Echo Detectability

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-)... 

For example, an echo detected 13 ]ls after the ultrasound pulse represents a boundary 10 cm away from the source. 

Figure 7.9 (a) Echo-detected X-band EPR spectrum of a powder sample of Gd001 Y099W30 at T = 6 K. (b) Decoherence times 7, (solid symbols) and T2 (open symbols) measured at fi0H = 0.347 T, as a function of the atomic concentration of Gd ions. 

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... 

When the primary electron donation pathway in photosystem II is inhibited, chlorophyll and p-carotene are alternate electron donors and EPR signals for Chl+ and Car+ radicals are observed.102 At 130 GHz the signals from the two species are sufficiently resolved to permit relaxation time measurements to be performed individually. Samples were Mn-depleted to remove the relaxation effects of the Mn cluster. Echo-detected saturation-recovery experiments were performed with pump pulses up to 10 ms long to suppress contributions from cross relaxation and spin or spectral diffusion. The difference between relaxation curves in the absence of cyanide, where the Fe(II) is S = 0, and in the presence of cyanide, where the Fe(II) is S = 2, demonstrated that the relaxation enhancement is due to the Fe(II). The known distance of 37 A between Fe(ll) and Tyrz and the decrease of the relaxation enhancement in the order Tyrz > Car+ > Chl+ led to the proposal of 38 A and > 40A for the Fe(II)-Car+ and Fe(II)-Chl+ distances, respectively. Based on these distances, locations of the Car+ and Chl+ were proposed. 

Catalysis. - Aluminophosphate molecular sieves (A1PO) form a family of synthetic zeotypes, containing many three dimensional framework structures. Metal substituted aluminophosphates (MAPO) have important applications as catalysts and HFEPR has been used to determine the catalytically active sites. Two very detailed papers on various MAPO have been reported recently22,23 using both echo-detected HFEPR at 95 GHz and 3H and 31P ENDOR. 

Figure 4. Coherent transients observed in gases and molecular beams. Shown are the photon echo (detected by spontaneous emission), the free induction decay, and Ti for different pressures (iodine gas and beam).

We need to apply temporal variation of magnetic gradient for localization of the spins between the 90° RF pulse and the echo detection, which is typically in a range of millisecond. In MRI experiments, the sample is divided into either a two-dimensional... 

This unsatisfactory situation motivated Hesselink and Wiersma to attempt to generate and detect picosecond photon echos. Using a picosecond synchronously pumped dye laser system for excitation, and optical mixing as an echo-detection scheme they succeeded in measuring directly photon-echo relaxation times in the picosecond time domain. In Fig. 20 we show the results of such a picosecond photon-echo measurement on the... 

Summaries of the information content of EPR spectroscopic methods (in particular on nitroxide radicals) and the length scales of interest are given in Fig. 3. Focusing on one radical ( observer spin ), the standard method continuous wave (CW) EPR at any temperature and echo-detected (ED) EPR at low temperatures give valuable information on the fingerprint of the radical. This is mainly the electronic but can also be the geometric structure of the radical center. From CW EPR spectral analysis and/or simulations, rotational motion on the time scale 10 ps - 1 ps can be characterized qualitatively and quantitatively. Furthermore, in CW EPR, radicals also intrinsically report on their immediate (usually up to a few solvation layers, maximum up to 2 nm) chemical environment (e.g., polarity, proticity, etc.). 

Fig. 6 Left Echo-detected longitudinal magnetization of Fea as a function of nutation pulse length. Right Absolute-value Fourier Transform, showing contributions due to ESEEM-like effects (sharp peak) and Rabi oscillations broad peak). Adapted from [150]. Used by permission of the PCCP Owner Societies...

Branca et alP reported a detailed and understandable analysis of the evolution of various coherence orders in a Correlated 2D spectroscopy revamped by asymmetric z-gradient echo detection (CRAZED) like pulse sequence, used to select a signal from intermolecular multi quantum coherences (iMQCs). Because the signal to-noise-ratio of iMQC is much lower than the signal from conventional single quantum coherence (SQC), an optimization of experimental parameters is a necessity when measurements are made with iMQC. For this purpose a phase cycle is shown that not only allows a simpler selection of a particular quantum coherence order, but also removes receiver artifacts. 

TOFD. data analysis times depend strongly on the complexity of the flow being analysed and the number of TOFD signals requiring reconstruction. Overall flow analysis time depends on the extent to which one needs to overlay pulse-echo detection data on the reconstructed TOFD data in order to obtain a consistent view of the flow region. An operator skilled in the use of the interactive data display facilities for TOFD and pulse-echo data can do a full analysis in approximately 30 minutes. 

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...

Figure 8.2(c) is an inversion-recovery quadrupole echo pulse sequence, which is used to measure the Zeeman spin-lattice relaxation time,, with quadrupole echo detection [8,9,115]. Pre-saturation (Figure 8.2(d)) or progressive saturation (variation of the delay between transients) are also used to measure T. Notably, pre-saturation with spectral subtraction can separate the spectra of domains with different and is used to obtain the individual spectra of the amorphous and crystalline regions of semicrystalline polymers [8]. Also, Void and co-workers have recently presented methods involving selective inversion for the measurement of slow molecular reorientation, which provide an alternative to spin alignment or multidimensional methods [116]. 

Figure 8.9(a) shows the pulse sequence used to obtain the deuteron exchange powder pattern [59]. The sequence differs from the spin 1/2 version (chapter 6) owing to spin 1 dynamics of the deuteron and the need for quadrupole echo detection to overcome probe ringing. The first two pulses, which bracket the evolution period, provide frequency selection of individual quadrupole splittings. As with the spin 1/2 experiment, in-phase and out-of-phase pulse pairs select the sine and cosine components of the evolving magnetization to yield quadrature detection in the FI dimension. One can use either the hyper-... 

A full description of the physics of spin echo sequences is beyond the scope of this chapter (Fig. 5.1). Important parameters that are adjusted to alter image contrast are the repetition time (TR) (measured in milliseconds) which is the time between the application of one RF excitation pulse and the start of the next RF pulse. The time to echo (TE) (measured in milliseconds) is the time between the application of the RF pulse and the peak of the echo detected. 

Echo detection of selectively-burned holes in photosynthetic systems led to estimates of distances between 25 and 50 A. If the contributions to spectral diffusion from motion, nuclear spin flip-flops, and instantaneous diffusion are smaller than the contribution from dipolar interaction between unpaired electrons, the spectral diffusion can be used to determine the interspin distance. (97-100). 

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... 

Fig. 6. The four-pulse DEER experiment, (a) Pulse sequence consisting of a refocused primary echo subsequence with fixed interpulse delays for the observer spins (top) and a pump pulse at variable delay t with respect to the first primary echo (bottom), (b) The pump pulse inverts the local field at the site of the observer spin (left arrow in each panel) imposed by a pumped electron spin (right arrow in each panel), (c) Observer and pump positions in an echo-detected EPR spectrum of a nitroxide.

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]. 

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