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Abstract:
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.
