Dependence of the S 0 on E i at normal incidence can be divided into two characteristic regions. In the first region below 2 eV, S 0 increases by more than 150 times as the E i changes from 0.28 eV to 2 eV. This indicates a translationally activated process with a high activation barrier. In the second region of 2 eV to 4.6 eV, S 0 increases by a factor of less than 1.5, and is 4.1 × 10 -2 at 4.6 eV. The values of the S 0 being low at large E i (much greater than the estimated barrier) is an indication of a severely constrained reaction pathway. Similar characteristics have been reported in other cases previously, for example, the well-studied N 2 /Ru(0001) system. Here too the S 0 increases very slowly with E i , with much larger than the activation barrier. Based on simulations over a six-dimensional PES it was found that increased surface corrugation and anisotropy near the minimum energy pathway creates a narrow bottleneck, thereby explaining the low reactivity, even at much higher energies than the dissociation barrier. A similar situation could be prevailing in this case as well. The slow increase in the S 0 even at E i well above 2 eV is consistent with a highly constrained reaction pathway, allowing only a small fraction of configurations to overcome the barrier, regardless of the available incident energy. Similar conclusions have been drawn from recent work by Yin and coworkers 14 based on simulations for the CO 2 /Cu(110) system on a multidimensional PES. Although, at the moment results from simulations are only available up to an E i of 2 eV, it will be interesting to look into the reaction probabilities predicted by extending this work to higher E i .