Radiofrequency radiation saturation

In the ENDOR methods, the spin system is irradiated by a microwave field to partially saturate the EPR transition while simultaneously sweeping the sample with radiofrequency radiation through nuclear resonance transitions (Makinen, 1998 Makinen et al. 1998). ENDOR provides a means of precisely measuring the hyperfine interaction (Ahf) between electron and nuclear spins. Within the strong-field approximation, the observed A f is given by Eq. 1.16 ... 

Consider a very simple AX system in which the two spins interact by a magnetic dipole-dipole interaction. We expect two lines in the spectrum, one from A and the other from X. However, when we irradiate the system with radiofrequency radiation at the resonance frequency of X using such a high intensity that we saturate the transition (that is, we equalize the populations of the X levels), we find that the A resonance is modified. It may be enhanced, diminished, or even converted into an emission rather than an absorption. That modification of one resonance by saturation of another is called the nuclear Overhauser effect (NOE). 

The saturation factor gives the degree to which the applied radiofrequency (RF) field saturates the electron transition of all radicals in the sample and can range from 0 to 1. The saturation factor can be written as a function of the applied radiation power P,... 

DNP at very low magnetic fields is attractive for two reasons. First, the electron saturation frequency at low fields is in the radiofrequency range, where it is much easier to obtain high-power radiation sources, amplifiers and transmission equipment. In fact, the first experimental verifications of the Overhauser effect were conducted between 1 and 5 mT,2/85/86 likely due to the ease of constructing a suitable magnet and equipment to perform ESR saturation. The second reason for adding DNP to a low-field system is to help overcome the limited thermal polarization at low... 

The excess nuclei are the ones that allow us to observe resonance. When the 60-MHz radiation is applied, it not only induces transitions upward but also stimulates transitions downward. If the populations of the upper and lower states become exactly equal, we observe no net signal. This situation is called saturation. One must be careful to avoid saturation when performing an NMR experiment. Saturation is achieved quickly if the power of the radiofrequency (RF) signal is too high. Therefore, the veiy small excess of nuclei in the lower spin state is quite important to NMR spectroscopy, and we can see that very sensitive NMR instramentation is required to detect the signal. 

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