ENDOTHERMIC DISSOCIATIVE CHEMISORPTION OF MOLECULAR D-2 ON AG(111)

The dissociative chemisorption of molecular D-2 On Ag is reported for the first time. Dissociation on the Ag(lll) surface is highly activate d with an approximately exponential dependence on the energy of the in cident molecules, the initial sticking probability increasing from app roximate to 10(-7) at 230 meV to 10(-2) at 465 meV, for a beam inciden t along the surface normal. The sticking probability is sensitive to t he internal temperature, or state distribution, of the D-2 beam, disso ciative chemisorption being more efficient for higher vibrationally ex cited states. The threshold for dissociative chemisorption of the grou nd state lies at too high an energy to be accessed, indicating an acti vation barrier > 0.8 eV for dissociation of D-2(v = 0). Comparison wit h the activation barrier to D recombinative desorption of 0.28 eV indi cates that dissociative chemisorption is endothermic. At large inciden ce angles and low nozzle temperatures adsorption is dominated by atoms formed in the nozzle, allowing an estimate of 0.6 for the initial sti cking probability of D atoms at Ag(lll).