EFFECT OF INTERNAL ENERGY ON THE DISSOCIATIVE CHEMISORPTION OF OXYGEN ON IR(110)

We have investigated the effect of internal energy on the initial probability (in the limit of zero surface coverage) of dissociative chemisorption of O-2 on Ir(110) using supersonic molecular beam methods. Measurements have been carried out at six beam energies ranging from 3.3 to 9.2 kcal/mol and at surface temperatures of 400 and 1000 K. Under these conditions the initial probabilities of chemisorption lie between approximately 0.70 and 0.85. Chemisorption is dominated by a trapping-mediated mechanism via a physically adsorbed state at 3.3 kcal/mol and below. With increasing translational energy, contributions to adsorption via direct access to a molecularly chemisorbed state become dominant. At each of the six translational energies investigated, beams were prepared with gas temperatures of 290 K (internally ''cold'') and 850 K (internally ''hot''). We have observed an increase in the probability of chemisorption due to internal energy for this highly reactive system. For a surface temperature of 400 K, there is an increase of about 0.06 in the initial probability of chemisorption of O-2 for the internally hot beam at the lowest translational energies. For a surface temperature of 1000 K, increased initial probabilities of chemisorption for the internally hot beam are also observed, although the enhancement (0.03) is not so large as at the lower surface temperature for the same translational energies. For each surface temperature the enhancement in the initial probability of chemisorption is greatest at the lowest beam energies. We conclude that the enhanced chemisorption is due primarily to increased reactivity, due to internal energy, of the physically adsorbed precursor in the trapping-mediated chemisorption of oxygen.