Isomeric nuclear states

Fig. 9.12 Photoionization yield as a function of the frequency of the first resonant transition of the atom in (a) a mixture of atoms in the ground and isomeric nuclear states...

A nuclide is an atomic species as determined by its atomic number (proton number) Z and mass number (nucleon number) A = Z+N, where N is the number of neutrons in its nucleus. Atomic species with the same nuclear composition but different nuclear energy states with measurable lifetime are considered independent nuclides in their own right. Nuclides can be classified in different ways. Nuclides having the same atomic number Z (but different mass number A) represent the same chemical element and are called the isotopes of that element. Nuclides with the same mass number A (but different atomic number Z) are called isobars. Nuclides of the same number of neutrons N (but different atomic number Z) are called isotones. Nuclides of the same nuclear composition but different nuclear states are referred to as (nuclear) isomers. The terms isotope, isotopic, isobar, isobaric, isotone, isotonic, isomer, and isomeric can also be applied to nuclei, but the terms nuclide and nuclidic can only be applied to atoms. 

The name isomer shift or isomeric shift is assigned due to the fact that this effect depends on the difference in the nuclear radii of the groimd and isomeric excited states. 

The half-life of excited nuclear states is typically of the order of picoseconds. However, sometimes one can observe states with significantly longer half-lives, nuclear isomers. In extreme cases the half-Ufe of the isomeric state can exceed the half-life of the ground state. Take as an example the case of Ta. This is one of the rarest isotopes that can be found in an isomeric state naturally on earth, where it is to all intents and purposes stable with a half-life Ti/2 > 1.2 x 10 y. However, its ground state decays rapidly with a half-Ufe of only Ti/2 = 8.125 h. 

Both unimolecular and bimolecular reactions are common throughout chemistry and biochemistry. Binding of a hormone to a reactor is a bimolecular process as is a substrate binding to an enzyme. Radioactive decay is often used as an example of a unimolecular reaction. However, this is a nuclear reaction rather than a chemical reaction. Examples of chemical unimolecular reactions would include isomerizations, decompositions, and dis-associations. See also Chemical Kinetics Elementary Reaction Unimolecular Bimolecular Transition-State Theory Elementary Reaction... 

Nuclear magnetic resonance (NMR) spectroscopy is a most effective and significant method for observing the structure and dynamics of polymer chains both in solution and in the solid state [1]. Undoubtedly the widest application of NMR spectroscopy is in the field of structure determination. The identification of certain atoms or groups in a molecule as well as their position relative to each other can be obtained by one-, two-, and three-dimensional NMR. Of importance to polymerization of vinyl monomers is the orientation of each vinyl monomer unit to the growing chain tacticity. The time scale involved in NMR measurements makes it possible to study certain rate processes, including chemical reaction rates. Other applications are isomerism, internal relaxation, conformational analysis, and tautomerism. 

Figure 6.2 Steering of photochemical reactions by coherent control of ultrafast electron dynamics in molecules by shaped femtosecond laser pulses. Ultrafast excitation of electronic target states in molecules launches distinct nuclear dynamics, which eventually lead to specific outcomes of the photochemical reaction. The ability to switch efficiently between different electronic target channels, optimally achieved by turning only a single control knob on the control field, provides an enhanced flexibility in the triggering of photochemical events, such as fragmentation, excited state vibration, and isomerization.

Nuclear decay processes that are often used to populate Mossbauer isotope excited states are (30) electron capture (electron + proton neutron), / decay (neutron - proton + electron), and isomeric transition (a long half-life nuclear excited state decays to the Mossbauer excited state). In addition, several of the parent nuclides of the heavy isotopes can be populated by a-particle emission. 

The quadrUpole interaction of the quadrupole moment of the I — 3/2 excited isomeric state of 57Fe with an electric field gradient at the nuclear position produces a characteristic line pair in Mossbauer spectra as shown in Fig. 2. The energy difference is called the quadrupole splitting... 

Nuclear spins and moments of ground and isomeric states are basic properties probing the structure and shape of atomic nuclei. The systematic experimental study of these quantities along isotopic and isotonic chains thus allows a mapping of the nuclear behaviour, to be compared with the predictions of different nuclear models. 

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