Indirect evidence for strong nonadiabatic coupling in N2 associative desorption from and dissociative adsorption on Ru(0001)

This paper reports the simultaneous internal state and translational energy resolved associative desorption flux of N-2 from Ru(0001) using two different experimental approaches. Both experiments show that the nascent N-2 is formed with little vibrational excitation and that the total excitation in all N-2 degrees of freedom accounts for only 1/3 of the barrier energy. Roughly 2/3 of the energy necessary to surmount the barrier is lost to the surface in desorption. This behavior, as well as the unusual behavior noted previously in direct measurements of dissociative adsorption, both imply strong vibrational quenching in reactive trajectories passing over the high exit channel (vibrational) barrier. Adiabatic quasiclassical dynamical calculations based on the ab initio potential energy surface and various models of coupling to the lattice are not qualitatively consistent with N-2 vibrational damping to phonons. However, including a strong nonadiabatic coupling of the vibrational coordinate to electron-hole pairs in the dynamics does yield qualitative agreement between experiments and calculated dynamics, and we suggest this as indirect evidence for strong nonadiabatic coupling. We argue that the nonadiabatic coupling is strong in this case because of the high vibrational excitation necessary to pass over the high exit channel barrier in the reactive processes and the large charge transfer inherent in making or breaking pi bonds. We believe that the same factors will be important in most activated dissociations of pi bonded molecules on transition metal surfaces, e.g., for O-2, NO, N-2, and CO, and if this scenario is correct then nonadiabaticity should be important in the activated dissociation dynamics of these systems as well. (C) 2002 American Institute of Physics.