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.

Infrared spectra have been determined of CF4 adsorbed on nanocrystals of cubic ice at 83 K to a thickness ranging from submonolayer to multilayer with a maximum thickness of ∼ four layers. For the multilayered adsorbate, the band of the CF4 antisymmetric stretch mode, known for its exceptional oscillator strength, appears as a transverse-longitudinal (T-L) doublet, with a splitting of ∼80 cm−1, closely matching that of the plastic solid phase. This doublet splitting is reduced when the amount of adsorbed CF4 is decreased, whether by reduction of the equilibrium vapor pressure or by dilution with CO as a coadsorbate; the response expected for a gradual decoupling of the oscillating dipoles as the local density of CF4 is decreased. A diminished but strong T/L splitting (∼55 cm−1 at 83 K and 72 cm−1 at 25 K) is apparently retained at near monolayer levels of CF4 coverage. Unlike CO, the coadsorbate acetylene was observed to have a relatively minor influence on the T/L splitting despite causing a similar reduction in the total amount of adsorbed CF4, an indication that, unlike CO, acetylene tends to phase separate from the adsorbed CF4 leaving the local CF4 molecular density largely unaffected. The intense absorption by the longitudinal mode, as reported here for the ν3 mode of adsorbed CF4, can be recognized as a Berreman effect from off-normal sampling of thin layers of adsorbates on the curved surfaces of the ice nanocrystals. Such an effect should be common for particulate samples in general that have a coating of a molecular species with an intensely dipole-active vibrational mode. An example of interest may be that of NaNO3 formed by reaction of NO2 or HNO3 at the surface of particles of NaCl as reported by Vogt and Finlayson-Pitts.