Impulsspektroskopische Untersuchungen der Elektronenspinrelaxation in freien Radikalen

Abstract

Direct measurements of the electronic relaxation times T₁ and T₂ have been carried out in crystalline free radicals, in radicals diluted in solids, and in liquid solutions of free radicals. For this purpose, a pulse spectrometer operating at 9.3 GHz has been constructed, which is able to deliver 90° pulses of 20 ns duration, and to determine relaxation times of 3·10^-8 s and longer. T₁- and T₂-measurements on DPPH (Diphenylpicrylhydrazyl) and BPA (Bis-diphenylenephenylallyl) have shown that the relaxation is governed by interactions between the Zeeman system and the exchange system of the electron spins down to the lowest temperature at which the ESR lines exist. The data are interpreted in terms of a simple model which takes into account exchange interaction between nearest neighbours of a linear chain of molecules. Furthermore, the electronic relaxation of organic crystals doped with free radicals has been investigated. - Finally, the relaxation of several free radicals has been studied in solutions of various organic liquids and of ammonia. From the temperature and concentration dependence it has been concluded that T₁ can be explained by dipolar interactions between the electrons the time dependence of which is due to translational motion of the molecules. In contrast to nuclear relaxation data in liquids, the electronic T₁ provides information for large ω τ-values, thus making possible a complete test of the theory of translational diffusion in liquids.