Larmor precession time

This equation corresponds to a static spectrum consisting of two Lorentzian lines centred around (co-I-Act)) and (co—Aco), which is shown schematically in Figure 5.2. In the context of magnetic splitting, the separation of the spectral lines, quantified by Aco, is related to the Larmor precession frequency and Tl the corresponding Larmor precession time, which can be thought of as the measurement time appropriate to the observation of a hyperfine interaction in a Mossbauer spectrum. It is clear that in order to detect the particular interaction it is necessary that Aco T-r, i.e. Tlbasic condition to be fulfilled for observing hyperfine structure in Mossbauer spectroscopy. In the case of Fe this implies that 10 s. 

The nucleus in a Mossbauer experiment is part of a many-body system consisting of the surrounding electrons and the quasiparticles corresponding to the various other degrees of freedom of the solid. Relaxation effects result from the various time-dependent processes in the vicinity of the nucleus. The nucleus thus acts as a local microscopic probe, which does not participate directly in the relaxation processes in its environment, but which senses these processes via the hyperfine interactions. Now, in interpreting the relaxation behaviour it is necessary to consider the nature and interrelationship of the important timescales of the problem. Some of these timescales are determined by the nature of the Mossbauer isotope and the interaction being studied, i.e., the mean lifetime of the Mossbauer excited state and the Larmor precession time t,. The other timescales relate to, and are characterised by, the nature of the fluctuations in the nuclear environment. These latter timescales are the inverse of the various relaxation rates and, as mentioned earlier, these can be controlled in the laboratory in various ways. The character of the relaxation spectra obtained obviously depends crucially on the interplay of the various timescales as discussed below. 

Experimental data at 4.2 K in zero external magnetic field are shown in Figure 5.8. Analysis of the data shows that relaxation phenomena must be considered. However, the presence of a magnetically split pattern implies that the relaxation time must be long in comparison with the Larmor precession time Tl (which corresponds to in the present case). Here is therefore an example in which a resolved magnetic splitting is seen, even... 

It is important to note that in the context of relaxation studies a combination of Mossbauer spectroscopy and the PAC and pSR methods allows a given problem to be approached over a range of experimental timescales. For example, it may be recalled that the experimental timescales in Mossbauer spectroscopy are the lifetime of the excited state of the nucleus and the Larmor precession time t, while in the case of PAC the... 

The range of timescales over which diffusion can be observed in Mossbauer spectroscopy is determined by the lifetime of the excited state of the Mossbauer nucleus and the Larmor precession time of the hyperfine interactions. For Fe the times involved are in the range between 5 x 10 and 5 X 10 s and the diffusional line broadening is equal to the natural linewidth when the diffusion constant D is of the order of 10 m s in a liquid. 

If the direction of the main component V z of the electric field gradient (EFG) fluctuates rapidly as a result of diffusion, the Mbssbauer spectrum may be changed in shape. Two atomic sites have very different isomer shift values and the jump between these two states also may show the time-dependent Mbssbauer spectra as a function of the jump firequency. The time scales over which the Mbssbauer effect can be used to observe the dynamical effect is determined by the characteristic times associated with the resonance the natural lifetime and the Larmor precession times of the hyperfine interactions. 

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