Over-barrier crossing in multidimensional tunneling

Studies of the dissociative chemisorption of N₂ using two-dimensional empirical potential energy surfaces that differ by the position of the saddle point, all show high tunneling probabilities at energies well below the saddle-point energy of the reaction. This is highly unusual for a heavy-atom system and contradicted by one-dimensional analysis along the minimum-energy path. A mechanism of over-barrier crossing, related to the high-momentum tail of the vibrational wave function of N₂, is demonstrated. We analyze the two-dimensional wave-packet propagation and compares to the probability of having vibrational momenta that exceeds the value required to overcome the relevant barrier.