ANGLE AND ENERGY-DISTRIBUTIONS OF THERMALLY DESORBING OXYGEN FROM PT(111) - THE INFLUENCES OF A DYNAMICALLY VARIABLE ACTIVATION BARRIER

Angular and velocity distributions of the desorbing flux of O-2 during temperature programmed desorption (TPD) were investigated for the mol ecularly chemisorbed and the dissociated oxygen species on the Pt(111) surface, starting at initial coverages of 0.47 O-2/Pt, and between 0. 17 and 0.20 O/Pt, respectively. The O-2 flux, which desorbs from the m olecularly chemisorbed state at surface temperatures, T-s, around 145 K in parallel with dissociation of O-2, shows the typical behaviour of non-activated desorption (Maxwell-Boltzmann velocity distribution wit h average kinetic energy [E] = 1.9 kT(s), where,is the mean desorption temperature; speed ratio SR = 0.97). In contrast, associative desorpt ion which occurs mostly at T-s between 700 and 900 K depending on cove rage, is more strongly peaked (angular distribution similar or equal t o cos theta(3.5)). The velocity distributions exhibit typical activate d behaviour with [E] = 3.3 kT, and speed ratios SR = 0.85 at angles of desorption up to 40 degrees, changing rapidly to [E] = 2 kT(s) and SR = 1 at a desorption angle of 60 degrees. Both findings are compatible with existing adsorption data if the relevant activation barrier is a ssumed between the molecularly chemisorbed and the physisorbed precurs or states. However, this barrier must be dynamically variable, dependi ng on the molecular orientation and its energy, so that for the parall el-oriented and slowly moving molecule no activation barrier exists. C overage dependences of the effective potentials are also very likely o f importance.