Role of Rotational alignment in Dissociative Chemisorption and Oxidation: O2 on bare and CO -precovered Pd(100)

The adsorption of gas-phase molecules that approach solid surfaces in well-defined quantum states represents a fundamental step for the understanding of heterogeneous chemical reactions and for better control over the growth of selfassembled layers. This objective has been achieved only in a few cases to date (NO, H2, CH4). Vibrational excitations and conversion of translational to vibrational energy were thereby found to account for strong enhancements in the dissociation probability. Less attention has been devoted to rotations because they involve less energy, and are therefore important only for physisorption. We show here that rotations as well as the alignment of the rotational axis play a role in the dissociative chemisorption of O2 onto a CO/Pd(100) surface. The effect, which arises from the requirements needed to pass through the CO adlayer, leads to different sticking probabilities and average O-CO distances, and has possible general applications for controlling reactions and film growth.