Product recoil energy distribution

Information of a different sort is obtained in a molecular beam experiment, although the means for producing the species undergoing unimolecular decomposition is also chemical activation. Whereas the conventional kinetic studies yield reaction rates for direct comparison with RRKM lifetimes, the beam technique yields product recoil energy distribution which, in principle, contain information regarding exit channel dynamics specifically ignored in RRKM. Comparison of experimental results with RRKM theory is indirect, requiring additional assumptions whose validity must be determined. Fortunately, however, statistical theories of a different sort exist which base their predictions on asymptotic (and therefore measureable) properties of the... 

Figure 9. Product recoil energy distributions for F + substituted benzenes...

Figure 12. IF product recoil energy distributions at Erei = 2.6 and 14.1 kcal/mol. Best fit and model calculation distributions as noted.

In order to obtain information about the energy distributions of reaction products, it is necessary to use a detection method that can determine either the internal state populations of the products or their recoil velocities. The methods employed to measure electronic, vibrational or rotational energy distributions are generally based on a form of emission or absorption spectroscopy, although there are other techniques that are sensitive to internal excitation. A variety of methods are used to measure recoil energy distributions these are commonly based on a mass spectrometric detection system used with some form of velocity analyser. 

For the reactions K + HBr and K + DBr, the KBr recoil energy distribution has been determined in a crossed-molecular beam experiment using a mechanical velocity selector. No difference was found in the form of the translational energy distributions for the two reactions for which a value of 0.30 may be derived. Although all the angular momentum appears in the product rotation, the moments of inertia for the alkali halides are large, which implies that the mean product rotational energy is quite small ( 0.21, 0.21 and 0.09 for K, Rb, Cs + HBr, respectively [3] these values are derived from the rotational temperatures obtained by electric deflection analysis). 

Herschbach [58] noted a striking similarity between the recoil energy distribution of Cl atoms in the H + CI2 reaction and that observed in the photodissociation of CI2. This suggests that the electron attachment to the molecule is essentially a vertical process, hence he proposed the DIP extension to the model, which makes the AB repulsion after the electron jump analogous to that encountered in photodissociation experiments. This provided the necessary empirical basis for estimating the parameter of the repulsive interaction. All the mathematical expressions relevant to the model were given by Truhlar and Dixon [62]. Zare and co-workers extended the model to chemiluminescent reactions and a full account of the new model is given in Ref. [81]. It was used to predict successfully the product state distribution in the reaction Ca( So) -I- F2 —> CaF(B ) + F. 

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