Magnetization-detected EPR

No direct information on the phase memory or quantum coherence time 7m has yet been obtained from magnetization detected EPR investigations. In one study, a spin echo measurement was attempted on frozen solutions of [Ni(hmp)(dmb)Cl]4 in toluene dichloromethane (1 1) at 5.5 K, but no echoes were observed. An upper limit of Tm = 50 ns was deduced from this observation [132]. For homogeneously... 

Investigations of the spin-lattice relaxation time of single crystals of Feg at T = 1.27 K, utihzing the stimulated echo sequence, revealed two relaxation processes with time constants of 1.0 ps and 0.95 ms, respectively [153]. The authors attribute the former time constant to spectral diffusion, and speculate that the latter may reflect true spin-lattice relaxation, although the time constant is two orders of magnitude longer than values found by magnetization-detected EPR (see above) [129, 131]. 

In electron-spin-echo-detected EPR spectroscopy, spectral infomiation may, in principle, be obtained from a Fourier transfomiation of the second half of the echo shape, since it represents the FID of the refocused magnetizations, however, now recorded with much reduced deadtime problems. For the inhomogeneously broadened EPR lines considered here, however, the FID and therefore also the spin echo, show little structure. For this reason, the amplitude of tire echo is used as the main source of infomiation in ESE experiments. Recording the intensity of the two-pulse or tliree-pulse echo amplitude as a function of the external magnetic field defines electron-spm-echo- (ESE-)... 

Electron paramagnetic resonance (EPR) is also referred to as electron spin resonance (ESR). In many respects, it is similar to NMR and the corresponding principles, discussed in the previous section, apply. The critical difference is that an unpaired electron spin is detected in this method instead of a nuclear spin. The method applies only to paramagnetic systems. The electron spin is more readily detected than is a nuclear spin and magnets on EPR instruments are correspondingly smaller and less expensive. 

Pulsed EPR experiments are typically performed with locally constructed instruments, although a commercially available X-band pulsed EPR/ENDOR instrument is now available from Bruker. A liquid helium immersion cryostat is generally employed. The pulsed EPR instrument creates short, high-power microwave and, for ENDOR, rf pulses, but the magnetization detected is of very small magnitude and requires a sensitive detector. This necessitates precise timing not only for creation and detec-... 

The method of determining T via amplitude modulation of Hi relies on variation of the modulation frequency, denoted by QJIti, until it exceeds T), at which point the magnetization cannot respond to the power variation and there is a loss in the detected EPR signal amplitude (Herve Pescia, 1960a). In a sense, this Ti measurement is analogous to that used to analyze the impedance of a nonlinear system, sueh as a passive filter. The precept is that the response of a system y t) to some perturbation x t) is determined by some differential equation of order n. In the case of a linear system and perturbation x(t)=A sin((oO, one observes a response y(t)=B sin(co -l-(t)) and one defines a transfer function as the ratio of output to input (in the frequency domain) H(j( i)= H(( i) wherey((o) and x(co)... 

The negative sign in equation (b 1.15.26) implies that, unlike the case for electron spins, states with larger magnetic quantum number have smaller energy for g O. In contrast to the g-value in EPR experiments, g is an inlierent property of the nucleus. NMR resonances are not easily detected in paramagnetic systems because of sensitivity problems and increased linewidths caused by the presence of unpaired electron spins. 

EPR absorption has been detected from zero magnetic field up to fields as high as 30 T corresponding to a... 

The low MW power levels conuuonly employed in TREPR spectroscopy do not require any precautions to avoid detector overload and, therefore, the fiill time development of the transient magnetization is obtained undiminished by any MW detection deadtime. (3) Standard CW EPR equipment can be used for TREPR requiring only moderate efforts to adapt the MW detection part of the spectrometer for the observation of the transient response to a pulsed light excitation with high time resolution. (4) TREPR spectroscopy proved to be a suitable teclmique for observing a variety of spin coherence phenomena, such as transient nutations [16], quantum beats [17] and nuclear modulations [18], that have been usefi.il to interpret EPR data on light-mduced spm-correlated radical pairs. 

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