Role of Vibrational excitation of the adlayer in Precursor mediated sticking probabilities of CO on metal surfaces: a kinetic model

The available experimental and theoretical evidence for vibrational excitation of the adlayer in CO chemisorption on metal surfaces raises the question on the validity of the generally accepted model of 'precursor-mediated' adsorption in the presence of adsorption–stimulated desorption (asd) as triggered by the hyperthermal conditions of the adlayer. The analysis shows that a 'precursor-mediated' kinetics still describes the total rate of adsorption, but that sticking probability measurements carried out in the presence of asd might actually measure the net adsorption rate, that is a lower sticking probability. Literature data, on the decrease of the sticking probability at increasing energies of the impinging CO molecular beams, have tentatively been interpreted by assuming an enhancement of the desorption rate caused by the energy transfer from the beam. The analysis is supported by a theoretical kinetic model of 'precursor- mediated' adsorption which takes into account, explicitly, the vibrational excitation of the adspecies.