Isomeric states, metastable

Contrary to the case of using the decay of the isomeric state of 2 Te is preferable to using the decay of the ground state for Mossbauer studies of This preference results from the more favorable y-ray branching to the 57.6-k.e.v. state from the metastable state. 

In most cases the emission of nucleons, electrons or positrons leads to an excited state of the new nucleus, which gives off its excitation energy in the form of one or several photons (y rays). This de-excitation occurs most frequently within about 10 s after the preceding or P decay, but in some cases the transition to the ground state is forbidden resulting in a metastable isomeric state that decays independently of the way it was formed. 

Furthermore, long-lived meta stable energy minima might exist in relation to the isomeric state of a critical peptide bond [184]. In the metastable state in which the folded forms of a polypeptide chain have similar structural characteristics but differ in their free energy level, the kinetically trapped species could demonstrate properties of a high energy peptide bond isomer [185]. 

The concept of an energy sink is further reinforced by the additional isomer (PB) which we have detected. It means, in fact, that an aromatic compound, following excitation, can not only dissipate its acquired energy into the nonlocalized ir system but in addition can decay into discrete, metastable isomeric states which are also interconvertible. These in turn can further react to form polymeric product or return to the ground state. 

Strutinsky developed an extension of the liquid drop model which satisfactorily explains the fission isomers and asymmetric fission. For such short half-lives the barrier must be only 2-3 MeV. Noting the manner in which the shell model levels vary with deformation ( 11.5, the "Nilsson levels"), Strutinsky added shell corrections to the basic liquid-drop model and obtained the "double-well" potential energy curve in Figure 14.14b. In the first well the nucleus is a spheroid with the major axis about 25 % larger than the minor. In the second well, the deformation is much larger, the axis ratio being about 1.8. A nucleus in the second well is metastable (i.e. in isomeric state) as it is unstable to y-decay to the first well or to fission. Fission from the second well is hindered by a 2 - 3 MeV barrier, while from the first well the barrier is 5 - 6 MeV, accounting for the difference in half-lives. 

Some half-lives of isomeric states can be very long, for example, lOmgj decays by alpha emission with a half-life of 3.0 X 10 year. Alpha decay is, however, a rare mode of decay from a metastable state gamma-ray emission is much more likely. A gamma transition from an isomeric state is called an isomeric transition (IT). On the Karlsruhe Nuclide Chart, these are shown as white sections within a square that is coloured (if the ground state is radioactive) or black (if the ground state is stable). 

ISOMERIC STATE Energy levels of a nucleus having different energies and half-lives. Often used specifically to refer to energy levels within an atom that have uncharacteristically long half-lives. (See Metastable State)... 

Gamma (y) rays are photons deriving from isomeric transitions. Isomeric transitions occur when a nucleus remains in an excited state after a particle emission or a decay by electron capture. These intermediate levels are referred to as isomeric states (or metastable states), and each decays to a lower state (either the ground state or another intermediate state) with lifetimes from picoseconds to years. Gamma ray emissions are characteristic of the radionuclide, and the energies of the emitted photons depend on the energy differences between the initial excited state and the next one. 

Azobenzenes have been integrated into PEMs to introduce Hght-responsive capabilities using polyelectrolyte grafted azobenzenes. Azobenzenes have two isomeric states a thermally stable trans configuration, and a metastable cis form. 

In the following sections, studies of isomeric ions are reported in which the ions are reactively probed. Where calculations are available, information on potential energy surfaces is given. This is usually the structure of the stable isomeric forms and transition states and their relative energies thus only points on the potential surface are known. The detailed form of the potential surface is almost never available nor is the connectivity between the various states usually established theoretically (chemical intuition is often used to connect the states). Pertinent experimental data on CID and metastable ions, isomers produced in binary reactions, and potential surfaces probed by binary reactions (with the excited isomeric ion as the reaction intermediate) are also given. 

W. H. Miller For a reaction such as H + O2 — OH + O, or ketene isomerization CC O — OCC H2, which involve a metastable intermediate, one can certainly do better than simple TST by using a model with two transition states and the unified statistical model to approximate the net reactive flux. Such an approach, though often useful, is nevertheless an approximate model that can never be made into a rigorous description. Such a model, for example, cannot describe the resonance tunneling structure in the ketene isomerization that I described. 

The chemical deactivation of photoexcited anthracenes by dimerization usually proceeds by 4re + 4re cycloaddition [8]. However, exceptions to this rule have become known in recent years [8], and a multitude of steps, including the formation of metastable intermediates such as excimers, may actually be involved in a seemingly simple photochemical reaction such as the dimerization of 9-methylanthracene [9, 10]. Moreover, substitution of the anthracene chromophore may affect and alter its excited state properties in a profound manner for a variety of reasons. For example, in 9-tert-butylanthracene the aromatic ring system is geometrically distorted [11,12] and, consequently, photoexcitation results in the formation of the terf-butyl-substituted Dewar anthracene [13-15], The analogous photochemical isomerization of decamethylanthracene [16] probably is attributable to similar deviations from molecular planarity. 

BitterwolfTE. Photochemical nitrosyl linkage isomerism/metastable states. Coord Chem Rev 2006 250 1196-207. 

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