XENON DIFFUSION ON A STEPPED PT(11,11,9) SURFACE

The surface diffusion of xenon was studied on a stepped Pt(11,11,9) su rface. Laser-induced thermal desorption (LITD) techniques were employe d to examine the coverage dependence and anisotropy of the Xe diffusio n. The coverage dependence of the Xe desorption rate was also measured using isothermal LITD and isothermal mass spectrometric methods. Both the diffusion and desorption rates of Xe on Pt(11,11,9) were independ ent of Xe coverage. These coverage-independent results provide no evid ence for Xe trapping on step sites and are inconsistent with the exist ence of Xe islands. The coverage-independent desorption kinetics were similar to the kinetics on Pt(111) at low coverage with E(des)=5.7 kca l/mole and nu(d)=3.7X10(10) s(-1). The Xe diffusion was anisotropic an d overwhelmingly parallel to the step edges. The diffusion kinetic par ameters parallel to the step edges were E(dif)=2.8 kcal/mole and D-0=5 0 cm(2)/s. This diffusion barrier is much larger than earlier theoreti cal predictions. The diffusion pre-exponential is also much larger tha n the values predicted by transition state theory. These experimental results suggest that Xe diffuses on the stepped Pt(111) surface with a multiple-site hopping mechanism. The temperature dependence of this m ultiple-site hop length may dominate the measured diffusion activation energy. Consequently, in accordance with recent calculations, the exp erimentally measured diffusion barrier may not directly reflect the un derlying potential energy surface.