Thermal desorption of hydrogen from the diamond C(100) surface

Temperature-programmed desorption (TPD) is used to measure the kinetics of hydrogen desorption from the C(100) surface. Two major desorption channels, attributed to hydrogen desorption from (2 × 1) domains (α sites) and disordered domain boundaries sites (β sites) were observed. The change in the morphology of the C(100) surface upon hydrogen adsorption/desorption leads to a variation in the intensity ratio between the two hydrogen desorption peaks. The degradation results in smaller (2 × 1) domains, as evidenced by deteriorated LEED patterns in which the characteristic (2 × 1) half-order spots disappear. The (2 × 1) LEED pattern can be restored on the degraded C(100) surface by repeated hydrogenation-annealing cycles in the temperature range 300-1100 K. Fitting the experimental TPD data with simulated curves from the two-site model shows that desorption from the α site obeys first-order kinetics with a prefactor of 1.4 ± 0.9 × 10¹³ s^-1 and an activation energy of 80.3 ± 1.3 kcal mol^-1, whereas the process giving rise to the β peak follows second-order kinetics with a prefactor of 2.3 ± 0.9 × 10¹³S^-1 (expressed in first-order units) and an activation energy of 75.1 ± 0.5 kcal mol^-1. Possible mechanisms for first-order recombinative hydrogen desorption from the C(100)-(2 × 1) surface are also discussed.