The active site for reaction V X, Y

The site dependence of the PES produces diffraction of scattered molecules, which we shall discuss in Section 5. The corrugation can also be probed by examining the incidence angle dependence of the dissociation. This is commonly discussed in terms of the scaling of the dissociation, writing 

The site dependence of the PES is more than just a variation of the barrier energy and distance from the surface, the early/lateness of the barrier and the curvature of the elbow PES can also change. This leads to a spatial separation of different processes, as noted above, for H2 on Cu surfaces, dissociation of the vibrational ground-state molecules is dominated by the bridge sites, while molecules vibrationally excite/de-excite largely at the atop site. In fact, site specificity of reactivity is also state specific [45]. Thus for H2/Cu(1 0 0), the bridge site is the most favoured for dissociation of vibrationally cold molecules, but the atop site is the most favoured for vibrationally excited molecules, as shown in Fig. 4. 

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