DESORPTION-KINETICS OF CO FROM EPITAXIAL PD FILMS ON W(110) IN THE LOW-COVERAGE LIMIT

The adsorption-desorption kinetics of CO from ultrathin epitaxial Pd f ilms has been studied using modulated molecular beam techniques. We fi nd that the rate constant for desorption, k(d), is nearly four orders of magnitude higher for the monolayer Pd film than that for the bilaye r film. The rate decreases further by a factor of 7 between 2 and 6 mo nolayers at which k(d) attains the value seen on Pd single crystals. W e conclude that although a bilayer film is similar to a bulk Pd(111) s urface in binding CO, true bulk-like behavior is only reached above si x atomic layers of Pd. Temperature programmed desorption at low covera ge can be fit using the beam determined kinetic parameters, showing th at defects play no major role in the beam experiments. Significant dep artures in the values of the activation energy for desorption and the preexponential factor for the monolayer film from expected values are attributed to a possible adsorbate induced surface reconstruction.