1. A fast positron in a condensed molecular medium initiates numerous chemical transformations. These are similar to chemical processes in tracks of electrons with the same initial energy. Therefore, joint analysis of experimental data of positron spectroscopy and radiation chemistry provides better insight into intratrack reactions which duration is comparable with the positron lifetime. That is why we discuss together the yields of radiolytic hydrogen and Ps in liquid hydrocarbons. Towards the positronium and radiolytic hydrogen formation mechanism in saturated hydrocarbons V.M. Byakov and S.V. Stepanov Our discussion will be based on our unified model describing formation of Ps in molecular condensed matter and that of radiolytic hydrogen in water and aqueous solution. The model consists in the following intratrack reactions: e + + e - => Ps (H 2 O, H 2 O) + + e - => (H 2 O, H 2 O)* => H 2 + 2OH According to this model it is seen that formation process of radiolytic hydrogen is very similar to that of Ps formation.

2. There are several effects which support the suggested mechanism: 1) An approximate equality between relative formation probabilities of Ps and radiolytic H 2 in aqueous solutions

3. 2) Identical isotopic effects re- placement on Ps and H 2 yields Experimental data of T.Hirade (Ps: +); J.-Ch. Abbe and G.Duplatre (Ps: Δ); Y. Katsumura (H 2 : ○, □) 3) Approximate equality of Ps and H 2 yields at different T

4. In hydrocarbons Ps and H 2 yields behave quite in another way. The hydrogen yields in c- alkanes (●) exceed the yields in normal alcanes (■). On the contrary, Ps yields are below than in normal alkanes. Therefore, Ps and H2 yield change oppositely, when the molecular structure is changed from the normal to the cyclic form. Thus, the proportionality between Ps and H2 yields is violated for saturated hydrocarbons. We will try to explain this anticorrelation effect basing on the above formulated concept of Ps and H2 formation.

5. Scheme of radiolytic hydrogen formation in cyclohexane From the scheme it is seen that hydrogen formation is initiated by a decay of excited molecules, C 6 H 12 *. They produce H 2 molecules directly or through intermediate formation of H atoms. The H 2 yield is equal to the yield of excited molecules: G(H 2 ) = G(exc). Explanation of the anticorrelation effect is based on a remarkable distinction of primary holes in cyclic hydrocarbons: their unusually high mobility, one or two orders of magnitude higher than in normal hydrocarbons. Because of that intratrack hole-electron recombination in cyclic hydrocarbons is much more rapid than in normal ones, the hydrogen yield is greater, the Ps yield is reduced. Note, that the formation of excited molecules occurs via electron-hole recombination. Its rate constant is controlled by a diffusion of reactants. In the e+ blob the electron-hole recombination competes with e+/e- recombination. Therefore, Ps formation probability depends on the ratio of their rate constants, k +e / k pe

6. Ps and H 2 yields given by quantitative model of intratrack processes based on the above reaction scheme For n-alkanes ( k +e / k pe <<1) both yields are constant in agreement with experiment. Therefore, As a result in n-alkane P Ps is higher than in cyclic whereas G exc is reduced.

7. Conclusion The unified (re)combination model for the Ps and radiolytic H 2 formation explains not only direct correlations between Ps and H 2 yields in aqueous solutions, but also their anticorrelations in liquid alkanes. The obtained results give an independent support of the idea on unusually high mobility of primary holes in cyclic hydrocarbons. They demonstrate that the role of intrablob electron-hole recombination may be important in the Ps formation processes. The anticorrelation effect is not a peculiarity of liquid alkanes only. It becomes apparent also in aqueous solutions. Alkalization of water increases the Ps yield, but reduces the yield of radiolytic hydrogen.

8. The approximate equality between relative formation probabilities of Ps and radiolytic H 2 is violated in alkaline aqueous solutions. Alkalization of water increases the Ps yield, but reduces the yield of radiolytic hydrogen

9. Similar linear dependencies of reciprocal Ps and hydrogen yields on various electron acceptor concentration The more a given solute S suppresses H 2 yield, the stronger it inhibits the formation of Ps.

10. Aggregate state similarly influences on the formation yields of Mu and Ps. In frozen amorphous aqueous solutions the yields are practically the same as in liquid ones. In crystalline systems the yields are increased against their values in liquid solutions due to much longer duration of the lifetime of presolvated electrons. Similar variations of Mu, Ps and 57 Fe 2+ yields with temperature and aggregate state

11. Ps and H 2 in saturated hydrocarbons The above experimental corroborations give weighty arguments in favour of the suggested mechanism. However, they all refer to pure water or to aqueous solutions only. Quite in another way Ps and radiolytic H 2 behave in saturated hydrocarbons. The proportionality between Ps and H 2 yields is violated for liquid alkanes. Instead of direct correlation between the yields which takes place in water and aqueous solutions anticorrelation between Ps and H 2 yields becomes apparent.

12. Scheme of the structure of a fast positron track in aqueous milieu Both (re)combination reactions compete against each other in the terminal positron blob. In addition, the reaction of H 2 formation occurs also in all track elements of any ionizing particle.