Influence of the pH on molecular hydrogen primary yields in He ²⁺ ion tracks in liquid water. A Monte Carlo study

Monte Carlo calculations are performed to investigate how the acidity of aqueous solutions at room temperature affects the molecular hydrogen (H ₂) yield as a function of time in 20 MeV-He ²⁺ ion track segments. For pH values varying from 1 to 13, the time dependence of the calculated yields is nearly independent of pH in the time range 10 ^-12-10 ^-8 s and only weakly dependent in the time range 10 ^-8-10 ^-6 s. To understand this behaviour, the kinetic mechanisms governing H ₂ formation are examined as a function of time. It is found that the main reactions responsible for the H ₂ yield as a function of time are strongly pH-dependent at low and high pH values. The pH-dependences of the reaction yields are however such that the variations in the yields compensate each other. This is why the time dependence of the H ₂ yield is only weakly pH-dependent.