What can be studied with Electron Paramagnetic Resonance?

EPR spectroscopy detects only paramagnetic species, i.e. species with unpaired electrons. Owing to its selectivity, an EPR study has the main advantage of providing insights into the nature of the paramagnetic centre, while also revealing detailed information on its environment and on the dynamical processes in which it is involved. In this chapter we will describe and provide a few examples of the various paramagnetic species that are studied using EPR methods for various experimental purposes

MOLECULAR-DYNAMICS OF RADICALS IN THE SOLID-STATE - ESR, ENDOR AND PULSED ESR STUDIES

In this communication I report the results of a series of studies by ESR, ENDOR and Electron Spin Echo on motions of radicals in solids. The radicals investigated were stable radicals obtained by gamma irradiation in single crystals (or crystalline powder)

Pulsed EPR of paramagnetic centers in solid phase

We present an overview of the most used Electron Spin Echo techniques and their applications to the study of structure and dynamics of paramagnetic centers in solid phases. A short theoretical section presents the tools necessary to understand the experiments. Three sections describe the experiments that are used for measuring longitudinal relaxation, transverse relaxation and hyperfine interactions. Many examples of applications to different research fields are given

Pulse EPR of Paramagnetic Centers in Solid Phases

We present an overview of the most used Electron Spin Echo techniques and their applications to the study of structure and dynamics of paramagnetic centers in solid phases. A short theoretical section presents the tools necessary to understand the experiments. Three sections describe the experiments that are used to get information on the spin and spatial dynamics of the system, on the distribution of paramagnetic centers in the solid matrix, and on their local environment. Many examples of applications to different paramagnetic centers in various research fields are given

Electron Spin-lattice Relaxation-time and Spectral Diffusion In Gamma-irradiated L-alanine.

We performed continuous (CW) wave and pulsed ESR experiments to obtain information on the relaxation behavior of the 1-alanine radical in an irradiated single crystal. The analysis of the CW saturation behavior gives a relaxation time of 2.8 micro s. The echo detected saturation recovery was obtained for a number of different experimental conditions. In any case only a portion of the 120 G wide ESR spectrum can be affected by the microwave (MW) pulses, spectral diffusion is active and a multi-exponential decay is therefore obtained. We measured characteristic spectral diffusion times of 1-10 and 20-50 micro s. We found that a long time of about 200 micro s can be measured only by using a train of long selective saturating pulses and short detecting pulses. The stimulated echo decay is bi-exponential, and the characteristic times are very short. A variable temperature investigation in the range 200 to 290 K showed that the decay is governed by the spectral diffusion and by the transverse nuclear spin relaxation time T-2n of the methyl protons

An EPR Study on Nanographites

We present the results of an electron paramagnetic resonance study (EPR) in the range of 4\u2013290 K on samples of nanographites obtained by ball milling graphite for different times. With a careful simulation of the spectral line shapes, we disentangled the EPR bands, providing the spectral profiles and intensities of the components on varying the temperature, their g tensors, and the homogeneous line widths of the contributing spin packets. We have been able to follow the effect of decreasing progressively the size of the flakes on the EPR bands due to mobile electrons and on Lorentzian lines due to nonbonding electrons on the zigzag edges of the crystallites. The temperature dependence of the EPR intensities shows a common trend for the signals attributed to edge electrons and to mobile electrons, showing that they belong to the same bath

STRUCTURE AND DYNAMICS OF RADICALS IN SOLIDS BY EPR AND ENDOR SPECTROSCOPIES

EPR and ENDOR (Electron Nuclear DOuble Resonance) spectroscopies give complementary information in the study of paramagnetic species in solids. The ENDOR technique allows the determination of hyperfine tensors an order of magnitude smaller with respect to the EPR one. The molecular motions in solids affect the relaxation of both the transverse and longitudinal magnetization of the paramagnetic probe. The measurement of the transverse relaxation time is traditionally available by the EPR Lineshape analysis. Population variations due to the longitudinal relaxation processes can be monitored by the study of the amplitude of the ENDOR spectra

ENDOR of Fremy's salt

The 14N ENDOR of the radical anion (SO3)2NO= has been observed in fluid solutions in water and in water-ethanol mixtures, and trapped in irradiated single crystals of potassium hydroxylamine disulphonate. The quadrupole coupling has been estimated from the solid-state spectra and it is argued that this interaction is too weak to provide a significant relaxation mechanism for the radical in solution. Spectra have been calculated assuming relaxation occurs through modulation of the g and dipolar tensors and the spin-rotation coupling, together with Heisenberg exchange, and the results agree quite well with the spectra observed for the radical in water. An additional relaxation mechanism seems to operate in the water-ethanol mixtures

Spin concentration in a possible ESR dosimeter: An electron spin echo study on X-irradiated ammonium tartrate

Several single crystals and powder samples of ammonium tartrate, recently proposed as a possible ESR dosimeter, have been X-irradiated with different doses. The total radical concentration has been determined by quantitative cw ESR, by comparison with a standard. The samples have been studied by electron spin echo spectroscopy. The two-pulse echo decay has been obtained and simulated by a single exponential function for different values of the microwave power of the pulses and for different pulse lengths. The dependence of the phase memory time TM on the microwave power has been exploited to get information on the contribution of the instantaneous diffusion to spin dephasing. At room temperature in the range of radical concentrations of 1E18-1E19 spins/cm3 the instantaneous diffusion is the dominant spin dephasing mechanism. The linear dependence of the instantaneous diffusion on the total concentration of the radicals is in agreement with the theory. From the latter result we conclude that the average radical-radical distance corresponds to a random distribution of the radicals in the matrix. A simple method of measuring the radical concentration by the ESE decays in powder samples of irradiated ammonium tartrate is described