OXYGEN ON THE PD(100) SURFACE - DESORPTION DYNAMICS WITH SURFACE PHASE-TRANSFORMATIONS AND LATERAL DIPOLE REPULSIONS

Phase transformations in oxygen overlayers on the Pd(100) crystal face reported earlier (Simmons, G. W.; Wang, Y.-N.; Marcos, J.; Klier, K. J. Phys. Chem. 1991, 95, 4522) express themselves in a distinct and interesting kinetic behavior during oxygen desorption. The thermal desorption spectra (TDS) from the dense (theta = 0.8) (square root 5 x square root 5)R27-degrees structure give rise to a sharp 'explosive'' desorption peak demonstrating that a very high population of surface oxygen is nearly simultaneously activated for desorption; this is consistent with the proximity of fixed pairs of oxygen atoms in the (square-root 5 x square-root 5)R27-degrees structure proposed by Simmons et al. When this dense structure is partially depleted, a phase splitting occurs such that dense and rare (theta almost-equal-to 0.4) phases coexist and desorption occurs from both until the dense phase is exhausted. The remainder of the rare phase obeys kinetic laws consistent with high mobility and strong lateral repulsion among the oxygen adatoms. Other phases of intermediate density such as c(2 X 2) (theta = 0.50) or p(5 x 5) (theta = 0.68) disproportionate into the dense (square root 5 x square root 5)R27-degrees structure and the rare phase only at temperatures just below desorption. A kinetic model that successfully describes the observed thermal desorption spectra involves a continuous equilibration of the dense and the rare phase during the desorption process. For the rare phase, both the King-Adams (King, D. A. Surf. Sci. 1975, 47,384. Adams, D. L. Surf. Sci. 1979, 42,12) and the Klier-Zettlemoyer-Leidheiser-Devonshire (Klier, K.; Zettlemoyer, A. C.; Leidheiser, H. J. Chem. Phys. 1970, 52, 589. Devonshire, A. F. Proc. R. Soc. London 1937, A]63, 132) models require lateral repulsion, 3.5 kcal/mol among nearest-neighbor occupied sites in the former and d2/a3 in the latter (d = -2.85 D, a = distance between nearest-neighbor adatoms, variable with surface coverage), and both models reproduce the high-temperature TDS peak shifts and widths. However, the latter model accounts better for the high activation energy for desorption from very low coverages experimentally determined by a trailing edge analysis (almost-equal-to 45-60 kcal/mol), large variation thereof with coverage, low value of activation energy at initial coverages, and overall line shapes of the TDS peaks. In the dipole repulsion model, the transition state is a molecule with 0-O distance 1.3 angstrom, close to that of the gaseous oxygen molecule, that undergoes rotation and hindered translation in the array of oxygen adatoms.