INTERACTION OF LOW-ENERGY ELECTRONS WITH SO2 LAYERS ON AG(111) - COMPARISON TO PHOTOCHEMISTRY

The interaction of low-energy electrons (8-54 eV) with SO2 layers on Ag(111) has been studied using temperature programmed desorption (TPD) and Auger electron spectroscopy (AES). For coverages up to one monolayer, the only electron-stimulated desorption (ESD) process is parent desorption, which has a cross section of 3.6 +/- 0.8 X 10(-17) cm2 using 54 +/- 1 eV electrons. This ESD process has an electron energy threshold of 18.0 +/-1.0 eV corresponding to ionization of the 6a, molecular orbital of adsorbed SO2. For coverages greater than one monolayer, ESD is accompanied by electron-induced decomposition (EID) and the total cross section for loss of SO2 is almost-equal-to 10(-16) cm2, independent of coverage up to 8 monolayers(ML). The differences between chemisorbed and physisorbed layers is attributed mainly to metal-induced quenching of electronically excited adsorbates, which is less important for those SO2 molecules further from the metal. Compared to photon-driven desorption, electron-driven desorption follows a different excitation pathway, which we attribute to the formation of transient positive ions.