The mechanism of positronim (Ps) formation in molecular substances is an important topic in positron chemistry as well as in radiation chemistry. Although ortho-positronium (o-Ps) lifetime in molecular substances can be related to the size of a cavity (free-volume holes in polymers and bubbles in liquids), the process of Ps formation is not fully understood. In order to account for Ps formation in molecular substances, several different models, the spur reaction model, the Ore model and the free volme model, were proposed [1].
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