A thermal desorption study of iodine on Pt(111)

Thermal desorption spectra of iodine atoms adsorbed on Pt(111) are reported for coverages theta 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 theta < 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 (root3 x root3)R30degrees phase and 155 +/- 15 K for the (root7 x root7)R19.1degrees phase. Lateral repulsions between iodine atoms are 10 kJ mol(-1) for theta < 0.28 ML. A feature at theta = 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 theta > 0.38 ML, and explains the zero-order desorption peak observed at these coverages. Thermodynamic quantities were estimated from the data, including Delta(des)Hdegrees = 219 +/- 20 kJ mol(-1) for (root3 x root/3)R30degrees-I --> I(g) and Delta(des)Hdegrees = 201 +/- 20 kJ mol(-1) for (root7 x root7)R19.1degrees-I --> 1(g). (C) 2002 Elsevier Science B.V. All rights reserved.