Early barrier

You can estimate when the barrier occurs (late or early) using thermodynamic information for the reaction (i.e. slopes and asymptotic energies). For example, an early barrier would be obtained for a reaction with the characteristics ... 

From the point of view of associative desorption, this reaction is an early barrier reaction. That is, the transition state resembles the reactants.46 Early barrier reactions are well known to channel large amounts of the reaction exoergicity into product vibration. For example, the famous chemical-laser reaction, F + H2 — HF(u) + H, is such a reaction producing a highly inverted HF vibrational distribution.47-50 Luntz and co-workers carried out classical trajectory calculation on the Born-Oppenheimer potential energy surface of Fig. 3(c) and found indeed that the properties of this early barrier reaction do include an inverted N2 vibrational distribution that peaks near v = 6 and extends to v = 11 (see Fig. 3(a)). In marked contrast to these theoretical predictions, the experimentally observed N2 vibrational distribution shown in Fig. 3(d) is skewed towards low values of v. The authors of Ref. 44 also employed the electronic friction theory of Tully and Head-Gordon35 in an attempt to model electronically nonadiabatic influences to the reaction. The results of these calculations are shown in... 

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 5.3 A potential energy contour diagram for an early barrier...

Reactions on attractive surfaces with early barriers are promoted by high translational energy in the reactants, with vibrational energy playing a minor role. Selective enhancement by translational energy is easiest when there is a straight run up the entrance valley to the critical configuration. 

Typical reactions with an early barrier studied by molecular beams include those of the alkali metals with halogens and other simple molecules. 

Since the M—N distance in the activated complex is only slightly greater than the intemuclear distance in the reactant molecule, then the reaction entity has only moved very slightly along the reactant valley. This means that the activated complex lies in the entrance valley, giving an early barrier. This conclusion is verified by the large P—M distance in the activated complex, which indicates that P is still far from MN in the activated complex. 

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. 

Fig. 3.1.4 Contour plot of a potential energy surface for the reaction A + BC —> AB + C. The surface is shown as a function of the two internuclear distances Rab and Rbc at a fixed approach angle. The barrier (marked with an arrow) occurs in the entrance channel, i.e., an early barrier.

This simple model would lead one to conclude that H2 dissociation on transition metals, where the unfilled d-states produce a low and early barrier (or even zero barrier), will show no vibrational enhancement, whereas dissociation on simple and noble metals, for which the barrier is high and late, will have vibrationally enhanced dissociation. This appears to be borne out in molecular beam experiments there is no observable increase in dissociation with internal state temperature for H2 on Ni(l 1 1), Ni(l 1 0), Pt(l 1 1) or Fe(l 1 0) [16-19], whereas dissociation on all surfaces of Cu shows an... 

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.

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