Magnetic fields resonance conditions

With help of the four-level diagram of the =I= system (see figure BL15.8 two conniion ways for recording ELDOR spectra will be illnstrated. In freqnency-swept ELDOR the magnetic field is set at a value that satisfies the resonance condition for one of the two EPR transitions, e.g. 4<- 2, at the fixed observe klystron frequency, The pump klystron is then turned on and its frequency, is swept. When the pump... 

Depending on how the secondary magnetic field is applied, there are two fundamentally different types of spectrometers, namely, continuous wave (CW) and pulse Fourier transform (PFT) spectrometers. The older continuous wave NMR spectrometers (the equivalent of dispersive spectrometry) were operated in one of two modes (i) fixed magnetic field strength and frequency (vi) sweeping of Bi irradiation or (ii) fixed irradiation frequency and variable field strength. In this way, when the resonance condition is reached for a particular type of nuclei (vi = vo), the energy is absorbed and... 

ESR spectroscopy can be transformed into an imaging method, ESRI, for samples containing unpaired electron spins, if the spectra are measured in the presence of magnetic field gradients. In an ESRI experiment the microwave power is absorbed by the unpaired electrons located at point x when the resonance condition, Equation (10), is fulfilled. 

The y-facior. The 0-factor takes into account the fact that the local magnetic field experienced by a particular atom in a molecule may not be the same as the applied field owing to the existence of local field effects. In the absence of such effects, g for any particular radical would simply have the same value as that of the free electron, 2.0023, and all radicals would come into resonance at the same applied field for a given microwave frequency. We can thus express the resonance condition (equation 2.173) as ... 

Experimentally there are two alternatives. One is to fix the magnetic field that determines the Larmor frequency and to sweep the radiation frequency through this value. Alternatively radiation at a fixed frequency is used while the field sweeps through the resonance value. When the resonance condition is reached absorption of radiation occurs. 

If one then defines the resonance condition, Br (alternately written as Bo or as Br when referring to the laboratory field associated with a particular EPR instrument system), as the magnetic field at which the energy of the transition comes into resonance with the field, one finds equation 3.16 or, more usefully, equation 3.17 ... 

Here 2tt Aft = trm is imposed to eliminate the effect by the zero-order interaction of the RF field, otherwise a term containing Ix will remain and it will distort the longitudinal magnetization, transverse magnetization, as well as the BSPS of the 13C . For the on-resonance condition, 5 = 0, all the higher-order average Hamiltonians vanish since [7f(/ ), = 0 for arbitrary l and... 

In paramagnetic resonance experiments, the sample is usually placed in a large constant magnetic field H0, whose direction is taken as that of the z axis. This field determines the quantum levels of the individual spins and polarizes them according to Curie s law. In a typical nuclear resonance experiment, a radiofrequency field H1( perpendicular to H0 and rotating in the x,y plane is applied to the sample. The response of the system is, under stationary conditions, described by the radiofrequency susceptibility %(co). The rotating field is given by... 

Figure 2. Electron spin levels in a magnetic field, showing the resonance conditions at H = Hq.

---