An infrared study of the symmetric P–F stretch of PF3 chemisorbed on the Pt(111) surface

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Abstract
We have used the technique of Fourier transform infrared reflection absorption spectroscopy (FT-IRAS) to characterize the symmetric P–F stretch of PF3 chemisorbed on the Pt(111) surface over the temperature range of 300–82 K. At 300 K and very low coverage we observe the P–F stretch at 899 cm−1. At intermediate coverages the P–F stretch shows two components of comparable intensity while at saturation coverage the spectrum is characterized by an intense band at 950 cm−1. The temperature dependence of the spectrum is distinctly different at different coverages. One of the two components of the broad band seen at 300 K after 0.8 L exposure becomes much sharper as the temperature is lowered while the other component becomes broader and the relative intensities of the two bands are reversed by 82 K. After an exposure of 0.5 L a symmetric band with a FWHM of 6.6 cm−1 at 300 K broadens upon cooling to 82 K into a band with an overall FWHM of 11.1 cm−1 which is asymmetric due to an unresolved high frequency shoulder. We attribute the two components of the P–F stretch to molecules within ordered ((3)1/2×(3)1/2)R 30° islands and to molecules in a lower density disordered lattice gas. Because of lateral interactions the higher frequency band is associated with molecules within the islands and the lower frequency band to the lattice gas. The relative intensity of the two components changes due to a shift in the equilibrium toward larger islands at lower temperatures and/or higher coverages.