Infra-red spectrum and spectral shifts of HCN adsorbed on evaporated alkali halides

Abstract

Infra-red spectra of HCN adsorbed on NaCl, NaI and CsCl films were recorded at low temperatures and for different surface coverages. The films were prepared in a specially designed low-temperature cell by deposition of vapours on a NaCl window cooled to –196°C. The surface areas were in the range 200–300 m²/g. HCN adsorbed on NaCl gave a strong and broad band at 3.18 µ with a shoulder at 3.29 µ, both due to the v₃ vibration. These bands are shifted with respect to the gas phase absorption by 170 and 270 cm^–1 towards lower energies. There appeared a narrow strong band at 4.77 µ due to the v₁ mode not present in the gas phase. Under better resolution this band splits into two absorptions about 10 cm^–1 apart. The v₂ bending mode appeared as two absorptions, at 12.5 µ and 13.4 µ, the latter as a well-resolved doublet with a spacing of 13 cm^–1 at –78°C and 23 cm^–1 at –196°C. This doublet is explained by the removal of the degeneracy of the bending mode in molecules adsorbed parallel to the surface. The 12.5 µ band probably results from molecules oriented perpendicular to the surface. On NaI films the spectrum in the high frequency region resembles that obtained with NaCl films. At low frequencies the v₂ band appears very flat without any signs of splitting. HCN on CsCl shows two absorptions for the v₃ mode at 3.16 µ and 3.32 µ due to adsorption of the molecule on two different sites arising from two different crystalline structures of the substrate. The v₂ band does not show any structure and its maximum is at 12.05 µ.

Spectral shifts of the v₃ mode were calculated using the perturbation method and anharmonic oscillator wave functions. The peturbing potential was taken as a sum of dispersion, electrostatic, induction and repulsion potentials. The magnitudes of these potentials were obtained through summation of interactions with hundreds of ions in the underlying crystal adsorbent.