Journal of Physical Chemistry, Vol.99, No.45, 16534-16548, 1995
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. : molecules with non-H-bonded or dangling-H(D) atoms, molecules with a dangling-O coordination, and 4-coordinated surface water molecules.
Keywords:AMORPHOUS SOLID WATER;INFRARED-SPECTRA;VIBRATIONAL-SPECTRA;LIQUID WATER;CLUSTERS;MOLECULES;QUANTUM;REGION;CO;ABSORPTION