Inhibition of Ps formation in benzene and cyclohexane by CH3Cl and CH3Br. Correlation to radiation chemistry results

Positron-annihilation lifetime spectra have been measured for mixtures of CH₃Cl and CH₃Br in cyclohexane and of CH₃Cl in benzene. The ortho-positronium (Ps) yield decreased monotonically from 38% and 43% in cyclohexane and benzene respectively to 11% in pure CH₃Cl and 6% in pure CH₃Br. The strength of the inhibition of Ps formation by CH₃Br was ten times that of CH₃Cl in cyclohexane, because the CH₃Br^? anion debrominates rapidly, while CH₃Cl^? is long-lived (= 30 ns) compared to the maximum time of Ps formation of 400–500 ps. as shown in radiation chemistry. The positron can pick off the electron from the CH₃X^? anions to form Ps. while it forms a bound state with the halides. X^?. CH₃Cl was a roughly three times weaker Ps inhibitor in benzene than in cyclohexane, which shows that CH₃Cl^? does not dechlorinate in times comparable to or shorter than 400–500 ps in benzene. An improved model for the explanation of Ps formation in mixtures, where the Ps yield versus electron scavenger concentration has a minimum, is proposed and discussed.