Angular distribution of products

Two teclmiques exist for measuring the angular distribution of products. In the crossed-beam setup, the... 

Since the velocities and angular distributions of products from collisional dissociations at low incident-ion energies have generally not been determined, the precise mechanism by which the products are formed is unknown. Thus in the collisional dissociation of H2+ with helium as the target gas, H+ may result from dissociation of H2+ that has been directly excited to the vibrational continuum... 

Finally, Fig. 4.1.15 shows the differential cross-section dan/dd, which gives the angular distribution of product molecules independently of their quantum state. The differential cross-section shows a peak for backward scattering (0 = 180°) with the... 

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. 

Generally in molecular beam studies, both beams have comparable velocities and intersect one another at 90°, and thus the CM velocity vector points at a wide angle intermediate between the two beams. Measurement of the displacement of the laboratory angular distribution of products from the centre-of-mass vector enables an estimate of the velocity of the products to be derived. Reaction products have been velocity analysed (e.g. see refs. 8 and 231) and the results support the view that the product relative translational energy is usually within ca. 1 kcal mole of the reactant relative translational energy. Most of the alkali metal reactions studied to date are exothermic, thus the products must be internally excited. It is believed [8] that, for most reactions, the internal excitation consists mainly of vibrational excitation however, the partition of the vibrational energy between, for example, KI and CH3 is as yet unknown. There are a few exceptions, e.g. the K + HBr reaction where KBr is rotationally excited rather than vibrationally excited [8], and the... 

As mentioned previously, the modified spectator stripping model (polarization model) of Herman et al. [103] explains the velocity distribution of products very well but does not predict the angular distribution, whereas the DIPR model explains both. Thus there had been no full comparison between the two models until Chang and Light [113] refined and extended the polarization model to yield the angular distribution of products as well. 

We can conclude by repeating our initial remark the amoimt of experimental data is still not sufficient and the further work is necessary. Moreover, in most actual studies carried out in the gas phase only the average cross-sections may be determined and this necessitates the averaging over the statistical distribution of velocities and of impact parameters in all theoretical treatments. From this point of view, crossbeam experiments, where the dependence of quenching cross-sections on the relative velocity of colliding particles (and even on the impact parameters, if the angular distribution of products is studies) may be determined, would be of highest interest. 

Reaction product imaging. In this technique, product ions are accelerated by an electric field toward a phosphorescent screen and the light emitted from the screen is imaged by a charge-coupled device. The significance of this experiment to the study of chemical reactions is that it allows for a detailed analysis of the angular distribution of products. 

D.A. Micha. Angular distribution of products of hydrogen atom-hydrogen molecule reactions. Ark. Fys.. 30. 437-47 (1965). 

The calculation of the angular distribution of products in magnetic predissociation is similar to that in photodissociation. In the above derivation, we have only demonstrated the calculation of the angular distribution for the case of the magnetic transition moment being along the molecular axis other cases can be treated in a similar manner as those in the photodissociation and will not be discussed here. Notice that when Hfi is independent of / values, Eq. (164) yields P v = 2. 

A broad definition of beam techniques includes devices such as doublechamber or tandem mass spectrometers. These are treated elsewhere in this book. We shall restrict ourselves here primarily to those studies in which velocity and/or angular distributions of products are determined. 

A detailed discussion of isotope effects, as such, is beyond the scope of this chapter, although mechanistic conclusions derived from them are incorporated in the preceding material. However, isotope effects in beam studies open such wide new perspectives that a few words are in order. The phrase isotope effect has usually connoted only such effects on rate constants or yields—and such information, in the form of cross sections, is still available from beams. What is important, however, is the fact that isotope effects on velocities and angular distributions of products can provide significant new sources of information. 

Third, the angular distribution of product recoil of the two channels can be plotted (lower left of... 

With the advent of the molecular beam technique it became possible to get valuable information on two-particle collision dynamics. Studies on the angular distribution of products and determination of their internal state at a fixed, sufficiently narrow initial distribution of reagents over velocities and rotational and vibrational states enable evaluation of the reaction cross sections. 

Another direction should be noted, namely the development of simple reaction models [185, 269, 372]. This approach provides qualitative and often even semi-quantitative results requiring no tedious calculation and thus permits the interpretation of some bimolecular reactions. The simple models are classified according to angular distribution of products and also by the extent to which the energy of the collision complex AB formed by collisions of reagents A and B is redistributed among various degrees of freedom before the reaction is completed. 

The angular distribution of products in these reactions is highly asymmetric with respect to -S = 90°. This implies that the rearrangement occurs in a time shorter than the period of rotation. For exchange reactions involving transfer of an atom or a group of atoms as an entity... 

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