Three-levels model kinetics for atom recombination at catalytic surfaces

A model kinetics is presented for describing adatom recombination ruled by the ‘hot atom’ mechanism. Hot atoms are identified with adatoms populating the excited vibrational states of the adsorption potential well. The disposal of vibrational quanta to the solid and the transfer of quanta among adatoms are taken into account by the kinetics. In the model case of three-levels the kinetics is solved analytically. It is shown that the competition between recombination and quantum transfer to the solid may drive the system far from equilibrium, leading to an enhancement of the reaction rate. The equilibrium non-equilibrium transition is studied in terms of control parameters, such as the gas flux and the vibrational relaxation time, and it is further characterized through the behaviour of the “apparent” activation energy of the reaction which is found to depend, strongly, on the vibrational state of the adlayer.