High-field ESR

This scheme has yet to be fully tested in a high-field ESR experiment but initial experimental tests and theoretical simulations by the Pisa group appear very promising. We have also tested this scheme in our own laboratory with very encouraging results and expect this type of resonator to become a common feature in high field systems. 

HIGH-FIELD ESR SPECTROSCOPY OF LOWDIMENSIONAL QUANTUM SPIN SYSTEMS... 

High-Field ESR Spectroscopy of Low-Dimensional Quantum Spin 241... 

Here, we report on high-field ESR properties of the 5=1 Heisenberg chain system Ni(C2H8N2)2Ni(CN)4 (NENC) [16,31], This material is characterized by strong in-plane anisotropy D/k= 6 K, and so far can be considered as the best known candidate for a quantum 5=1 chain in the large-D limit (D/J=1.5) [32],... 

Fig. 30 The temperature dependence of the high-field ESR spectra of powdered TDAE-C70. Here the Larmor frequency is 245 GHz...

Let us briefly mention here that proton hyperfine splittings have been observed in the high-field ESR spectra of the naphthalene triplet (33,40) in single crystals. Recently Grivet (43) has succeeded in observing hyperfine splittings in the intermediate field (AMg=2) line of the triplet of partially deuterated naphthalene in rigid solution with a conventional spectrometer. 

Although all laboratories that perform high-field ESR have experimented with Fabry-Perot resonators instead of fundamental mode microwave cavities, few laboratories have as yet explored quasioptical implementations of common microwave devices such as a magic T or circulator in an FIR-ESR spectrometer (see Earle and Freed, 1995 Earle et al. 1996b Smith, 1995). Part of the problem is the unfamiliar appearance of optical circuits to spectroscopists who are only familiar with... 

Large line-widths, sometimes in conjunction with insufficiently resolved hyperfine structure, can preclude the determination of individual g factor components close to g = 2 under conventional ESR conditions (X band, 9.5 GHz, 330 mT). As in NMR spectroscopy, an increase in the field and frequency is then appropriate via the Q band (35 GHz) to the W band (95 GHz) or even higher frequencies and magnetic fields [48]. An application of such high-field ESR for the inorganic radical 10 with sizeable metal participation has been reported [49a]. 

Not all ESR laboratories can afford the price and running costs of a high field ESR spectrometer equipped with a superconducting magnet. Applications like those considered in Chapter 5 are also most conveniently performed at X-band. Thus the need for analysis beyond first order remains. 

Exercise E3.14 for a discussion of this point and the use of high field ESR in Chapter 4 to increase the resolution. 

As mentioned in Chapter 1 high field ESR gives a better spectral resolution of anisotropic g-factors, and allows simpler analysis than is possible at X-band of spectra with large zero-field (another term is fine structure) and hyperfine splittings. These features, (l)-(3) below, are the most important in general applications, but some other characteristics of high field ESR are also of relevance [2] ... 

Scheme 4.2 Schematic representation of g-factor resolution by high-field ESR spectroscopy applied to radicals in humic acids. The scheme is reproduced from [14] with permission from the American Chemical Society...

---