorbate spectroscopy is used to probe both the adsorbate layer and the ice surface structure. These results are potentially informative of basic questions, such as cooperative aspects of H-bonding and the mechanism of ice vaporization, and of diverse practical questions, such as the role of icy particles in atmospheric chemistry and physics as well as the chemistry of interstellar space. Methods are described for the preparation and spectroscopic study of microporous amorphous ice and cubic ice nanocrystals with surface to volume ratios that make it possible to obtain low-noise infrared and Raman spectra of the vibrational modes localized near the surfaces and of the fundamental modes of small molecule adsorbates. The assignment of the bands of several of the surface-localized modes is reported, on the basis of primarily the calculated vibrational excitations for simulated structures of both amorphous and crystalline ice. The usefulness of these spectra is enhanced by conversion to difference spectra that compare high surface area and low surface area samples. Bands have been assigned to each of the three important types of surface water molecules, as revealed by the simulated structures and spectra: molecules with non-H-bonded or dangling-H(D) atoms, molecules with a dangling-0 coordination, and 4-coordinated surface water molecules. The experimental difference spectra have also been used to display the influence of small adsorbate molecules on surfacelocalized vibrations of each type of water molecule. This influence is apparent through the shifting and enhancement of bands of surface-localized modes, the response of the modes of the adsorbate molecules, and the determination of site-selective heats of adsorption of small molecules using the assigned ice modes. Computer modeling in conjunction with ab initio calculations was used to analyze and interpret adsorbate spectra and to elucidate the influence of factors such as the extent of surface disorder on gas-surface interactions. The results suggest significant modification of the ice surface structure with respect to the cubic crystalline interior, toward loss of lateral order.

An overview of snow photochemistry: evidence, mechanisms and impacts

“…Adsorption of various organic molecules on ice surfaces can be described well with a multi-parameter linear free energy relationship, based on the van der Waals and the electron donor/acceptor interactions (such as H-bonding) (Roth et al, 2004 (Devlin, 1992;Devlin and Buch, 1995). Many organic halocarbon compounds have also been shown to adsorb on water-ices by interactions with the ice surface dangling bonds (Holmes and Sodeau, 1999).…”

Adsorption of CO on amorphous water-ice surfaces

“…In the other three orientations, the water molecule has both its OH groups pointing obliquely down to the water molecules in the second monolayer: both these OH groups are classified as "bonded OH" in this case. The lone pairs of electrons on the oxygen atoms of these water molecules provide the so-called dO adsorbing site for CO (Devlin & Buch 1995). In the second monolayer, each water molecule forms four-hydrogen bonds, so all the OH groups are involved in the hydrogen bonding network, and can also be labelled as "bonded OH".…”

“…In the second monolayer, each water molecule forms four-hydrogen bonds, so all the OH groups are involved in the hydrogen bonding network, and can also be labelled as "bonded OH". The molecules in this second surface monolayer provide the s4 adsorbing sites for CO (Devlin & Buch 1995), see Fig. 2.…”

Atomistic simulations of friction at an ice-ice interface

“…However, the temperature range in which this disorder is seen in experiments varies over the techniques applied, as each of them measures different physical properties of the system [37]. Extensive molecular dynamics (MD) simulations have been performed on the surface of ice demonstrating the presence of a quasi-liquid layer at the surface [40][41][42][43][44][45], although it's thickness and temperature dependence is somewhat a function of the potential that is used in simulations to represent the intermolecular interactions.…”