Late barrier

An important further consequence of curvature of the interaction region and a late barrier is tliat molecules that fail to dissociate can return to the gas-phase in vibrational states different from the initial, as has been observed experunentally in the H2/CU system [53, ]. To undergo vibrational (de-)excitation, the molecules must round the elbow part way, but fail to go over the barrier, eitlier because it is too high, or because the combination of vibrational and translational motions is such that the molecule moves across rather than over the barrier. Such vibrational excitation and de-excitation constrains the PES in that we require the elbow to have high curvature. Dissociation is not necessary, however, for as we have pointed out, vibrational excitation is observed in the scattering of NO from Ag(l 11) [55]. 

The thermoneutral reactions mostly follow the same behaviour as with the endothermic reactions. If there are late barriers, vibrational energy is most effective, otherwise the translational energy is relatively more important. 

Nonempirical LCAO-MO-SCP study for linear HeH surface demonstrates a late barrier, or potential-energy step characteristic for reactions showing vibrational enhancement (Fig. 61) 453... 

Other situations can occur where the activated complex lies in the entrance valley, an early barrier, or in the exit valley, a late barrier. 

Figure 5.5 A potential energy contour diagram for a late barrier...

Figure 5.10 A potential energy contour diagram for a light atom attacking in a reaction with a late barrier, showing energy requirements for reaction and energy disposal in products...

Since these two cross sections have similar magnitudes, this would correspond to a rebound mechanism where the two molecules have to come very close together before reaction will occur. The activated complex thus lies in the exit valley and the potential energy profile has a late barrier. 

A barrier that occurs in the entrance channel while the reactants are approaching each other is denoted as an early barrier, whereas a late barrier occurs in the exit channel as the products are separating. 

Note that for two reactions that only differ by the position of the saddle point along the reaction path, i.e., early and late barriers (Section 3.1), respectively, transition-state theory will predict exactly the same rate. 

In contrast to H2/Pd, the vibrational effects in the adsorption of H2/Cu(l 0 0) are mainly caused by the curved reaction path. The basic mechanism can be discussed within a two-dimensional elbow plot shown in Fig. 2b. The PES corresponds to a so-called late barrier system which refers to the fact that the barrier is located after the curved region of PES. If the molecule is already initially vibrating, i.e. if it is oscillating back and forth in the d-direction, then the vibrational energy can be very efficiently used to make it around the curve and enter the dissociation channel. Nevertheless, adiabatic effects as just discussed in the context of the hydrogen dissociation on Pd(l 00) also contribute to the vibrational effects for H2/Cu(l 0 0). 

Figure 1 The ubiquitous elbow potential energy surface showing for the dissociation of a diatomic molecule on a surface. This is a function of die molecular bond length and the molecule-surface distance. The reactants are intact molecules, while die products are the atoms chemisorbed separately on the surface. The two extreme cases are shown, an early barrier for which the initial vibration of the molecule is ineffective in overcoming the barrier, and a late barrier for which vibration assists in the dissociation process.

Overall this leads to an increase in dissociation with increasing rotational state, in competition with the reduction in dissociation due to orientational hindrance [30, 33], For the H2/Cu(l 1 1) system, experiments indicate that there is likely a small upshift of the dissociation threshold with increasing rotational state, /, at small J, but then a downshift of the threshold as J increases further, i.e. orientational hindrance wins out at low rotational states, but eventually the centrifugal enhancement from a late barrier dominates [34-36],... 

The orientation dependence of the PES couples the rotational states of the molecules, consequently the scattered flux should show strong rotational excitation and de-excitation. For late barrier systems this also couples to vibrational motion, i.e. there are combined vibrational and rotational transitions. We shall return to this topic in Sections 5 and 6. 

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