SURFACE-DEFECT VIBRATIONAL-MODES OF LARGE ICE CLUSTERS

A comparison of FT-IR spectra of ice clusters of approximately 25-nm diameter with spectra of larger clusters permits a closer look at the surface-defect modes of crystalline ice clusters. In addition to the dangling-OH group infrared absorption at 3694 cm-1, a broader band of comparable intensity is revealed at 3563 cm-1. This band is assigned to the in-phase stretch of surface molecules having a dangling-OH group on the basis of an apparent analogy between the modes of such molecules and the donor water molecule of the water dimer. Further, an examination of the water bending-mode region, including comparisons with (a) spectra of larger clusters and (b) spectra of clusters for which the dangling-OH groups are hydrogen bonded to a surface dopant, indicates the presence of two surface-defect-mode absorptions. A band at 1650 cm-1 is assigned to the bending mode of the surface water molecules containing the dangling-OH groups. A broader band at 1700 cm-1 has been tentatively identified with the surface water molecules containing the ''dangling'' oxygen atoms.