Infrared spectra of ice surfaces and assignment of surface-localized modes from simulated spectra of cubic ice

Abstract

The use of a new method of preparing micron-thick deposits of nanocrystals of ice for Fourier transform infrared sampling, with the nanocrystals supported on a vertical infrared window, has greatly improved the signal-to-noise levels of the spectra available for large ice clusters. High quality spectra of modes of the surface molecules are reported, even for regions that underlie the intense bands of the bulk ice modes. These experimental features are most clearly displayed through the use of difference spectra. For example, the difference between spectra obtained for nanocrystals, before and after an annealing cycle that significantly increases the average cluster size, reflects the decrease in number of surface groups and the corresponding increase in number of interior molecules. Similarly, differences between spectra of bare and adsorbate-covered nanocrystals, obtained at the same temperature for the same ice sample, show the significant shifts of ‘‘surface-localized’’ ice modes caused by the adsorbate molecules. These difference spectra, and similar spectra for amorphous ice, are rich with information about the (three) distinct types of ice surface water molecules and their interactions with small adsorbate molecules. The extraction of that information has been initiated by comparison of the experimental difference spectra from two sizes of D2O cubic ice nanocrystals with simulated difference spectra for a relaxed cubic ice surface compared to bulk cubic ice. From these comparisons specific experimental features have been assigned to modes of the three categories of surface D2O(HDO) molecules: (a) three-coordinated molecules with dangling-D—2725 (2713) cm−1; (b) three-coordinated D2O molecules with dangling-O—2645 (∼2600) cm−1; (c) relaxed four-coordinated molecules—∼2580 (∼2550) cm−1. Also, information has been obtained on the approximate positions (cm−1) of other modes of surface molecules: (a) D-bonded part of dangling-D(H) molecules; ∼2350; (b) dangling-O molecules; ∼2500; (c) four-coordinated molecules; 2300–2500. The computations also indicate that, of the various modes of the surface molecules, only the higher frequency modes of the dangling-D and dangling-O are strongly localized; and only the dangling-D mode is localized on individual surface molecules.