Temperature of Spin-Spin Interactions in Electron Spin Resonance

Abstract

The dielectric behaviour of Poly(n-butyl methacrylate) (PnBMA) has been studied by thermo-stimulated currents (TSC) and dynamic dielectric spectroscopy (DDS). The analysis of the TSC complex spectrum shows the existence of three dipolar relaxations around the glass transition. The first one, labelled β, near −25˚C, has been assigned to a secondary relaxation mode. The second one, α, at 30˚C, has been associated with the glass relaxation. The third one, above T g at 90˚C, might be assigned to the dielectric manifestation of the liquid-liquid transition. The use of fractional polarizations allows us to determine the fine structure of these three modes. The relaxation times of the β and α modes are widely distributed and they follow an Arrhenius equation. We also report a compensation phenomenon for the α mode. As for the α mode, the relaxation times are narrowly distributed and they obey a Vogel equation. By using a blocking electrode, the study by DDS exhibits three relaxational phenomena: the β and α modes which merge approximately at 10 4 Hz and lead to an (αβ) mode, and an additional α phenomenon. By using the Havriliak-Negami parameters, we characterize the shift of their maxima with frequency and temperature. The consistent set of data obtained from TSC and DDS allows us to follow the relaxation modes of PnBMA in a very broad temperature and frequency range. They significantly improve the knowledge of molecular mobility/dynamic structure relationships.

Dynamic nuclear polarisation by thermal mixing: quantum theory and macroscopic simulations

“…Three basic mechanisms are well-known to be responsible for DNP in solids 5 , depending on the effective number of electron spins: solid effect (SE) 9 , cross effect (CE) [10][11][12] and thermal mixing (TM) [13][14][15] . The SE involves single electron spins transferring their polarisation to nuclei via either zero-or double-quantum transitions.…”

Oxygen effect on spin relaxation of the surface paramagnetic centers in carbon chars

“…Such bottleneck phenomena are well known. One can find examples in processes such as nuclear spin relaxation via dipolar reservoir of paramagnetic impurities [12] and relaxation of localized PCs through conductive electrons in metals [13].…”

“…Note that the characteristic coupling times obey the relation r~s/rsE = CE/C s, where C s and C E are heat capacities of the S and E subsystems, respectively. The formula for C s is weU known [12,14]:…”

The Spin Coherence Relaxation in the Rotating Frame as a Microscopy Parameter for Strongly Coupled Spin Systems

“…It is known that in the ordinary weak collision theory the direct correlation between T 1 and the spin dynamics characterized by a rate c Ϫ1 applies only in a range outside ␥B e c ӷ 1, where ␥ is the gyromagnetic ratio (16,17). An interesting calculation developed by Atsarkin and colleagues (18,19) shows that the explicit dependence of T 2 on the magic angle from molecular motion extends to the limit…”