Time domain ESR

The electron spin echo (ESE) method is a kind of time-domain ESR using pulsed microwave radiation and it directly observes the relaxation behavior of electron spins. Since the construction of an ESE spectrometer was first reported by Kaplan early in 1962 [8], the ESE method has become more and more popular slowly but steadily. Owing to the progress in microwave technology and to the continual effort of pioneering workers such as Tsvetkov, Mims, Kevan and others [9, 10]. the ESE method has been extensively developed and improved. It is now considered to be requisite for studying the paramagnetic relaxation of radical species. 

CW experiments (sometimes called stationary or steady state ) are ones in which either no modulations are used, or they are so low in frequency that no spectral complications ensue. (This is only approximately the case if 100 kHz field modulation is employed. This frequency gives rise to modulation sidebands and, under saturating conditions, rapid passage effects.) Time-domain ESR involves monitoring the spin system response as a function of time. Pulse ESR can be divided into two broad categories the response of spin systems to sequences of microwave pulses (spin echo) and the response of spin systems to step changes in resonance conditions (saturation recovery). 

To this list can be added various major specialized sub-systems that are required for some of the more sophisticated experiments. Some of the basic sub-systems also require modification for these experiments. (7) Pulse programmer for time domain ESR (8) programmable radio frequency source for electron-nuclear double resonance (ENDOR) (9) pump microwave source for electron-electron double resonance (ELDOR). 

The reader is referred to the book by Kevan and Kispert [19] for a thorough discussion of ENDOR and ELDOR, to the book by Kevan and Schwartz [20] for a discussion of time domain ESR, and to the review by Hyde and Froncisz [242] of multifrequency ESR. 

Pulse or time domain ESR can be divided into two categories the transient response of spin systems to abrupt or step changes in resonant condition and the transient response to sequence of pulses [20]. The step response, as in saturation recovery, is used commonly to measure T, and the pulse response, as in 90-180° spin echo, to measure Tj. 

Solid-state time-domain ESR is nsnally based on echo experiments. The transverse magnetization is excited by a pnlse with a flip angle of n 2 and decays within the deadtime ti of the spectrometer following the high-power pnlse. In the two-pnlse echo experiment this transverse magnetization is refocnsed hy a second pnlse with a flip angle of tt, which is applied after a delay t > fa with respect... 

Fig. 5. Pulse sequences for basic time-domain ESR and pulsed ENDOR experiments, (a) Primary echo experiment, (b) Inversion recovery experiment (variation of T) or Davies ENDOR. (c) Stimulated echo experiment or Mims ENDOR. For ENDOR experiments, the horizontal bar in (b) and (c) indicates a radiofrequency pulse, whose frequency is varied while all interpulse delays are fixed.

Klug CS, Eaton SS, Eaton GR, Feix JB. 1998. Ligand-induced conformational change in the ferric enterobactin receptor FepA as studied by site-directed spin labeling and time-domain ESR. Biochemistry 37(25) 9016-9023. 

The time domain ESR line, G(t) = (5 t)S (0)), should be, in principle, directly observable using pulsed techniques, as the free induction decay signal after a ttI2 pulse. The spin echo generated at time /e = 2t after a sequence rr/2 - t - tt decays with a time constant T2 in simple cases of fixed or slowly moving spins. For rapidly moving spins in a field gradient G there is an additional decay as exp [-(1/12)7 G D Ie ] [26]. 

A more direct method to obtain values of the spin-lattice and spin-spin relaxation times is to use time-domain ESR methods, which are briefly described next. 

Time-domain ESR methods have become quite important for modem applications of ESR. In a time-domain experiment one uses microwave pulses rather than steady-state microwaves. It is possible to use pulses as short as a few nanoseconds, which makes fast kinetic processes involving paramagnetic species observable and allows the direct measurement of relaxation times. In addition, time-domain ESR methods have been useful for more direct determination and study of relaxation mechanisms and for developing new methods for obtaining stmctural information in disordered systems. 

Two methods of time-domain ESR will be briefly discussed. The oldest method is that of saturation recovery, which is a direct method to determine spin-lattice relaxation times. The idea is to perturb the steady-state population of spins with a partially saturating pulse of microwaves and then to observe with a very weak microwave field the recovery of the perturbed spin population to equilibrium. In the absence of complications, the recovery process is exponential and can be related to the time constant for spin-lattice relaxation. Exponential recoveries are generally observed in liquids and in some cases in solids. If the spin-lattice relaxation time is not much longer than the spin-spin relaxation time, which is atypical in paramagnetic systems, the interpretation of saturation recovery data becomes more complex. 

This chapter is organized as follows the fundamentals of ESR spectroscopy are described in Sections 2.08.2.1-2.08.2.3, with emphasis on the experimentally available parameters relevant for duddation of the polymer stmcture, dynamics, transport, and stability. line-shape analysis of nitroxide spin probes is given in detail in Section 2.08.2.4. Advanced ESR methods, induding HF-ESR and time-domain ESR as wdl as double resonance methods are outlined in Sections 2.08.2.5-2.08.2.7. The spin trapping method is described in Section 2.08.2.8. The transition from ESR spectroscopy to ESRI is presented in Section 2.08.3. Spedfic and important, applications of ESR spectroscopy and ESRI to polymeric systems are described in Sections 2.08.4.1-2.08.4.4. We will condude with an evaluation of the strengths of ESR methods, and our view on further applications of this approach and on areas that require further devdopment. 

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