Apparent tunneling in chemical reactions

A necessary condition for tunneling in a chemical reaction is that the prob ability of crossing a barrier is non-zero, when the energy of the reactants is below the potential energy of the barrier. Due to the non-classical nat ure (i.e, momentum uncertainty) of vibrational states this is, however, not a sufficient condition in order to establish genuine tunneling as a result of quantum dynamics. This proposition is illustrated for a two-dimensional model potential describing dissociative sticking of N-2 on Ru(s). It is su ggested that the remarkable heavy atom tunneling, found in this system, is related to the high momentum tail of the initial vibrational state of nitro gen, allowing for amplitude to cross over the barrier. (C) 2000 Elsevier Sc ience B.V. All rights reserved.