ANNIHILATION RESPONSE OF THE ORTHO - POSITRONIUM PROBE FROM POSITRON ANNIHILATION LIFETIME SPECTROSCOPY AND ITS RELATIONSHIPS TO THE FREE VOLUME AND DYNAMICS OF GLASS - FORMING SYSTEMS

Positron annihilation lifetime spectroscopy ( PALS) becomes gradually a standard technique for detecting the local regions of reduced electron density of condensed systems via the formation and annihilation behaviour of ortho - positronium (o-Ps). First we give a short overview of the free volume concept and the principles of PALS method. Then, we present typical o-Ps annihilation responses of organic small molecular and simple polymer systems and their operational classification with a discussion of the microscopic origin of the distinct features between the Type I and Type II glass formers. Further, we give a free volume interpretation of o-Ps annihilation behavior by means of two model dependent treatments of the annihilation parameters, i.e., the o-Ps lifetime, tau(3), and the relative o-Ps intensity, I(3), in terms of free volume hole size, or free volume hole fraction, f(h), respectively. Finally, an example of correlation between effective free volume hole characteristics and macroscopic dynamics will be demonstrated on the case of dielectric relaxation data of glycerol.