M. Martinelli

Longitudinally detected electron spin resonance: Recent developments

Multiple-quantum spectroscopies are reviewed in the frame of electron paramagnetic resonance. Some properties of different nonlinear techniques are discussed for both transverse and longitudinal detection. The connections of effects recently presented with the procedure of longitudinal detection of electron paramagnetic resonance (LODESR) in presence of double transverse irradiation are stressed. Peculiarities of LODESR spectroscopy and its capabilities in facing problems related to relaxations in presence of very slow dynamic processes are evidentiated. Recent results show the vitality of the LODESR technique, that in the future could be applied to new fields, owing to experimental updating.

A wide-band Whispering Gallery Mode dielectric resonator having multiple high-Q cavities for high-field ESR spectroscopy

The behaviour of a disc-shaped dielectric resonator made of sintered alumina and working in the Whispering Gallery Mode (WGM) has been studied in the frequency interval from about 8 up to about 26 GHz. Many equidistant resonances have been detected in this frequency interval each of which being characterized by a satisfactoryQ-value due to the very good self-confinement of the microwave field which is typical of WGM propagation. Particular care has been devoted to the optimization of the coupling between the resonating disc and the microwave source. Examples of ESR spectra obtained on each of the detected resonances are reported which show the variation of the lineshape with the increase of the frequency. The obtained results allow one to foresee that the same dielectric disc can be used for ESR spectroscopy also at frequencies much higher than those up to now exploited.

Non-linear electron paramagnetic resonance spectroscopy: direct observation of slow dynamics effects at polymer glass transition

Abstract Non-linear ESR schemes can be devised with high sensitivity to slow time scales. The glass transition of a polymeric mesophase is investigated by longitudinally detected ESR (LODESR) spectroscopy. By direct linewidth measurements the orientational correlation time of paramagnetic probes is measured. The temperature dependence is found of the Williams-Landen-Ferry form.

Dynamical Line-Shifts in High-Field Electron Spin Resonance: Applications to Polymer Physics

Abstract High-field high-frequency Electron Paramagnetic Resonance spectroscopy (HF2-EPR) is a powerful tool to investigate, with ultra-high angular resolution, the rotational dynamics of complex systems like polymers, viscous fluids and glasses. Usually, information is drawn by detailed numerical analysis of the overall lineshape. Here, we present a simplified analytical model of the line shifts due to the rotational dynamics of the paramagnetic centre. The model captures the basic features of the reorientation process (time scale and size of the angular jump). It is compared with experimental results concerning the reorientation of a paramagnetic guest molecule dissolved in polystyrene. We find that, if the rotational model to describe the reorientation of the radical is consistent, the best-fit parameters yield equally acceptable best-fits of the overall spectrum by numerical simulations and dynamical line shifts by independent analytic expressions.

A Study of the Glass Transition in Polymeric Mesophases via Calorimetric and Non-Linear ESR Techniques

Abstract In undercooled polymeric mesophases, being annealed at a temperature higher than Tg , nucleation and crystal growth processes take place. We investigate the ageing of comb-like polyacrilates both by calorimetry and non-linear ESR After annealing, the local transport properties of the polymers with quenched disorder exhibit relevant changes which are interpreted in terms of a decreased cooperativity of the amorphous part.

Electron resonance investigation of a cholesteric mesophase induced by a chiral probe

An induced cholesteric phase has been produced by dissolving the chiral nitroxide probe 5α-cholestane-3-spiro-2′-N′-oxyl-3′,3′-dimethyloxazolidine in the nematic phase of 4,4′-bis-n-heptylazoxy-benzene (HEPTAB). The chiral solute has been studied by e.s.r., and order parameters and correlation times obtained by spectral simulation using angular-dependent linewidth theory. Inhomogeneous broadening has been taken into account by using a Voigt convolution lineshape function. The unwinding of the induced cholesteric helix when cooling to the smectic phase has been followed, and the distortion parameter for the helix has been determined at various temperatures.

A NEW NON-LINEAR HIGH-SENSITIVITY SPECTROSCOPY IN THE SLOW MOLECULAR-MOTION REGIME

Abstract The application of doubly modulated E.S.R. (DOMESR) to viscous samples is discussed. An experimental study on a model system is reported which points out, in contrast to E.S.R. spectroscopy the very high sensitivity of DOMESR to slow molecular motions. It is shown that a satisfactory phenomenological interpretation may be given in terms of a dressed atom formalism.