Forward peaked angular

In the remainder of this paper, we wish to consider the conditions which may lead to broad, or even forward peaked, angular distributions of products for energies near the reaction threshold. Such angular distributions have been observed In recent F+H2 beam experiments (23), and within the context of the BCRUI, we have observed this phenomenon In several systems. 

Figure 15 shows a contour map for the F2 + ICl system which is typical of all the data. Very forward peaked angular distributions for the products suggest that abstraction of an F atom from F2 by XI proceeds through a somewhat bent geometry, i.e., F-F-I or F-I-X angle less than 180° in the system F-F-I-X. 

In summary, the H + HD reaction shows little sign of resonance scattering in the ICS. Furthermore, the product distributions without angle resolution show no unusual behavior as functions of energy that might indicate resonance behavior. On the other hand, the forward peaking in the angular product distribution does appear to reveal resonance structure. Since time-delay analysis is at present not possible in a molecular beam experiment, it is the combination of a sharp forward peak with the unusual... 

Since the forward peak is clearly from high J collisions, it is clearly produced via a rapidly rotating intermediate exhibiting an enhanced time delay. Further insight into the associated dynamics is provided by a classical trajectory simulation by Skodje. The forward peak results from the sideway collisions of the H atom on the HD-diatom (see Fig. 37). At the point where the transition state region is first reached, the collision complex is already oriented about 70° relative to the center-of-mass collision axis. The intermediate then rotates rapidly with an angular frequency of u> J/I, where / is the moment of inertia of the intermediate. If the intermediate with a time delay of the order of the lifetime r, the intermediate can rotate... 

LAB angular distributions for F H2(J=0) and F H2(J°1) at 1.84 kcal/mol in Figure 9 show that there is considerably less forward peaking of the HF(v 3) product from H2(J 1) than from H2(J 0). It appears that resonance effects are less pronounced... 

In an indirect reaction [2] A + BC —t B-A-C —t AB + C or AC + B. In a first step, the A atom inserts into the BC diatom forming an ABC complex. Two new bonds (AB and AC) are formed while the BC bond is broken. Then the complex dissociates with a breaking of one of these two bonds. This reaction mechanism is called insertion. In contrast with abstraction reactions, all three bonds in the triatomic molecule ABC participate actively in the reaction. Two bonds are formed teni] )orarily while only one exists for the reactants and products. Thus, the potential energy surface involves a very deep well (several eV) which correspond to a stable ABC molecnle or radical. When the lifetime of the ABC molecule is larger than its rotational period, angular distributions of the products are symetric with a backward/forward peak and the population of rovibrational states of the products presents a statistical character. 

Kwei et al. [172] have observed differences in the reactions of Li -i- KF and Li + KBr which may be explained in terms of the difference in exothermicity of the two reactions. For KF the large exothermicity removes any basin which might exist and reaction proceeds by the stripping mechanism. The KBr reaction is less exothermic and a shallow basin remains. The presence of a small forward peak in CM angular distribution suggests that the LiBrK triangular complex has a lifetime of >2 X 10 sec. 

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