Unstable ions

AES Auger electron spectroscopy After the ejection of an electron by absorption of a photon, an atom stays behind as an unstable Ion, which relaxes by filling the hole with an electron from a higher shell. The energy released by this transition Is taken up by another electron, the Auger electron, which leaves the sample with an element-specific kinetic energy. Surface composition, depth profiles... 

Unstable ion. An ion that is sufficiently excited to dissociate within the ion source, under stated experimental conditions. 

Figure 4.6. Photoemission and the Auger process. Left An incident X-ray photon is absorbed and a photoelectron emitted. Measurement of its kinetic energy allows one to calculate the binding energy of the photoelectron. The atom becomes an unstable ion with a hole in one of the core levels. Right The excited ion relaxes by filling the core hole...

Figure 4.5. Longitudinal section through a quadrupole mass filter (showing two of the four rods), illustrating stable and unstable ion trajectories.

Fig. 12.1 Oxidation states of the actinide elements most stable ions in aqueous solutions ++ oxidation states observed in aqueous solutions +, unstable ions observed only as transient species. In solids precipitated from alkaline solutions.

No stable compound of helium has ever been found. However, it is possible for an atom of hydrogen to combine with helium (and other light noble elements) under special conditions to form HeH, an unstable ion. 

Reduction by pulse radiolysis of Mn04 under acidic conditions has allowed a study of the UV-visible spectra of the unstable ion 03Mn (0H) and the determination of the pAa of this ion, 7.4 0.1." Abrasive stripping voltammetry has been used to characterize solid barium and... 

The mathematical treatment of these equations of motion uses Mathieu s differential equations. It is demonstrated that there are stable and unstable ion paths. With the stable paths, the distance of the ions from the separation system center line always remains less than r (passage condition). With unstable paths, the distance from the axis will grow until the ion ultimately collides with a rod surface. The ion will be discharged (neutralized), thus becoming unavailable to the detector (blocking condition). 

Certain limitations were noted however. First, the plot did not predict the zero value of selectivity for highly unstable ions, expected from the reactivity- selectivity principle. Secondly, as discussed on p. 77, solvolytic rate constants are only a very approximate measure of substrate reactivity. Furthermore, the possibility exists of solvent sorting, in which the concentration of potential nucleophiles around the reaction intermediate differs from that in the bulk solution. Such a phenomenon would, of course, seriously diminish the significance of selectivity as a measure of reactivity, and, while such a possibility... 

The success of a carbocation preparation in superacid is frequently very technique dependent. Despite this fact, rather few detailed accounts have been published of the special techniques that have been developed. To be able directly to observe reactive (unstable) ions by nmr, for example, the ions not only have to be studied at low temperatures (where non-degenerate rearrangements are slow) they also have to be prepared at low temperatures for the very same reason. Furthermore, side reactions have to be suppressed. Common reactions of this type are dimerizations and polymerizations water and oxygen also have to be excluded. 

Reactions of this type involve the oxidation/reduction of a reactive intermediate for which the one-electron redox potential may not be known (C02 in Eq. 5). Determination of the free energy for formation of the emissive state requires knowledge of the potentials of both ion radicals and the energy of the emitting excited state (Eq. 2). It is, in principle, possible to find approximate values for one-electron potentials of very unstable ion radicals (e.g., the C02 / potential of Eq. 5) by use of a series of chromophores with varying potentials for forming the excited state from the radical ion (varying RUB+/ potentials). 

The e.p.r. spectra of unstable ion centres produced in polycrystalline MgO and CaO have been studied. A theoretical model has been described which provides a satisfactory basis for the interpretation of the electronic spectrum of the [V(H20) ] ion. ... 

Figure 16.12 Modelling of the ion pathway in a trap, (a) Trajectories of a stabilized ion (b) Pathway of an unstable ion terminating in contact with one of the electrodes (the layout of the trap is the same as in Figure 16.11).

If we consider the internal energy distribution of the ions, as shown by Fig. 3.2, for E < E, we will have stable ions, which will reach the detector undecomposed. For E > E2, we will have unstable ions, which will decompose inside the ion source and whose fragments will be... 

Aminium radicals were not considered until now as intermediates in photosta-bilizing mechanisms of HAS. These unstable ion-radicals were reported to be formed transiently by photolysis of HAS [172],... 

The second stage of the Bloch-Bradbury mechanism includes collision with the third-body particle M (density no), leading to relaxation and stabilization of O2 or collisional decay of the unstable ion ... 

By the ab initio method Hehre and Hiberty have calculated different conformations of the homoallylic ions C H. and the isomeric bisected cyclopropylcarbinyl cation to show all the homoallylic ions I-IV being energetically less stable than ion V by 20-30 kcal/mole. The solvolysis products whose composition points to the intermediate formation of homoallylic rations can 1% attribute either to the fast trapping of unstable ions by counterions or to the change of the stability ratio of the isomeric ions 4 by solvation effects. 

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