MOLECULAR AND DISSOCIATIVE CHEMISORPTION AND CONDENSATION OF SO2 ON CU(111)

The chemisorption and condensation of SO2 on a Cu(111) surface have been studied in the temperature range of 75-860 K using angle-resolved TPD, temperature-programmed change of the work function (TPDELTAPHI), molecular-beam backscattering (MBBS), LEED and AES. SO2 molecules mainly adsorb molecularly on clean Cu(111) with a sticking probability of unity at 75-90 K. During heating up, two first-order desorption peaks alpha1 and alpha2 at approximately 150 and 280 K, accompanied for higher exposures by distinctly separated peaks for the bi-, tri- and multilayers, are recorded. A small amount of SO2 was found to adsorb dissociatively, controlled by defects of the Cu(111) surface, as could be shown by AES and isotope-exchange experiments. Assisted by specific TPD experiments and numerical simulations, it could be shown that the unusual small width of the desorption peak at 280 K is caused by a reversible, coverage-dependent step of the binding energy. The desorption of the bi- and trilayer, the latter of which surprisingly desorbs at lower temperatures than the multilayers, shows zeroth-order kinetics, and from the leading edges of the different peaks the desorption parameters could be determined.