Field-swept electron spin echo

The field-swept electron spin echo (FS-ESE) spectrum (which yields a spectrum similar to that acquired by a CW experiment, but is acquired using a simple two-pulse sequence) of VO(ema)2-treated rat bone yielded spin Hamiltonian parameters quite different from those obtained with the other two vanadyl compounds (Table 4). As with VO(pic)2, it was clear that the eoordination state of the orally administered VO(ema)2 had changed [95]. Despite the difference in spin Hamiltonian parameters determined from CW-EPR (VOSO4, VO(pic)2) and FS-ESE (VO(ema)2) spectra, it appears that all three compounds share the same metabolic fate after uptake into bone mineral. 

Electron spin resonance (ESR) spectroscopy is a very powerful and sensitive method for the characterization of the electronic structures of materials with unpaired electrons. There is a variety of ESR techniques, each with its own advantages. In continuous wave ESR (CW-ESR), the sample is subjected to a continuous beam of microwave irradiation of fixed frequency and the magnetic field is swept. Different microwave frequencies may be used and they are denoted as S-band (3.5 GHz),X-band (9.25 GHz), K-band (20 GHz), Q-band (35 GHz) and W-band (95 GHz). Other techniques, such as electron nuclear double resonance (ENDOR) and electron spin echo envelope modulation (ESEEM) spectroscopies, record in essence the NMR spectra of paramagnetic species. 

Figure 17 Physical crosslinking of PDADMAC by TAM. (a) Electron-spin echo detected field-swept ESR spectra at W-band frequencies of TAM in glassy frozen water/glycerol (temperature 80 K) without polyelectrolyte (black), with a big excess of PDADMAC (/ = 0.0028, blue) and with a moderate excess of PDADMAC (/ = 0.1, red), (b) Proposed model of zipper-like crosslinking. Favorable attachment sites for TAM are those where two chain sections are sufficiently close for crosslinking. Further TAM ions tend to attach near existing TAM ions as they also seek for proximal chain sections. This leads to clustering of TAM as well as to growth of parallel chain sections.

If the formally forbidden electron-nuclear transitions are weakly allowed, the two-pulse echo decay is modulated by the corresponding nuclear frequencies. For a spin system of two weakly coupled electron spins, it is modulated with the coupling between the two spins. Measurement of the echo amplitude as a function of the external magnetic field Bq yields the absorption ESR line shape. This field-swept echo-detected ESR experiment is a useful alternative to CW ESR for systems with strong anisotropic line broadening. For example, in the situation in Fig. 4b the g feature can be easily missed, in particular if it is broadened by g strain. The strong anisotropy is then revealed more clearly in the absorption line. 

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