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 molecularly 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 molecularly 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 with average kinetic energy [E] = 1.9 kT(s), where,is the mean desorption temperature; speed ratio SR = 0.97). In contrast, associative desorption which occurs mostly at T-s between 700 and 900 K depending on coverage, is more strongly peaked (angular distribution similar or equal to cos theta(3.5)). The velocity distributions exhibit typical activated 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 assumed between the molecularly chemisorbed and the physisorbed precursor states. However, this barrier must be dynamically variable, depending on the molecular orientation and its energy, so that for the parallel-oriented and slowly moving molecule no activation barrier exists. Coverage dependences of the effective potentials are also very likely of importance.