Spin echo detection

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-)... 

The result is an enhancement of all the advantages of spin-echo detection. The number of usable data points in each acquired data array can in this case exceed in an FID by a factor much larger than two. Since T2 relaxation is going on during the detection, it is again important to keep 8 rigorously constant during the whole multi-block experiment. 

As in the case of the single spin-echo detection, the multiple spin-echoes method attenuates magnetization components with short 72(5 ) values and the magnitude of the attenuation increases with increasing echo... 

Figure 5 X-band (9.101GHz) two-pulse spin-echo detected absorption spectrum of 2.0 mM Cu(dtc)2 in glassy toluene solution at 30 K, obtained with 20 and 40 ns pulses and a fixed time between the pulses...

The local structure of iron sites in Fe-mazzite and Fe-ZSM-5, in which iron was incorporated during zeolite synthesis, was studied by X- and Q-band ESR, electron spin echo detected ESR (ED-ESR), electron spin echo envelope modulation (ESEEM), and diffuse reflectance UV-vis [94G1]. The X-band ESR spectra of Fe-MAZ (100 Fe/(Fe + A1 + Si) = 1.20) render three signals at g = 4.3, g = 2.3, and g = 2.0 - Table 14a. The Q-band spectra testifies only the signal atg = 2.0. The linewidths of the g = 2.0 signals are smaller in the Q-band spectra - Table 14. This narrowing indicates that the linewidth is at least partially due to the second-order broadening of the -l/2> l/2> transition. The X-band spectrum of Fe-MAZ with 100 Fe/(Fe + A1 + Si) = 0.07 exhibits the... 

If more sophisticated experiments are aimed, such as those involving multiple pulse sequences, spin echo detection, rotor synchronization, or 2D methods, additional calibration steps, trial tests, and optimization procedures are required. The reader is referred to the specific literature dealing with each particular method for a proper description of the experimental setup. A general account of these methods can also be found in the monographs by MacKenzie and Smith [4] and Duer [15]. 

ESRI is a relatively new technique with unique capabilities to map the distribution of paramagnetic species in macroscopic systems. An ESR image is a representation of the spatial distribution of the ESR signal intensity in a heterogeneous sample. Various ESRI techniques have been reported such as spin-echo-detected imaging and spatial-spectral ESR imaging. 

Figure 21. Pulsed ELDOR data of E. coli R2 and Rnr2Rnr4 from yeast (A) low-temperature (5 K) spin-echo detected spectrum of Tyr in E. coli R2, (B) time traces normalized with the echo signal intensity at zero time, and (C) frequency spectrum. Reprinted with permission from [334]. Copyright 2003, American Chemical Society.

This limitation is not there for pulsed ENDOR methods, which can be used at all microwave frequencies. In ENDOR, the sample is irradiated with a combination of microwaves and radio waves. Continuous-wave ENDOR was already introduced in 1956 by Feher [1] and for a long time remained an important tool to determine the hyperfine and nuclear quadrupole interactions. However, nowadays this technique is largely replaced by the pulsed counterparts, which are more versatile. Two of the most commonly used ENDOR pulse sequences are Davies ENDOR [18] and Mims ENDOR [ 19]. In these techniques a combination of microwave pulses and a n radio frequency (RF) pulse with variable RF is used. A first set of microwave pulses creates electron polarization. When the RF matches one of the nuclear transitions, the populations of the different energy levels will be affected. This will change the electron polarization that is read out by a last sequence of microwave pulses, usually via electron spin echo detection as a function of the radio frequency. In this way, the nuclear frequencies can be directly detected. 

Carbon. - Pulsed HFEPR and ENDOR measurements at 95 GHz have been reported for a frozen solution of fully C-enriched Cm molecules in their photoexcited triplet state. The triplet state was populated by irradiation at 532 nm using the second harmonic of a Nd YAG laser. The shape of the electron spin echo detected HFEPR spectrum showed that the g factor was anisotropic. There are four types of symmetry-inequivalent C atom in the triplet state and the ENDOR measurements showed that the electron spin density was largely on the equator atoms (3.8%) and on atoms of type 3 (1.1%). The calculated triplet wave function was found to reproduce the large spin density on the equator atoms. 

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.

Electron Spin Resonance Spectroscopy. Several ESR studies have been reported for adsorption systems [85-90]. ESR signals are strong enough to allow the detection of quite small amounts of unpaired electrons, and the shape of the signal can, in the case of adsorbed transition metal ions, give an indication of the geometry of the adsorption site. Ref. 91 provides a contemporary example of the use of ESR and of electron spin echo modulation (ESEM) to locate the environment of Cu(II) relative to in a microporous aluminophosphate molecular sieve. 

Pulsed ENDOR offers several distinct advantages over conventional CW ENDOR spectroscopy. Since there is no MW power during the observation of the ESE, klystron noise is largely eliminated. Furthemiore, there is an additional advantage in that, unlike the case in conventional CW ENDOR spectroscopy, the detection of ENDOR spin echoes does not depend on a critical balance of the RE and MW powers and the various relaxation times. Consequently, the temperature is not such a critical parameter in pulsed ENDOR spectroscopy. Additionally the pulsed teclmique pemiits a study of transient radicals. 

The delay is generally kept at Vi x> The coupling constant Jcc for direcdy attached carbons is usually between 30 and 70 Hz. The first two pulses and delays (90J -t-180 2-t) create a spin echo, which is subjected to a second 90J pulse (i.e., the second pulse in the pulse sequence), which then creates a double-quantum coherence for all directly attached C nuclei. Following this is an incremented evolution period tu during which the double quantum-coherence evolves. The double-quantum coherence is then converted to detectable magnetization by a third pulse 0,, 2, and the resulting FID is collected. The most efficient conversion of double-quantum coherence can... 

Smith, W.E., Reglinski, J., Hoey, S., Brown, D.H. and Sturrock, R.D. (1990) Changes in glutathione in intact erythrocytes during incubation with penicillamine as detected by proton spin-echo NMR spectroscopy. Inorganic Chemistry, 29, 5190-5196. 

Figure 1.5 High-field (aliphatic) regions of 400 MHz H Hahn spin-echo NMR spectra of (a) E-199 culture medium (b) as (a) but following a 2 h incubation with neutrophils at 37°C (c) as (b) but incubated in the presence of 1.00 x 10 mol/dm phorbol 12-myrlstate 13-acetate (PMA). For abbreviations, see Fig. 1.2 with Met, methionine-S-Ctl3 group resonance MetSO, methionine sulphoxide-SO-CHs group resonance. The 2.245 p.p.m. singlet detectable in spectrum (c) arises from the -CH3 groups of acetone, the solvent in which PMA was...

Fig. 2.7.2 Diffusion-relaxation correlation se- The detection (2nd) segment for both is a quences using pulsed field gradients, (a) The CPMG pulse train that is similar to that in first segment is a spin-echo with the echo Figure 2.7.1. The amplitude or the duration of appearing at a time 2tcpi after the first pulse, the gradient pairs in both sequences is (b) The first segment is a stimulated echo incremented to vary the diffusion effects, appearing at a time tcpi after the third pulse.

The spin-echo successfully imaged water held in the cracks of cement paste. This technique can be used to resolve cracks much smaller than the nominal resolution by relying on water saturation of the crack, the connectivity of the crack structure and the fact that it is relatively easy to detect a high intensity structure on a low intensity background. 

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