Quantum-state-resolved reactivity of overtone excited CH4on Ni(111): Comparing experiment and theory.
Quantum state resolved reactivity measurements probe the role of vibrational symmetry on the vibrational activation of the dissociative chemisorption of CH4on Ni(111). IR-IR double resonance excitation in a molecular beam was used to prepare CH4in three different vibrational symmetry components, A1, E, and F2, of the 2ν3antisymmetric stretch overtone vibration as well as in the ν1+ν3symmetric plus antisymmetric C-H stretch combination band of F2symmetry. The quantum state specific dissociation probability S0(sticking coefficient) was measured for each of the four vibrational states by detecting chemisorbed carbon on Ni(111) as the product of CH4dissociation by Auger electron spectroscopy. We observe strong mode specificity, where S0for the most reactive state ν1+ν3is an order of magnitude higher than for the least reactive, more energetic 2ν3-E state. Our first principles quantum scattering calculations show that as molecules in the ν1state approach the surface, the vibrational amplitude becomes localized on the reacting C-H bond, making them very reactive. This behavior results from the weakening of the reacting C-H bond as the molecule approaches the surface, decoupling its motion from the three non-reacting C-H stretches. Similarly, we find that overtone normal mode states with more ν1character are more reactive: S0(2ν1) > S0(ν1+ ν3) > S0(2ν3). The 2ν3eigenstates excited in the experiment can be written as linear combinations of these normal mode states. The highly reactive 2ν1and ν1+ ν3normal modes, being of A1and F2symmetry, can contribute to the 2ν3-A1and 2ν3-F2eigenstates, respectively, boosting their reactivity over the E component, which contains no ν1character due to symmetry.
Laboratoire de Chimie Physique Moléculaire, Ecole Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland.
P Morten
PM
Hundt
Laboratoire de Chimie Physique Moléculaire, Ecole Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland.
Maarten E
ME
van Reijzen
Laboratoire de Chimie Physique Moléculaire, Ecole Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland.
Rainer D
RD
Beck
Department of Chemistry, University of Massachusetts, Amherst, Massachusetts 01003, USA.
Han
H
Guo
Department of Chemistry, University of Massachusetts, Amherst, Massachusetts 01003, USA.
Bret
0000000271271735
B
Jackson