A thermal desorption study of iodine on Pt( [formula omitted

Abstract

Thermal desorption spectra of iodine atoms adsorbed on Pt(1 1 1) are reported for coverages θ up to 0.43 monolayers (ML), the saturation coverage at 300 K. Analysis of the spectra as a function of heating rate and initial coverage shows that for θ<0.28 ML, desorption is first order with a coverage-dependent desorption energy. These data are fitted quantitatively to a Bragg–Williams statistical–mechanical transition-state model that includes the effect of vibrations of the adsorbed iodine layer according to the Debye theory. Measurements of the temperature variation of low-energy electron diffraction beam intensities gave surface Debye temperatures of 105±15 K for the (√3×√3) R30° phase and 155±15 K for the (√7×√7) R19.1° phase. Lateral repulsions between iodine atoms are 10 kJ mol −1 for θ<0.28 ML. A feature at θ=0.28 ML is interpreted in the hard-hexagon model as a phase transition between 2-D gas and 2-D solid phases. Another such phase transition occurs at θ>0.38 ML, and explains the zero-order desorption peak observed at these coverages. Thermodynamic quantities were estimated from the data, including Δ des H o=219±20 kJ mol −1 for (√3×√3) R30°-I→I(g) and Δ des H o=201±20 kJ mol −1 for (√7×√7) R19.1°-I→I(g).