Multi-resonance hyperfine

Abstract Multi-resonance involves ENDOR, TRIPLE and ELDOR in continuous-wave (CW) and pulsed modes. ENDOR is mainly used to increase the spectral resolution of weak hyperfine couplings (hfc). TRIPLE provides a method to determine the signs of the hfc. The ELDOR method uses two microwave (MW) frequencies to obtain distances between specific spin-labeled sites in pulsed experiments, PELDOR or DEER. The electron-spin-echo (ESE) technique involves radiation with two or more MW pulses. The electron-spin-echo-envelope-modulation (ESEEM) method is particularly used to resolve weak anisotropic hfc in disordered solids. HYSCORE (Hyperfine Sublevel Correlation Spectroscopy) is the most common two-dimensional ESEEM method to measure weak hfc after Fourier transformation of the echo decay signal. The ESEEM and HYSCORE methods are not applicable to liquid samples, in which case the FID (free induction decay) method finds some use. Pulsed ESR is also used to measure magnetic relaxation in a more direct way than with CW ESR. 

Multi-resonance and pulsed ESR techniques can provide better spectral resolution than conventional ESR. Multi-resonance involves ENDOR, TRIPLE and ELDOR. In an ENDOR experiment a radiofrequency (RF) field is applied in addition to the microwave (MW) employed in standard continuous wave (CW) ESR. ENDOR is mainly used to increase the spectral resolution, so that overlapping or unresolved hyperfine structure in the ESR spectra can be detected. In the classical work by Feher [1] the radiofrequency was continuously swept. CW X-band spectrometers with an ENDOR attachment have been commercially available for a long time. Accessories for other frequency bands and for pulsed ESR have been developed more recently. In a TRIPLE experiment two RE fields are applied [2, 3]. A theoretical application has been to determine the relative signs of two hyperfine couplings. In an ELDOR experiment two MW frequencies are applied. Early applications using... 

The use of multi resonance techniques such as ENDOR together with general / special TRIPLE resonance techniques has now become much more common as the techniques move more into more general use, certainly towards the end of the review period. The use of such techniques has enabled very small hyperfine coupling constants (smaller than the linewidth) to be measirred which would normally be inaceessible to... 

Lunsford [3b] and Hoffman and Nelson [23] first reported the ESR spectra for adsorbed NO molecules. Then, Kasai [4b] revealed that ESR spectra of NO probe molecules are very sensitive to the interaction with metal ions and Lewis acid sites in zeolites. The earlier ESR studies of the NO/zeolite system have been summarized in several review papers [3a, 4a, 8]. A number of ESR studies have been also carried out for NO adsorbed on metal oxides such as MgO and ZnO as reviewed by Che and Giamello [5]. Modern ESR techniques such as pulsed ESR [25-27], ENDOR (Electron Nuclear Double Resonance) [26], and multi-frequency (X-, Q-, and W-band) ESR [28] are especially useful for an unambiguous identification of the ESR magnetic parameters (g, hyperfine A, and quadrupole tensors, etc.) and, consequently, for a detailed characterization of structural changes and motional dynamics involved. Some recent advancements in ESR studies on NO adsorbed on zeolites are presented in this section. 

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