Advanced EMR ENDOR vs. ESEEM

The principal advantage of ENDOR spectroscopy is the much finer energy scale upon which the state-to-state transitions are recorded. As discussed in 3, conventional cw-EMR spectroseopy detects the weak hyperfine interactions of the spin Hamiltonian as a perturbation of the electronic Zeeman effect in many practical situations, inhomogeneous broadening will wash out the hyperfine structure of the speetrum. In such cases of inhomogeneously broadened EMR spectra, interaction can only be deconvoluted for practical analysis via simulations (Hyde Fron-cisz, 1982). The ENDOR method, however, records a spectram that represents the 

Electron spin echo modulation spectroscopy (Norris et al., 1980 Dikanov Tsvetkov, 1992) is sometimes called FT-ENDOR because the echo modulation time series yields a frequency spectrum that corresponds to transitions among nuclear sublevel (Rowan et al., 1965). The ESEEM technique is often said to be complementary to ENDOR (Tsvetkov Dikanov, 1987) beeause ESEEM tends to yield well-resolved spectra in the low-frequency range ( 4 MHz) of the nuclear hyperfine spectrum, where cw-ENDOR is often problematie. The eonverse is likewise true ESEEM tends to be problematic at recording hyperfine frequencies above 10 MHz. 

The echo method is valuable because it provides a second means of determining spin relaxation rates that is an alternative to spin induction decay. The ampli- 

PULSE excitation BANDWIDTH SPANS ALL ALLOWED TRANSITIONS 

EMR transitions - /2,- /2) -i- /2,-i/2) and -V2,+V2) - -i- /2,-i- /2) may be simultaneously driven by ineident mierowave field H if the energy difference of these transitions is less than the speetrometer bandwidth, which is affected by the modulation or exeitation method. In sueh a case quantum beats or modulation effects will be deteeted on the deeay profile. For example, a 20-ns square pulse will have an excitation bandwidth of approximately 50 MHz, which is quite broad relative to the EMR linewidths of many organic radicals, and one might expect to detect the entire ENDOR spectrum of an organic radical in an echo modulation time series. The state diagram of the proton hyperfme i.e., ENDOR, Bender et al., 

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