Resonance capture

In this work the reacting negative ions were produced with electrons of low energy leading to resonance capture processes, rather than at higher... 

High concentrations of chlorine give the same value of Gw., within experimental error, as is observed with bromine97. However, there is an important difference between the two halogens. Although it is exothermic by 26 kcal.mole-1, the resonance capture process... 

The MCI spectrum of psoralen (Fig. 9.25) indicates that resonance capture is occurring so that most of the ion current is carried hy the molecular ion at m/z 186. The associated GC trace indicates that ionisation is ca 10 times more efficient for the same amount of psoralen in comparison with El. In addition, since most of the ion current is channelled into the molecular ion, an analytical method which selectively monitored the molecular ion of psoralen would be ca 40 times more sensitive if NICI conditions were used rather than El. The technique only works for compounds which are electron capturing. The small ion at m/z 218 indicates that the psoralen has also formed an adduct with traces of oxygen present in the instrument, i.e. true chemical ionisation. 

C2F3 -55.9 Resonance capture processes Lifshitz and Grajower107a... 

The cross sections for (n,y) reactions common in reactor thermal neutron activation generally decrease with increasing neutron energy with the exception of resonance-capture cross section peaks at specific energies. This reaction is, therefore, not important in most 14 MeV activation determinations. However, some thermalization of the 14 MeV flux may always be expected due to the presence of low Z elements in the construction materials of the pneumatic tubes, sample supports, sample vial, or the sample itself (particularly when the sample is present in aqueous solution). The elements Al, Mn, V, Sn, Dy, In, Gd, and Co, in particular, have high thermal neutron capture cross sections and thermal capture products have been observed in the 14 MeV neutron irradiation of these elements in spite of care taken to reduce the amount of low Z moderating materials in the region of the sample irradiation position 25>. 

Self-shadowing and resonance capture effects. The use of small samples and standards so that the neutron flux is not appreciably attenuated between the exterior and interior of the irradiation unit is to be desired. When large samples are used or appreciable high cross section material is present in the matrix, it is important that the standard be prepared with a matrix physically and chemically similar to that of the sample. 

It was obvious that process 3 was the most normal - a resonance capture of slow neutrons to form 239U, a beta emitter which necessarily decays to element 93. If the Berlin team had been able to detect this element 93 and determine its chemical properties, they would have realized that processes 1 and 2 were incorrect. But they did not do it. In Berlin, as Hahn later wrote, they were not very interested because... 

Assumed type of dissociative resonance capture Reference... 

In addition to formation of radical anions via process (2), they can sometimes also be generated by the dissociative resonance capture process... 

The dependence of upon E has been found to be linear, at least at all bar the highest energies, for quite a number of polyatomic molecules [310, 369, 675]. It is clear from the slopes of these plots that eqn. (42) does not, in general, hold for dissociative resonance captures. 

These electrons can be captured by a molecule. The process can be associative or dissociative. The associative resonance capture that leads to the formation of negative molecular... 

The associative resonance capture is favoured for molecules with several electronegative atoms or with possibilities to stabilize ions by resonance. The energy to remove an electron from the molecular anion by autodetachment is generally very low. Consequently, any excess of energy from the negative molecular ion as it is formed must be removed by collision. Thus, in Cl conditions, the reagent gas serves not only for producing thermal electrons but also as a source of molecules for collisions to stabilize the formed ions. 

In this chapter we summarize our present knowledge of chemical compounds that contain positrons or positronium. We do not give an exhaustive review of the literature, but rather we try to give information that is correct up to at least January 1, 2002. Since most of our knowledge of bound positronic systems comes from theoretical work, this chapter is primarily concerned with quantum mechanics. The most conspicuous exception is provided by recent observations of vibrational shifts of compounds that have resonantly captured a positron [l], implying the existence of bound states. 

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