Angular distribution forward-backward symmetry

After these two examples, it should not be thought that reactions described by the harpoon model are necessarily direct. This was the case with reactions 4 and 5 because they are very exoergic and the molecular negative ion is unstable. In contrast, in the almost thermoneutral reaction of Cs with NO2, NOj is a stable ion, and the charge-transfer complex Cs - NOj corresponds to a deep well along the reaction coordinate. Hence the reaction proceeds from a persistent complex, with a forward-backward symmetry in the angular distribution of the reaction product CsO [72]. 

Complex-mediated reaction An elementary bimolecu-lar reaction proceeding via a long-lived collision complex having lifetimes ranging from several vibrational periods (100 fs) to many rotational periods (>10 ps). If complex lifetimes exceed several rotational periods, product angular distributions from crossed beam reactions exhibit forward-backward symmetry in the center-of-mass frame of reference. 

The experimentally measured CO2 angular distribution, shown in Fig. 16, is broad with products scattered to both sides of the center-of-mass of the system. Such angular distributions are said to exhibit forward-backward symmetry in the center-of-mass frame of reference. From analysis of product translational energy distributions (not shown), the degree of CO2 vibrational excitation has been inferred for this reaction. 

Since the isotropic distribution of the reaction products occurs in the plane of the collision, and since it has cylindrical symmetry about the relative velocity vector, the isotropic character leads to a non-uniform distribution in space. As shown in the bottom of the figure, the angular distribution shows a typical forward-backward symmetry. As a result, there is much more intensity near the poles (0 = 0, tt) than the equator (0 = tt/2). 

Figure 10. shows the P(E) and T(0) used in the simulation of reaction (iii). The angular distribution is quite broad. The forward-backward symmetry in the... 

The F + CH3I reaction proceeds via a potential hypersurface with a well. At a collision energy of 10.9 kJ mol" the IF angular velocity distribution as determined by Farrar and Lee for F + CH3I shows forward-backward symmetry. 

The key point is that, for a long-living complex, the angular distribution has a forward-backward symmetry. The dynamics appears to have forgotten the initial direction. The only thing that it remembers is that the angular momentum (and, of course, energy) is conserved. Problem D and Section 10.3.1.1 provide a quantum version of this result. 

The aS3rrametric forward-backward peaking may be modelled by assuming that the angular distribution, 1(0), can be factored into a term with forward-backward symmetry and an "osculation" term ... 

The angular distribution of the KBr product was found to be strongly forward-peaked (see Fig. 7 of Ref. 1). This indicates immediately a substantial contribution ftom (la) since (lb), from symmetry, can only give a forward-backward symmetric product distribution. However, measuring the K atom product separately it was found that it matched the KBr only in the forward direction, but fell short at 9(P. Thus there must also be a contribution from (lb) which gives KBr predominantly at wider angles. A branching ratio (la) (lb) = 3 1 was estimated. 

The angular distribution from Ca + NaCl depends on the CaCl vibrational level backward-forward symmetry peaking at 0 and 180° occurs for... 

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