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Observation of an exceptionally high apparent kinetic order in the thermal desorption of D2 from the Si(100) surface

Isothermal desorption and angular distribution measurements of D2 recombinatively desorbing from the Si(100)-(2 ×  1) surface are reported for the coverage range of 0.5 to 1.0 monolayer (ML). Above 0.9 ML, the apparent kinetic order of the desorption process is ≥5, far larger than the order of ∼1, for which this reaction is now well known in the surface science community. Below 0.9 ML, the order declines monotonically until it reaches a constant value of ∼1 at a coverage of 0.7 ML. Correspondingly, as the coverage decreases, the activation energy for thermal desorption increases from 1.96 eV to 2.45 eV, while the angular distribution of desorbing D2 molecules sharpens from a cos2 θ det distribution to a cos5 θ det distribution. The changing kinetics and dynamics as the coverage decreases from 1 ML can be understood in terms of a transition at ∼0.8–0.9 ML from the 4H (and perhaps 3H) desorption mechanism to the 2H mechanism. We propose that the high apparent kinetic order is due to an unexpectedly rapid decline in 4H configurations (four D atoms bound as Si–D in adjacent Si dimers) caused by competition from the slower, first-order 2H desorption pathway.