Maximum recoil product

Tang was the first to report the formation of Si-l-silacyclopent-3-ene, a product whose yield passes through a maximum of only 5%, as shown in Figure 2 (4,39). An empirical formula, SiC4He, was deduced for the major product, and the silacyclopentadiene structure was favored, but the authentic compound could not be synthesized (4). The experience of other workers with substituted silacyclopentadienes suggests that this molecule will dimerize quickly in macroscopic quantities (41), but for a carrier-free recoil product this process would be slow. 

An important characteristic of radioactive decay is that the momentum of the emitted particle must be balanced by the momentum of the product nucleus. In suitable cases it should be possible to detect the recoil nucleus. Thorium-228, which decays to radium-224 having an energy of 97 keV, has been used to study the oxidation of a number of metals. Its advantages are its low volatility (as thorium oxide) and its relatively low rate of diffusion in lighter metals. The maximum range of recoils in solids is of the order of 300-500 A and for thinner oxide layers, its distance from the surface can be measured. One difSculty with quantitative work is that the radium undergoes a sequence of further decay, which complicates calculation of recoil ranges, and calibration may be necessary. 

Shortly after these results were published, Bernstein and coworkers (10,11) completed a crossed beam study of the reaction of K atoms with HBr and DBr, with velocity selection of the incident K-atom beam and velocity analysis of the KBr product. Analysis of the data showed that the product KBr c.m. angular distributions are broadly backward-peaked for K + HBr and nearly isotropic for K + DBr and that the recoil velocity distributions are broad and extend to the maximum value allowed energetically. These results differ considerably from the results for K + TBr since (i) when taken together, the angular distributions imply that the KBr product shifts nonmonotonically from broadly backward-peaked to nearly isotropic to sharply backward-peaked with the isotopic substitutions HBr DBr TBr and (ii) the mean recoil energy is much lower (and therefore the product excitation much higher) for K + TBr. These differences are still unresolved at this writing. 

Stopping potential analyzers offer maximum sensitivity at lowest beam intensities. However, the fact that all ions below a certain energy are detected makes it difficult to distinguish low-energy components which are relatively weak (e.g., those particles recoiling backwards in the center-of-mass system from a reaction which yields mostly forward-directed products). 

The separation characteristic of SHIP is shown in Fig. 19.7. The upper panel displays the reaction Lu( Ar, 5n), which has its maximum within the velocity window of SHIP while the lower panel shows the reaction Lu( Ar, a4n), which has a minimum in this velocity window. The recoil from the a particles emitted from the compound nucleus is large enough to drive the remaining residue out of the velocity window. This results in a strong suppression of a-channels and all channels, which involve the emission of a fragment like an a particle or a heavier product in the exit channel (Faust et al. 1979). 

Probing product state distributions by multiphoton ionization is one of the most sensitive methods for the analysis of both bimolecular and photofragmentation dynamics. For example, by using REMPI one can measure the rotational state distribution in the N2 fragment produced in the photofragmentation of N2O it was found that the maximum in the rotational state population is near J 70. This reveals that although the ground electronic state is linear, the excited state is bent and thus the recoil from the O atom results in rotational excitation of the N2 molecule... 

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