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

Abstract

This paper reports the simultaneous internal state and translational energy resolved associative desorption flux of N 2 from Ru͑0001͒ 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 ͑vibrational͒ 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 bonds. We believe that the same factors will be important in most activated dissociations of 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.

Multidimensional Effects on Dissociation ofN2on Ru(0001)

“…The barrier V to reaction is much higher for N 2 Ru0001 (2 eV) [13] than for H 2 Cu (0:5 eV) [19]. In contrast to H 2 =Cu [17] comparison between adiabatic theory and experiment for N 2 reaction on Ru(0001) has so far presented major discrepancies, for dissociative chemisorption, associative desorption [11], and inelastic scattering [18], supporting the idea of a large influence of nonadiabatic effects.…”

“…Also, it has recently been shown that nonadiabatic [diabatic with [8] or without [9] couplings] models describe the dissociation of O 2 on Al(111) well, whereas an adiabatic description fails. It has even been argued that indirect evidence exists that nonadiabatic effects decrease the reactivity of N 2 on Ru(0001) (a low spin molecule with low electron affinity reacting on a general type transition metal surface) by more than an order of magnitude [10,11].…”

Vibrational deexcitation and rotational excitation of H2 and D2 scattered from Cu(111): Adiabatic versus non-adiabatic dynamics

“…15 Motivated by this controversy, an important number of theoretical methods that include electronic excitations have been recently developed. [16][17][18][19][20][21][22][23][24] In some cases, non-adiabatic dynamics based on low-dimensional calculations has resulted in overestimation of the effects associated with e-h pair excitations. For example, for N 2 /Ru(0001) it was claimed, 16 based on low-dimensional non-adiabatic calculations, that the huge discrepancy between low-dimensional adiabatic calculations and experiment was mostly due to e-h pair excitations, whereas a subsequent adiabatic high-dimensional dynamics study 25,26 showed that most of the discrepancy vanishes when the six degrees of freedom of the molecule are taken into account in the dynamics.…”

The reaction rate for dissociative adsorption of N2 on stepped Ru(0001): Six-dimensional quantum calculations

“…All six nitrogen degrees of freedom are included in this study, while the positions of the surface atoms are frozen. The possible effect of electron-hole pairs 59 has not been included in the present description. In future work the effect of the motion of the Ru surface will be investigated.…”

“…52 Due to its importance, the N 2 -Ru system has been extensively studied experimentally. [53][54][55][56][57][58][59][60] Density-functional theory ͑DFT͒ calculations 52,61-65 explored the electronic structure and the reaction path.…”