Highly Rotationally Excited N₂ Reveals Transition-State Character in the Thermal Decomposition of N₂O on Pd(110)

We employ time-slice and velocity map ion imaging methods to explore the quantum-state resolved dynamics in thermal N₂O decomposition on Pd(110). We observe two reaction channels: a thermal channel that is ascribed to N₂ products initially trapped at surface defects and a hyperthermal channel involving a direct release of N₂ to the gas phase from N₂O adsorbed on bridge sites oriented along the [001] azimuth. The hyperthermal N₂ is highly rotationally excited up to J = 52 (v″ = 0) with a large average translational energy of 0.62 eV. Between 35 and 79% of the estimated barrier energy (1.5 eV) released upon dissociation of the transition state (TS) is taken up by the desorbed hyperthermal N₂. The observed attributes of the hyperthermal channel are interpreted by post-transition-state classical trajectories on a density functional theory-based high-dimensional potential energy surface. The energy disposal pattern is rationalized by the sudden vector projection model, which attributes to unique features of the TS. Applying detailed balance, we predict that in the reverse Eley-Rideal reaction, both N₂ translational and rotational excitation promote N₂O formation.