CHEMISORPTION OF CARBON-MONOXIDE ON THE IRIDIUM(111) SURFACE - IN-SITU STUDIES OF ADSORPTION AND DESORPTION-KINETICS VIA VIBRATIONAL SPECTROSCOPY

The IR absorption peak position of the CO intramolecular stretching vi bration, measured with Fourier transform infrared reflection-absorptio n spectroscopy, has been employed for the first time as a nonintrusive coverage probe to study the adsorption of CO on Ir(111). The observed temperature and coverage dependence of the adsorption probability is consistent with an extrinsic precursor model. Isobars were obtained un der steady-state conditions for pressures ranging from 10(-9) to 10(-5 ) Torr. Isosteres were constructed from these data and used to determi ne both isosteric heats of adsorption and the adsorbate entropy as fun ctions of coverage. The isosteric heat of adsorption decreased from 44 +/- 2 kcal/mol at 0.05 ML to 37 +/- 1 kcal/mol at 0.6 ML. A pronounce d decrease in the binding energy at 0.35 ML coincides with the weakeni ng of the (root 3 x root 3)R30 degrees LEED pattern and is attributed to near-neighbor CO repulsions. The entropy of the adsorbate layer ind icates two ordering regions which correspond to the formation of the ( root 3 x root 3)R30 degrees and (2 root 3 x 2 root 3)R30 degrees overl ayer structures.