Three pulse electron spin echo spectra

Experimental three pulse electron spin echo spectra of Ag (A) in Lij2 A zeolite. The two large sharper peaks below 1 ps are due to two pulse interference. 

Cu isotopes both with nuclear spin I-3/2. The nucle r g-factors of these two isotopes are sufficiently close that no resolution of the two isotopes is typically seen in zeolite matrices. No Jahn-Teller effects have been observed for Cu2+ in zeolites. The spin-lattice relaxation time of cupric ion is sufficiently long that it can be easily observed by GSR at room temperature and below. Thus cupric ion exchanged zeolites have been extensively studied (5,17-26) by ESR, but ESR alone has not typically given unambiguous information about the water coordination of cupric ion or the specific location of cupric ion in the zeolite lattice. This situation can be substantially improved by using electron spin echo modulation spectrometry. The modulation analysis is carried out as described in the previous sections. The number of coordinated deuterated water molecules is determined from deuterium modulation in three pulse electron spin echo spectra. The location in the zeolite lattice is determined partly from aluminum modulation and more quantitatively from cesium modulation. The symmetry of the various copper species is determined from the water coordination number and the characteristics of the ESR spectra. 

Experimental three pulse electron spin echo spectra of two types of Chi in Na -A zeolite. Chi (( (D-CO-is the dominant copper species in site S2 in freshly prepared, hydrated Nai2 A and Cu +(0z)j(D20)2 in site S2 is the dominant species after partial dehydration under vacuum at room temperature. The different deuterium modulation depths characterize the different numbers of coordinated waters in these two Cu species. 

ESE-ENDOR. In ESE-ENDOR, introduced by Mims1181, a rf pulse in a three-pulse electron spin echo experiment is applied during the time interval between the second and third microwave pulse. The ENDOR spectrum is obtained by monitoring the decrease... 

Figures 1 and 3 show that although the modulations of the three-pulse, or stimulated echo are less intense than those of its two-pulse counterpart, the resolution is much higher and the spectrum is simplified because combination peaks only enter into the data through the presence of multiple ESEEM-active nuclei. Equation (8) shows that for an S = 1 /2, 7 = 1/2 spin system, judicious selection of the r-value can control the ESEEM amplitudes of the hyperfine frequencies from a and electron spin manifolds allowing them to be optimized or suppressed. For weakly coupled protons, where the modulation frequencies from both electron spin manifolds are centered at the proton Larmor frequency, x can be set at an integer multiple of the proton Earmor frequency to suppress the contributions of this family of coupled nuclei from the three-pulse ESEEM spectrum. It is common for three-pulse ESEEM data to be collected at several r-values, including integer multiples of the proton Larmor period, to accentuate the other low frequency modulations present in the data and to make sure that ESEEM components were not missed because of T-suppression.

The field-swept electron spin echo (FS-ESE) spectrum (which yields a spectrum similar to that acquired by a CW experiment, but is acquired using a simple two-pulse sequence) of VO(ema)2-treated rat bone yielded spin Hamiltonian parameters quite different from those obtained with the other two vanadyl compounds (Table 4). As with VO(pic)2, it was clear that the eoordination state of the orally administered VO(ema)2 had changed [95]. Despite the difference in spin Hamiltonian parameters determined from CW-EPR (VOSO4, VO(pic)2) and FS-ESE (VO(ema)2) spectra, it appears that all three compounds share the same metabolic fate after uptake into bone mineral. 

The two-pulse echo decay is sometimes too fast to obtain a satisfactory frequency spectrum after Fourier transformation. In this case the three-pulse sequence shown in Fig. 2.21(a) is an alternative. It gives rise to a stimulated echo at time r after the third k/2 pulse. The decay rate is limited by the electron spin-lattice relaxation time Ti, which is usually longer than the phase memory relaxation time Tm for the two-pulse decay. 

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