Continuous-Wave and Pulsed ESR Methods

G. Jeschke and S. Schlick, Continuous-wave and pulsed ESR methods, in S. Schlick, ed.. Advanced ESR Methods In Polymer Research, John Wiley Sons Inc., 2006, pp. 3-24. 

Detroit Mercy, Detroit, Michigan, Continuous-Wave and Pulsed ESR Methods (Chapter 1), ESR Imaging (Chapter 4), ESR Methods for Assessing the Stability of Polymer Membranes Used in Fuel Cells (Chapter 8), Spatially Resolved Degradation in Heterophasic Polymers From ID and 2D Spectral-Spatial ESR Imaging Experiments (Chapter 9). 

Gunnar Jeschke, MPI for Polymer Research, Mainz, Germany, Continuous-Wave and Pulsed ESR Methods (Chapter 1), Double Resonance ESR Methods (Chapter 2), Site-Specific Information on Macromolecular Materials by Combining CW and Pulsed ESR on Spin Probes (Chapter 7). 

Numerous nanocomposite characterization methods are available thermogravimetric analysis (TGA), differential scanning calorimetry [DSC], transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), nuclear magnetic resonance (NMR), IR spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, dielectric relaxation spectroscopy, atomic force microscopy [AFM], electron spin resonance, continuous-wave and pulsed ESR spectroscopy and others [59]. Among all of the methods, TGA, DSC, wide-angle scattering diffraction [WAXS], and TEM are the most commonty used and will be discussed in detail. 

Analysis of a CIDEP pattern with time-resolved ESR (TR ESR) spectra provides a solid conclusion to be made on the spin multiplicity of molecular precursors of polarized free radicals (a singlet or a triplet excited molecule) and the tracking of fast reactions of polarized radicals leading to secondary radicals. Thus, TR ESR is a convenient method in mechanistic photochemistry and free radical chemistry. Continuous wave TR ESR (CW TR ESR) devices are widely used for detection of photogenerated radicals. They usually consist of a pulsed ns laser with detection of transients by their ESR spectra with a X band ESR spectrometer in the direct detection mode (no hied modulation).Time-resolved Eourier transform ESR (FT ESR) has some advantages and drawbacks with respect to CW TR ESR. Rather sophisticated FT ESR devices have become available, and FT ESR studies become more common. 

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. 

Most of the specialized techniques involve the use of fairly sophisticated instrumentation these can be divided roughly into two categories continuous wave (CW) and pulse methods. Electron-nuclear double resonance (ENDOR) and ESR imaging will be discussed in the CW category, while a variety of... 

In NMR experiments, T and T2 are determined using pulse methods. Here, a variety of specific pulse sequences has been developed. ESR experiments, especially in metals, are performed by steady-state (continuous wave, cw) methods only. In these experiments the linewidth A77 of the resonance absorption is determined by the characteristic relaxation time. In metals we ean assume o>o tc-Cl and hence Ti = T2- Here o o is the resonance frequency and Tc is the correlation time of the conduction electrons. An extensive diseussion on the comparison of NMR, ESR and MSssbauer techniques has been given by R.G. Barnes (1979). 

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