SURFACE OF ICE AS VIEWED FROM COMBINED SPECTROSCOPIC AND COMPUTER MODELING STUDIES

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-O coordination, and 4-coordinated surface water molecules. The experimental difference spectra have also been used to display the influence of small adsorbate molecules on surface-localized 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 hears 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.