Spin-echo pulse delayed detection

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... 

It must be stressed that the spin-echo sequence is applied only during the detection period and its unique purpose is to estimate the signal amplitude (in a sense, it is a replacement for the simple 90° pulse). Consequently, in an arrayed multi-block experiment whose purpose is to measure Ti(Br), only the X value is varied, while the delays 5 and 8 are kept constant in order to make sure that no T2(Ba) effects leak into the experimental relaxation curves. Moreover, to avoid contamination of the echo by FID residues due to imprecise settings of RF pulses and to Bi inhomogeneity, proper phase cycling is highly recommended. 

Prepare the spin echo application with appropriate acquisition parameters. Typically, 90° to 180° pulse separation 3.5 msec, moisture plus oil sample window 0.05 to 0.06 msec, oil sample window 7 msec, number of scans 16, recycle delay 2 sec, detection mode magnitude , and phase cycling (if available) ON . 

Use of a surfactant allows solubilization of the polyoxometalate cluster K6[Vi5As6042(H20)] 8H20 (V15) in the organic solvent chloroform. Spin echo measurements revealed a phase memory time of Tm = 340 ns, which was attributed to resonances in the 5 = 3/2 excited state of the cluster [166]. No quantum coherence was detected in the pair of 5 = 1/2 ground states [151]. By measurement of the z-magnetization after a nutation pulse, and a delay to ensure decay of all coherences, Rabi oscillations were observed. From the analysis of the different possible decoherence mechanisms, it was concluded that decoherence is almost entirely caused by hyperfine coupling to the nuclear spins. 

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]. 

To measure the longitudinal relaxation time Ti, an inversion or saturation pulse is applied, followed, after a variable time T, by a two-pulse echo experiment for detection (Fig. 5b). The inversion or saturation pulse induces a large change of the echo amplitude for T < T. With increasing T, the echo amphtude recovers to its equilibriiun value with time constant Ti. The echo amphtude of the stimulated echo (Fig. 5c) decays with time constant T2 when the interpulse delay T is incremented, and with the stimulated-echo decay time constant Tse < T1 when the interpulse delay T is incremented. A faster decay, compared to inversion or saturation recovery experiments, can arise from spectral diffusion, because of a change of the resonance frequency for the observed spins, of the order of Av = 1/t on the time scale of T. Quantitative analysis of spectral diffusion can provide information on the reorientation dynamics of the paramagnetic centers. 

The remote-echo detector is shown in Figure 11. In this method the electron spin echo at the end of the pulse sequence, which uses Vi < rnuclear coherence generator, is not recorded. Instead, at the time of echo formation an additional nil pulse transfers the electron coherence to longitudinal magnetization. The echo amplitude information can thus be stored for a time interval up to the order of T. After a fixed time delay h < T l, the z-magnetization is read out using a two-pulse echo sequence with a fixed time interval X2 > r. Remote echo detection can be applied to many experiments, including three-pulse ESEEM and HYSCORE, and thus can eliminate blind spots with an appropriate choice of small ri. Note, however, that it may suffer from reduced sensitivity due to the increased sequence time. 

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.

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