Frequency domain EPR

Fig. 3 Frequency domain EPR spectra of pressed pellet of Mnl2 at variable temperature and zero applied magnetic field. At 2 K, only Ms = 10 are populated and only the Ms = 10 to 9 transition is observed Ms = 9 to 8, etc. observed on warming. Figure from [46]...

Pulsed, or time-domain, EPR spectrometers have also been developed at higher frequencies up to 140 GHz [55. 56]. They are generally low-power units with characteristically long pulse lengths (typically 50 ns for a n/2-pulse) due to tire limited MW powers available at millimetre wavelengths and the lack of fast-switching... 

Bar, G., M. Bennati et al. (2001). High-frequency (140-GHz) time domain EPR and ENDOR spectroscopy The tyrosyl radical-diiron cofactor in ribonucleotide reductase from yeast. J. Am. Chem. Soc. 123 3569-3576. 

On the basis of its high g-value it was suggested that the tyrosine in PS E (g =2.0045) is a neutral hydrogen-bonded oxidation product. To shed more light on the question whether D is a cation or the neutral hydrogen-bonded species we performed an Electron Spin-Echo Envelope Modulation (ESEEM) study on D . Such a study can reveal new information about the weak hf interactions of D that can not be resolved by cw EPR. Here we report the results of ESE experiments of Signal E in different systems, protonated, deuterated and perdeuterated in the frequency range of 8.6 to 9.3 GHz, and the ESEEM spectra in the time and frequency domain are analyzed. 

Figs. 3c and 3d display the three-pulse ESEEM patterns for the multiline EPR signal of untreated PSII membranes prepared in 2H2O buffer and illuminated at 195 K. The time and frequency domain patterns again show clear evidence of deuterons located on inner sphere molecules, presumably water... 

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)... 

In principle, the technique can also be extended to the fitting of pulsed EPR spectra using the frequency domain approach [62]. Each peak in the ESEEM spectrum is given by one or more frequencies, a complex intensity, and a linewidth. Derivatives of the ESEEM frequencies with respect to each of the spin Hamiltonian parameters can be calculated and utilized in an identical fashion to that used for CW spectra simulations. 

The method to determine defect concentrations, defect mobilities, and their influence on the diffusion is to use conductivity measurements at different temperatures and measuring frequencies. In a single experiment the activation energies for formation and displacement of the defects can be established if both the intrinsic and extrinsic domain can be covered. Some defects can be identified spectroscopically, e.g., with electron paramagnetic resonance (EPR or ESR) if the defect has unpaired electrons. 

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