MULTIPLE IONIZATION IN THE EARLIER STAGES OF WATER RADIOLYSIS

We have studied the fragmentation of water vapour molecules induced by collision with a Xe44+ beam at 6.7 MeV/u. From the measurement of the fragment time of flight, we show that the amount of fragmentation due to multiple ionization is very large. In the case of single ionizatio n, we are able to reproduce accurately the experimental cross sections by calculating for each molecular level the single-ionization cross s ection in the framework of the CDW-EIS theory and with a diagram of di ssociation modified with respect to the diagram obtained in the case o f dipolar ionization. By using qualitative arguments based on the abil ity of the medium to neutralize a charged species, we tentatively exte nd our result to liquid water. From our analysis, we show that ionizat ions involving three or more ejected electrons could enhance the oxyge n production. For the physicochemical phase we estimate that the rate of oxygen production by multiple ionization represents approximately 1 8% of the OH rate produced by single ionization.