Abstract:

Thermal desorption spectra for oxygen chemisorbed on electrolytic silver have been obtained using a temperature programmed reaction (TPRS)technique with mass analysis in an ultrahigh vacuum (UHV) system. More than twenty times cycles of pretreatments were carried out at a partial pressure of oxygen equal to 1×10~(-3) torr and at 650 ℃ while desorption measurements were obtained in a total pressure enviroment 2-5×10~(-9) torr. For oxygen exposure 90 L three distinct species appear in the desorption spectra. Isotope-mixing, CO titration and kinetic behaviour studies lead to the following assignments: weakly molecularly chemisorbed oxygen (E_d=92 kJ mol~(-1), v=4.1×10~(11) s~(-1)), strongly chemisorbed stomic oxygen (E_d=134 kJ mol~(-1), v=4.7×10~(13) s~(-1)) and oxygen diffused to the bulk of silver (see Fig.2). Detailed information about the variation of states of adsobed oxygen with exposure, between 5 L and 10~8 L, have been obtained and for θ=0.2~0.6 a “break” of atomic oxygen desorption is also found, in agreement with other studies. The desorption of molecularly adsorbed oxygen exhibited complex kinetics due to lateral interactions between the adsorbed molecularly oxygen and stomic oxygen. It is suggested that the adsorption of molecularly oxygen due to the presence of surface impurities, such as sodium, carbon, sulfur etc.. The behavior of the desorption of bulk oxygen has been studied by isothermal dessorption technique. The result shows that the higher temperature of oxygen adsorption favors the diffusion of oxygen.