A note on the vibrational efficacy in molecule-surface reactions

Abstract

The effectiveness of vibrational energy in promoting dissociation of molecules colliding with surfaces can be measured through the so-called vibrational efficacy. It is by many thought to be a pure “energetic” measure and therefore believed to be limited from below by zero (in the case that there is no increase in dissociation probability upon vibrational excitation) and from above by one (in the case that all of the vibrational excitation energy is used to promote reaction). However, the quantity vibrational efficacy is clearly linked to the detailed dynamics of the system, and straightforward considerations lead to the conclusion that it is not limited either from below or above. Here we discuss these considerations together with a quasi-classical dynamics study of a molecule-surface system, N2/Ru(0001), for which a vibrational efficacy bigger than one has been found both experimentally and theoretically. We show that an analysis of the vibrational efficacy only in terms of energy transfer from vibration to translation can be too simple to describe the behavior of systems for which the potential energy surfaces present (high) reaction barriers, potential corrugation and anisotropy, and curved reaction paths.