ESR, continuous wave

TR ESR, (CW) TR ESR-tune resolved continuous wave ESR FT ESR-time resolved Fourier transform ESR SS-steady state... 

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. 

H. Kurreck, B. Kirste, W. Lubitz, ENDOR Spectroscopy of Radicals in Solution, VCH, 1988 E. R. Davies, Phys. Lett. A 1974, 47, 1-2 Arthur Grupp and Michael Mehring, in Modem Pulsed and Continuous Wave ESR , 1979, 195-229. 

Figure 4 Block diagram of a simple continuous-wave ESR spectrometer.

This contribution reviews the basic tools which are currently employed for interpreting ESR and NMR observables in condensed phases, with an emphasis on stochastic modeling as key for the prediction of continuous-wave ESR (cw-ESR) lineshapes and NMR relaxation times of proteins. Section 12.2 is therefore devoted to the definition of reduced (effective) magnetic Hamiltonians and the stochastic (Liouville) approach to spin/molecular dynamics in order to clarify the basic stochastic approach to cw-ESR observables. Section 12.3 provides a short overview of rotational stochastic models for the evaluation of relaxation NMR data in biomolecules. Conclusions are briefly summarized in Section 12.4. 

Kevan, L. and Bowman, M.K. (eds) (1990) Modern Pulsed and Continuous-wave ESR, Wiley, New York. 

An example of typical ESR spectra, measured in the first derivative mode, is shown in Fig. 12. Just like NMR, ESR can be used to detect phase transitions and to study the orientation and dynamics of liquid crystals. The spectra shown in Fig. 12, for example, are from a study comparing the dynamics of the spin label at the end of the polymer chain and the freely dissolved spin probe in a liquid-crystalline polyether by continuous wave ESR (Fig. 12) and 2D Fourier transform ESR experiments [137]. The end label showed smaller ordering and larger reorientational rates than the dissolved spin probe. Furthermore, it was demonstrated that the advanced 2D FT ESR experiments (see below) on the end-labeled polymer chain could not be explained by the conventional Brownian model of reorientation, although this model could explain the ID spectra. This led to the development of a new motional model of a slowly relaxing local structure, which enabled differentiation between the local internal modes experienced by the end label and the collective reorganization of the polymer molecules around the label. The latter was shown to be slower by two orders of magnitude. 

A magnetic resonance spectroscopic technique used for detect hyperfine interactions between electrons and nuclear spins. In its continuous-wave mode, the ESR signal intensity is measured as radio frequency is applied. In pulsed mode, pulses of radio frequency energy are applied, and the ESR signal is detected as a spin-echo. In either case, enhanced EPR signal strength occurs when the radio frequency is in resonance with the coupled nuclei. 

Electron paramagnetic resonance spectroscopy, also known as electron spin resonance (ESR) spectroscopy, detects the excitation of electron spins in an applied external magnetic field.13 Conventional continuous-wave (CW) EPR is based on resonance of a fixed-frequency standing microwave to excite some of the electrons in Zeeman-split spin multiplets to undergo a transition from a lower Ms level to a higher... 

Levanon, H. and Stehlik, D. (1992) Time-resolved continuous wave-vs. Fourier transform-ESR, Israel J. 

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. 

For the examples discussed so far, continuous wave (cw) ESR was utilized, where the magnetic field is swept during the measurement. In the pulsed free induction decay attenuation method [47, 48], the sample in the applied steady field is subjected to a powerful microwave pulse which causes the tipping of the magnetization vector. This method is analogous to pulsed NMR measurements. After the pulse, the... 

The magnitude of the interactions in ESR, exceeding that in NMR by several decades, makes pulsed excitations rather difficult. Consequently, most dynamic ESR studies are performed as continuous wave (CW) experiments, employing a sequential excitation of the spin system. It can be shown, however, that the frequency spectra from the CW technique are identical with those obtained by FT of the FID, following a single pulse [45, 65, 7. ... 

In pulsed ESR samples are exposed to a series of short intense microwave pulses, e.g. the 2-pulse sequence shown in Fig. 1.14 in place of the continuous radiation with microwaves at low power in the traditional continuous wave spectrometer. 

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... 

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