LAYER GROWTH-MECHANISM AND DIPOLE DIPOLE INTERACTIONS OF CH3CL ON PD(100) - AN INFRARED AND THERMAL-DESORPTION STUDY

Fourier transform infrared spectroscopy (FTIRS) and temperature programmed desorption (TPD) have been used to study the layer growth mechanism of CH 3Cl on a Pd(100) surface at 85 K. Methyl chloride adsorbs and desorbs in molecular form without decomposition. Three different adsorbed states are readily distinguishable by both methods. The chemisorbed state (first layer) saturates at Θ = 0.5 coverage, with the characteristic C-H vibrations of 1011, 1330, 1426, 2927 cm-1. The desorption spectrum for this state suggests a coverage-dependent activation energy between 54.5 and 42 kJ/mol. Nearly the same amount of CH3Cl is bonded in the physisorbed state (second layer) with characteristic C-H vibrations at 1013, 1338, 1428, and 2925 cm-1. The activation energy of the desorption in this case is 40-35 kJ/mol. The condensation phase appears only after the completion of the first two layers and exhibits characteristic C-H vibrations at 1023, 1338/1346, 1444, and 2955 cm-1. In a model calculation {developed by Albano [J. Chem. Phys. 86, 1044 (1986)]} it is shown, that the intermolecular dipole-dipole interaction may explain correctly the coverage- dependent activation energy for desorption of methyl chloride. © 1990 American Institute of Physics.