ESEEM blind spots

Muns ENDOR mvolves observation of the stimulated echo intensity as a fimction of the frequency of an RE Ti-pulse applied between tlie second and third MW pulse. In contrast to the Davies ENDOR experiment, the Mims-ENDOR sequence does not require selective MW pulses. For a detailed description of the polarization transfer in a Mims-type experiment the reader is referred to the literature [43]. Just as with three-pulse ESEEM, blind spots can occur in ENDOR spectra measured using Muns method. To avoid the possibility of missing lines it is therefore essential to repeat the experiment with different values of the pulse spacing Detection of the echo intensity as a fimction of the RE frequency and x yields a real two-dimensional experiment. An FT of the x-domain will yield cross-peaks in the 2D-FT-ENDOR spectrum which correlate different ENDOR transitions belonging to the same nucleus. One advantage of Mims ENDOR over Davies ENDOR is its larger echo intensity because more spins due to the nonselective excitation are involved in the fomiation of the echo. 

Another advantage of ReMims (Doan) ENDOR is the ability to observe this implicit-TRIPLE effect for a wider range of hyperfine couplings. A given hyperfine coupling could be deleferiously affecfed by Mims blind spots, which would prevent observation of implicit TRIPLE. However, use of ReMims (Doan) ENDOR would allow selection of T value(s) that do not cause troublesome blind spots while simultaneously suppressing ESEEM, so that the presence of the implicit TRIPLE effect could be explored. 

In a three-pulse ESEEM experiment the time T between the second and the third pulse is increased while the time x between the first and second pulse is kept constant. In contrast to the two-pulse ESEEM experiment, the three-pulse ESEEM spectra do not contain sum and difference frequencies as illustrated schematically in Fig. 2.21 for an S = Vi species with anisotropic hyperfine coupling due to a proton. Both spectra contain lines with nuclear frequencies and v expected for = /2. The combination lines at v v seen as satellites in the two-pulse spectrum do not appear in the corresponding 3-pulse spectrum. On the other hand lines can escape detection in the 3-pulse spectrum for certain values of the time x between the first and second pulse at so called blind spots. It is therefore customary to record several 3-pulse specfra with different values of x. 

Considering the resolution of the nuclear frequency spectrum, this two-pulse echo experiment is not optimal. The nuclear frequencies are here measured as differences of frequencies of the ESR transitions, so that the line widths correspond to those of ESR transitions. The nuclear transitions have longer transverse relaxation times Tin and thus smaller line widths. In fact, if the second mw pulse is changed from a n pulse to a Ji/2 pulse, coherence is transferred to nuclear transitions instead of forbidden electron transitions. This coherence then evolves for a variable time T and thus acquires phase v r or vpT. Nuclear coherence cannot be detected directly, but can be transferred back to allowed and forbidden electron coherence by another nil pulse. The sequence (jt/2)-x-(Jt/2)-r-(jt/2)-x generates a stimulated echo, whose envelope as a function of T is modulated with the two nuclear frequencies v and vp. The combination frequencies v+ and v are not observed. The modulation depth is also 8 211. The lack of combination lines simplifies the spectrum and the narrower lines lead to better resolution. There is also, however, a disadvantage of this three-pnlse ESEEM experiment. Depending on interpulse delay x the experiment features blind spots. Thus it needs to be repeated at several x values. 

When T is varied the echo envelope is modulated only by the two basic frequencies CDa and (Up, the sum and difference frequencies do not appear, in contrast to the two-pulse ESEEM experiment. This is usually advantageous, as it simplifies spectra, but it may also be a disadvantage for disordered systems where the sum-combination line is often the only narrow feature in the ESEEM spectrum. Another important difference is the dependence of the three-pulse ESEEM amplitudes on r, as is apparent from Eq. (17) by the factors 1 - cos(copr) and 1 - cos(cOcir). Due to this suppression effect, individual peaks in the spectrum can disappear completely. These blind spots occur for the a(P) peak when r = 2n /(Up(a) (k = 1, 2,. ..). In principle they can be avoided by using r < Inlco, where (Umax is the maximum nuclear frequency however, this is usually precluded by the spectrometer deadtime. Consequently, the three-pulse ESEEM experiment has to be performed at several r values to avoid misinterpretation of the spectra due to blind-spot artifacts. 

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. 

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. 

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