Magnetic field spinning electrons

In electron-spin-echo-detected EPR spectroscopy, spectral infomiation may, in principle, be obtained from a Fourier transfomiation of the second half of the echo shape, since it represents the FID of the refocused magnetizations, however, now recorded with much reduced deadtime problems. For the inhomogeneously broadened EPR lines considered here, however, the FID and therefore also the spin echo, show little structure. For this reason, the amplitude of tire echo is used as the main source of infomiation in ESE experiments. Recording the intensity of the two-pulse or tliree-pulse echo amplitude as a function of the external magnetic field defines electron-spm-echo- (ESE-)... 

Another surprise was that nuclear spins also become polarized by the magnetic layer after several minutes. This polarization acts like a magnetic field causing electron spin to precess at a frequency and in a direction controlled by the magnetic layer. 

When one electron is assigned to an orbital in an atom, the electron can spin in either direction. In a magnetic field, this electron acts as a micromagnet and aligns with the magnetic field. An element with an unpaired electron is usually magnetic. 

The theoretical framework for a discussion of the hyperfine interactions in radicals is given by the so called spin Hamiltonian, which describes the interaction between the unpaired electrons and the magnetic nuclei (I>l/2) in the sample. When the radical is placed in a static magnetic field, the electrons and the magnetic nuclei will interact with the field. These interactions give rise to the electronic and nuclear Zeeman terms,... 

If the collision time RJv is short compared to the period of the Larmor precession of the electronic spin in the intra-atomic magnetic field, spin orientation can be considered to be fixed during the collision. Then the scattering matrix S(jm,j m )connecting 2Pjm and 2P,connecting components of a spinless P state [71, 72]. This interrelation for depolarization probabilities is given in Table 5.4 (last column). 

Afree radical (with just one unpaired electron) is described as an electronic doublet because, in an external magnetic field, the electron can only exist in one of two possible spin states ( up or down ). By contrast, a pair or radicals, or a biradical (a species with two unpaired electrons in the same molecule) can exist in either of two electronic states singlet or triplet. In the singlet state the electrons are paired (opposite spin) a singlet radical pair (or biradical) is thus diamagnetic (W5 = I -1 = 0) and not observable by EPR. The radical pair above is shown in the singlet state. 

An unpaired electron, like a proton, can adopt either of two spin orientations when immersed in a magnetic field. An electron in either orientation will precess at a frequency given by Eq. (11.1). The g factor is similar in some respects to the magnetogyric ratio (y) used in NMR spectroscopy. The value of g, used as the position parameter in EPR spectroscopy, depends on the exact structure of the free radical possessing the unpaired electron. 

In EPR spectroscopy, samples are brought into a homogeneous external magnetic field. The electron spin vector of the paramagnetic species in the sample can only take on distinct orientations with respect to the direction of this external field. 

In an atom subjected to an external magnetic field, the electron spin and nuclear spin will interact. All combinations of electron and nuclear spin are allowed, thus for [Pg.145]

In a strong magnetic field, the electron and nuclear spin vectors, 1 and S are fully decoupled and each have their axis parallel to the applied field. If 6 is the angle between the axis of the dipoles (Fig. 5) and the line joining them, and r is their separation, then the Hamiltonian representing the energy of dipolar interaction is... 

The 5 ground state of the Eu ion has a low spin-lattice relaxation rate, and the spin-spin relaxation frequency becomes of the order of the Mossbauer state lifetime at concentrations of the order of 5% Eu " " in a diamagnetic lattice. Consequently paramagnetic relaxation phenomena have been found for Eu ions in both CaPj and CaS [49]. In zero applied field at 4-2 K the lines are partially narrowed by isotropic spiir-spin relaxation among electronic or nuclear levels, but an external magnetic field causes electronic polarisation and an increased resolution of hyperfine components. 

When the energy of the system depends upon the magnetic field B, electron spin S and nuclear spins 7y, the set ofmagneticparameters is covered by the second-rank tensors ic, D, Ag, oN, TN and KNM. These are the reduced magnetic parameters, interrelated with the observed (measured) quantities according to Table 5.1. 

The two possible orientations of an electron s spin and its associated spin magnetic moment with respect to an axis produce two energy levels in an externally applied magnetic field. In electron-spin-resonance (ESR) spectroscopy, one observes transitions between these two levels. ESR spectroscopy is applicable to species such as free radicals and transition-metal ions that have one or more unpaired electron spins and hence have a nonzero total electron spin and spin magnetic moment. 

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