1. Introduction

The history of radiation chemistry is briefly described. A general description is presented of the nature and sequence of initial events taking place during the absorption of ionizing radiation at molecular level. Primary interaction takes place within the range of 10-18 to 10-16 s and after this period of time the initial localized or collective electron excited state has formed at the point of interaction. At 10-14 s molecular vibrations are evident, at 10-12 s thermal diffusion of heavy particles and solvated electrons is activated in the dense matter. The chemical stage of radiolysis starts at this moment. (E.S.)