Decay after-effects

As already indicated at the beginning of this chapter, it is not intended to treat Co doping of metals in detail, but to discuss typical illustrative examples. The chemical isomer shift of an isolated Fe impurity atom in a metallic lattice will reflect to some degree the electronic structure of the metal. The high mobility of electrons will ensure that any decay after-effects are completely nullified within the time-scale of the Mossbauer event. 

We have already mentioned that the electron-capture decay is prone to decay after-effects. Comparative experiments were first made with in copper, NalOs, Nal, and Ij matrices [49]. The recoil-free fractions of the last two named proved too small for serious use, whereas the copper matrix is the narrow-line source commonly used for this isotope. The NalOs source gave an emission profile which was complex. An analysis was suggested involving at least two distinct charge states of Te formed by the Auger cascade which follows the electron capture. 

Less data are available on decay after-effects from "Te. A matrix of PbTe irradiated with neutrons was found to have a chemical isomer shift of over +0-1 mm s relative to a PbTe absorber [56]. Furthermore this shift difference decreased exponentially with a time constant of about ten days, the implication being that the difference is due to radiation damage causing a defect structure which anneals at room temperature. It was proposed that a radiation-induced distortion of the band structure in the PbTe semiconductor alters the s-electron density at the nucleus. 

The nuclear decay of radioactive atoms embedded in a host is known to lead to various chemical and physical after effects such as redox processes, bond rupture, and the formation of metastable states [46], A very successful way of investigating such after effects in solid material exploits the Mossbauer effect and has been termed Mossbauer Emission Spectroscopy (MES) or Mossbauer source experiments [47, 48]. For instance, the electron capture (EC) decay of Co to Fe, denoted Co(EC) Fe, in cobalt- or iron-containing compormds has been widely explored. In such MES experiments, the compormd tmder study is usually labeled with Co and then used as the Mossbauer source versus a single-line absorber material such as K4[Fe(CN)6]. The recorded spectrum yields information on the chemical state of the nucleogenic Fe at ca. 10 s, which is approximately the lifetime of the 14.4 keV metastable nuclear state of Fe after nuclear decay. 

When M and Q cannot change their positions in space relative to one another during the excited-state lifetime of M (i.e. in viscous media or rigid matrices), Perrin proposed a model in which quenching of a fluorophore is assumed to be complete if a quencher molecule Q is located inside a sphere (called the sphere of effective quenching, active sphere or quenching sphere) of volume Vq surrounding the fluorophore M. If a quencher is outside the active sphere, it has no effect at all on M. Therefore, the fluorescence intensity of the solution is decreased by addition of Q, but the fluorescence decay after pulse excitation is unaffected. 

The NMR and UV spectra of the aforementioned adducts in DMSO solution decay after some time and are eventually replaced by the spectra of the conjugate bases of the corresponding hydroxypyrimidines. The lower the stability of the adduct, the more effectively the demethylation reaction appears to compete with adduct formation. 

The principle of presaturation relies on the phenomenon that nuclei which are unable to relax, because their population in the ground state a and the excited state (3 is the same, do not contribute to the free induction decay after pulse irradiation. Prior to data acquisition, a highly selective low-power pulse irradiates the desired solvent signals for 0.5 to 2 s, thus leading to saturation of the solvent signal frequency. During data acquisition, no irradiation should occur. NOESY-type presaturation is an effective pulse sequence of presaturation. The pulse sequence consits of three 90° pulses (similar to the first increment of a NOESY experiment) ... 

MODIFICATIONS OF THE METHOD Other authors (e.g. Glassman 1971) injected the dose which induced a full antinociceptive effect in mice twice daily for a period of 21 days and evaluated the stepwise decay of effectiveness. After 21 days, the effect of 10 mg/kg morphine or 30 mg/kg meperidine i.p. decreased to approximately 50 % of the value of the first day. 

When radical generation is stopped abruptly, polymerization dies out at a rate proportional to the decreasing concentration of radicals. This is called the post or after effect. In a defined manner it can be brought about only in photochemically initiated polymerizations simply by switching off the light. In the course of polymerization decay, radicals are consumed by mutual termination... 

So-called after effects from the decay of the mother isotope can give rise to difficult and even incorrect interpretation of the data. This is often the case for electron capture but not for isomeric transitions. 

As an alternative to the above method for eliminating the NOE an instrumental technique is available. This depends upon the realization (247) that the time-dependent behaviour of the NOE and of spin decoupling are different. Thus the NOE takes a time comparable for Tj to build up or to decay after application or removal of a rf field, whereas spin decoupling effects appear or disappear almost instantaneously. Consequently if the proton decoupler is gated off immediately prior to... 

In the nonlinear case, the function ij t does not vanish (at times intermediate between t = 0 and t = oo). The itinerant oscillator with an effective potential harder than the linear one is shown to result in ri(t) < 0 in accordance with the results of CFP calculations which show its decay after excitation to be faster than the corresponding equilibrium correlation function (see Fig. 11). 

Figure 6.20 (a) Comparison of the ESR signal decay after 0.3 and 24h from the plasma coating of 1 min TMS followed by 1 min of plasma treatment of O2, H2, Ar, HFE and the plasma coating of CH4 (b) Comparison of the effect of H2 plasma treatment on plasma pol5mier of TMS and of CH4, where the number in parenthesis indicates the treatment time in minute. 

Mdssbauer spectrometry gives information about the chemical environment of the Mdssbauer nuclide in the excited state at the instant of emission of the photon. It does not necessarily reflect the normal chemical state of the daughter nuclide, because of the after-effects that follow the decay of the mother nuclide (recoil and excitation effects, including emission of Auger electrons). At very short lifetimes of the excited state, ionization and excitation effects may not have attained relaxation at the instant of emission of the y-ray photon this results in a time-dependent pattern of the Mdssbauer spectrum. 

The Mossbauer emission spectrum reproduces two effects the situation of the implanted atom in the host and the so-called after-effect. The latter expression means that, for example, in the case of Co having been implanted, we first get the radioactive decay of Co Fe and then the Mossbauer transition to the Fe ground state. The electron capture or jS-decay itself very often causes a perturbation of the environment of the isotope in question Thus some knowledge about the pure after-effect is needed for the interpretation of the spectra of implanted samples. 

Recently a new technique has come into use which avoids radioactivity as well as after-effects conversion electron spectroscopy This method uses the fact that in most Mossbauer transitions not only 7-rays but also conversion electrons are emitted. In the case of Co the electron conversion is the major decay mode. Thus, instead of measuring the 7-rays, absorbed or reflected from an absorber, one measures the emission of electrons from the absorber as a function of the velocity... 

The former spectrum decayed after treatment at -78° C for 30 sec. Effect of oxygen on radical formation with the light of 300 nm was also reported (67). 

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