Remote echo detection

The use of a remote-echo detector allows r values shorter flian the spectrometer deadtime to be employed [55]. This is important in two-pulse ESEEM experiments where the deadtime prevents the signal for times r from being recorded. Also in the deadtime-free four-pulse experiments described in 3.3, a small T value is often needed to avoid blind spots. Bhnd spots are a particular concern for flie measurement of proton spectra at X-band, where flie signals typically extend from 5 to 25 MHz, and with a r = 100 ns blind spots occur at nh = 0, 10, 20. MHz. 

JEFFREY HARMER, GEORGE MITRIKAS, and ARTHUR SCHWEIGER 

In the HYSCORE experiment only nuclear frequencies in different manifolds belonging to the same paramagnetic center are eorrelated with each other. For multinuclear spin systems the assignment of nuelear frequencies is often not straightforward, since some of the correlation peaks may not be observed in the HYSCORE speetrum due to the small intensity of the nuelear transitions in one of the two Ws manifolds. Additional information can be gained if correlations of nuclear frequencies within the same manifold can be obtained. Cross-peaks that represent such correlations can be created by replacing the nonselective transfer n pulse in the HYSCORE sequence by the double nuclear-coherence transfer (DONUT) mixer % - t - n [58]. This DONUT-HYSCORE experiment with the pulse sequence ji/2 -ti nil - echo results in crosspeaks and (twp i, copj). The presence of these cross-peaks in the DONUT- 

HYSCORE spectrum is a proof that p and j belong to the same paramagnetic center, and this information can support their unambiguous assignment. An experimental example for nitrogens that are close to the exact cancellation condition has been published for the complex Co(II)TPP(py) [59], where the DONUT-HYSCORE experiment revealed one of the NQR frequencies that was missing from the HYSCORE speetrum. 

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Figure 11. Pulse sequence for remote-echo detection. Modified with permission from [7]. Copyright 2001, Oxford University Press.

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. 

Cho H, Pfenninger S, Gemperle C, Schweiger A, Ernst RR. 1989. Zero deadtime pulsed ESR by remote echo detection. Chem Phys Lett 160 391-395. 

Pulsed techniques that operate in the so-called spin-echo mode are particularly good for applied NQR, and they naturally lend themselves to remote detection.12... 

Passive piezoelectric sensors can also be applied to detect cracking and crack growth via acoustic emission or to identify damage proceedings via impact detection. Active piezoelectric sensors in SHM structures are suited for the detection of remote damage via pulse-echo, pulse-transmission and phased-array methods or for the identification of damages nearby the sensors via high-frequency electromechanical impedance methods. 

Electron spin-echo studies indicate that pink uteroferrin s unpaired electron interacts with at least one and possibly two classes of nitrogen nuclei, one of which is the imidazole nitrogen of an iron-coordinated histidine These results support similar conclusions drawn from NMR studies showing histidine ligation to both ferrous and ferric ions of the binuclear cluster . Recent ENDOR experiments have also detected N hyperfine interactions with the S = 1/2 center of uteroferrin, presumably due to the remote nitro-gen(s) of an iron coordinated histidine(s) ... 

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