A SIMS STUDY OF THE INFLUENCE OF LOW-LEVELS OF SILICON AND CALCIUM ON THE ADSORPTION PROPERTIES OF O-2 ON PT(111)

Surface impurity levels of Ca, Si and Al, which are at or below the detectability limits of Auger electron spectroscopy, have been followed in secondary ion mass spectrometry (SIMS) and correlated with oxygen adsorption on Pt(111). It is shown that oxidation of Si to SiO//x, monitored by the rise in the SIMS Si** plus ion intensity, takes place above 500 K during oxygen TPD. The SiO//x starts decomposing above 1100 K and can be reduced within 200 s. by 2 multiplied by 10** minus **8 Torr of H//2 at 1200 K. The molecular, atomic and total amount of oxygen adsorbed/desorbed in a temperature programmed desorption (TPD) cycle depends on the immediate past history of the impurity levels on the surface. In particular, the presence of SiO//x is correlated with an increase in the overall sticking coefficient of oxygen. These results highlight the importance of impurity levels below the detectability limits of AES and suggest pretreatment methods for obtaining better reproducibility.