THERMAL-DESORPTION OF HYDROGEN FROM THE DIAMOND C(100) SURFACE

Temperature-programmed desorption (TPD) is used to measure the kinetic s of hydrogen desorption from the C(100) surface. Two major desorption channels, attributed to hydrogen desorption from (2 x 1) domains (a s ites) and disordered domain boundaries sires (beta sites) were observe d. The change in the morphology of the C(100) surface upon hydrogen ad sorption/desorption leads to a variation in the intensity ratio betwee n the two hydrogen desorption peaks. The degradation results in smalle r (2 x 1)domains, as evidenced by deteriorated LEED patterns in which the characteristic (2 x 1) half-order spots disappear. The (2 x 1) LEE D pattern can be restored on the degraded C(100) surface by repeated h ydrogenation-annealing cycles in the temperature range 300-1100 K. Fit ting the experimental TPD data with simulated curves from the two-site model shows that desorption from the a site obeys first-order kinetic s with a prefactor of 1.4 +/- 0.9 x 10(13) s(-1) and an activation ene rgy of 80.3 +/- 1.3 kcal mol(-1), whereas the process giving rise to t he beta peak follows second-order kinetics with a prefactor of 2.3 +/- 0.9 x 10(13) s(-1) (expressed in first-order units) and an activation energy of 75.1 +/- 0.5 kcal mol(-1). Possible mechanisms for first-or der recombinative hydrogen desorption from the C(100)-(2 x 1) surface are also discussed. (C) 1998 Elsevier Science B.V. All rights reserved .