Effect of rotation on the translational and vibrational energy dependence of the dissociative adsorption of D2 on Cu(111)

Abstract

We have investigated the dependence on the rotational and vibrational states of the translational energy of D₂(v,J) formed in recombinative desorption from Cu(111). These results provide information about the effect of rotational energy relative to that of vibrational and translational energy on the dissociative chemisorption of D₂ on Cu(111). The range of rovibrational states measured includes rotational states J=0–14 for vibrational state v=0, J=0–12 for v=1, and J=0–8 for v=2. D₂ molecules were detected in a quantum‐state‐specific manner using three‐photon resonance‐enhanced multiphoton ionization (2+1 REMPI). Kinetic energies of desorbed molecules were obtained by measuring the flight time of D₂⁺ ions in a field‐free region. The mean kinetic energies determined from these measurements depend strongly on the rotational and vibrational states. Analyzing these results using the principle of detailed balance confirms previous observations that vibrational energy is effective, though not as effective as translational energy, in promoting adsorption. Rotational motion is found to hinder adsorption for low rotational states (J≤5) and enhance adsorption for high rotational states (J≥5). Even for high J states, however, rotational energy is less effective than either vibrational energy, which is 30%–70% more effective than rotational energy, or translational energy, which is 2.5–3 times more effective than rotational energy in promoting adsorption. The measured internal state distributions for the rovibrational states listed above are consistent with the observed dependence of the kinetic energy of the de‐ sorbed molecules with the rotational state. In addition, the analysis performed yields the dependence of the adsorption probability on kinetic energy separately for each rovibrational state. These functions have very similar sigmoidal shapes for all states examined. Changing the quantum state is primarily associated with a shift in the position, or threshold energy, for the curves. The level at which these functions saturate or level off at high energy is independent of rotational state but varies nonmonotonically with the vibrational state.