Infrared spectra of CF4 adsorbed on ice: Probing adsorbate dilution and phase separation with the ν3 transverse-longitudinal splitting

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Abstract
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.