Scattered products

The most important observable is the angular distribution of the scattered products with respect to the initial approach direction of the reagents, which is called the state-to-state differential cross-section (DCS). The DCS can be written [57-61]... 

We will also need to consider the reaction probability, which is a measure of the amount of scattered product at a given value J, defined by... 

Fig. 37. Reaction mechanism for the forward scattering product from the H + HD H2 + D reaction at the collision energy of 1.200 eV.

The beams of reactant molecules A and B intersect in a small scattering volume V. The product molecule C is collected in the detector. The detector can be rotated around the scattering centre. Various devices may be inserted in the beam path, i.e. between reactants and scattering volume and between scattering volume and product species to measure velocity or other properties. The angular distribution of the scattered product can be measured by rotating the detector in the plane defined by two molecular beams. The mass spectrophotometer can also be set to measure a specific molecular mass so that the individual product molecules are detected. 

Note that if a channel is closed, kn is imaginary [see Eq. (2.46)] and the (R kn) wave function is proportional to a decaying exponential exp(—/cnft). Closed channels are therefore not observed in the ft —> cc limit, i.e., they do not contribute to the scattering products. 

Ab initio [515, 516] and semi-empirical calculations [517] of the reaction potential-energy surface show that the potential-energy barrier for reaction depends on the angle of the H—H—F transition state and is lowest for the collinear configuration, having a value 4 kJ mole-1. Thus, collisions involving a nearly collinear approach of F to H2 make the major contribution to reaction and give backward-scattered products. All the surfaces are of a repulsive type. 

The yield of recoil energy from the M + X2 reaction appears to be independent of the nature of X2 but does increase up the series from Cs to Li. This is paralleled by a decrease in aR and an increase in the amount of backward scattered product. Parrish and Herm [63] have speculated that this may simply arise from a mass effect, although they have indicated that other explanations are possible. 

Wilson and Herschbach [75] have discovered several systems, involving polyhalide molecules, where the peak intensity of the scattered product corresponded to wide c.m. angles and the dynamics is intermediate between the limiting cases represented by rebound and stripping. The preference for forward scattering could be correlated with large values for aR. E was estimated to lie between 10 and 30% of the total energy. 

Fig. 10. Representative time-of-flight distributions for inelastically scattered oxygen atoms (rnfz = 16) following impact with a squalane surface. The incident atomic-oxygen beam had an average energy of 504 kJ mol and impinged on the surface at an incident angle of 60°. The scattered O atoms were detected at three final angles 70°, 45°, and 10°. Two populations of scattered products are identified, those with thermal (dashed line) and hyperthermal (solid line) energies.

The reactions of vdW molecules and clusters can be divided into intra- and intercluster processes, and further into neutral and ionic cluster reactions. The latter were recently reviewed by Mark and Castleman. Therefore the scope of this contribution will be limited to neutral species only. We distinguish between intra- and intercluster reactions. In intracluster processes reactions are induced inside a cluster, usually by light. Examples of such reactions are the reaction of excited mercury atoms with various molecules attached to them, reactions that follow photodissociation in the cluster, and charge transfers inside a large cluster. In intercluster reactions the cross molecular beam technique is usually applied in order to monitor scattered products and their internal energy. The intercluster reactions may be divided into three major categories recombination processes, vdW exchange reactions, and reactions of clusters with metal atoms. 

However, this measure of the scattered product at a given value of J (equivalently, at a fixed classical impact parameter) is not an experimental observable, and therefore one needs to sum up the different contributions of different impact parameters leading to reaction to be able to compare theory to experiment. By coherently summing different S matrix elements corresponding to different values of J, one obtains the scattering amplitude given by the expression... 

To establish whether the product thermodynamically favored is the one really formed, we carried scattering experiments at two different collision energies of 21.1kJ/mol and 27.0kJ/mol [75,82], We were able to observe scattered products at two different mass-to-charge ratios, 51 and 50, which correspond to the ions and C2N , respectively. However, the LAB... 

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