ANGLE-RESOLVED TPD, DELTA-PHI AND MBBS MEASUREMENTS OF CHEMISORPTION AND MULTILAYER ADSORPTION OF SO2 ON CU(111)

The interaction of SO2 with a Cu(111) surface has been studied in the temperature range between 80 and 860 K using angle-resolved TPD, TP-DELTA-phi and molecular beam backscattering (MBBS) measurements, assisted by LEED and AES experiments. After exposing the clean surface to SO2 at 80 K, the TPD spectra consist of two distinctly separated, successively appearing desorption peaks at 280 and at 130 K accompanied for high exposures by an unsaturable multilayer peak. The angular distribution of the desorbing SO2 molecules is in accordance with the cosine law for all observed desorption peaks. Besides the desorption of molecular SO2, a small accumulation of sulphur has been observed by LEED and AES. It could only be removed by Ar+ sputtering, increases with the number of exposure-TPD cycles and yields a (square-root 7 x square-root 7)R +/- 19.1-degrees LEED pattern at saturation. Even very small amounts of sulphur cause drastic changes in the TPD spectra. Isothermal DELTA-phi measurements during SO2 exposure at 80 K show a stepwise increase of the work function by 240 meV, in contrast to a decrease of 220 meV measured at 200 K. A comparison of the TP-DELTA-phi results, obtained during temperature programmed heating of the sample after SO2 exposure, with corresponding TPD measurements indicates a reorganization within the chemisorbed adlayer.