Endothermic dissociative chemisorption of molecular D2 on Ag(111)

The dissociative chemisorption of molecular D2 on Ag is reported for the first time. Dissociation on the Ag(111) surface is highly activated with an approximately exponential dependence on the energy of the incident molecules, the initial sticking probability increasing from ≈ 10−7 at 230 meV to 10−2 at 465 meV, for a beam incident along the surface normal. The sticking probability is sensitive to the internal temperature, or state distribution, of the D2 beam, dissociative chemisorption being more efficient for higher vibrationally excited states. The threshold for dissociative chemisorption of the ground state lies at too high an energy to be accessed, indicating an activation barrier 0.8 eV for dissociation of D2(ν = 0). Comparison with the activation barrier to D recombinative desorption of 0.28 eV indicates that dissociative chemisorption is endothermic. At large incidence angles and low nozzle temperatures adsorption is dominated by atoms formed in the nozzle, allowing an estimate of 0.6 for the initial sticking probability of D atoms at Ag(111).