Effect of CCl4 on positronium formation in isooctane

The effect of CCl4 on positronium (Ps) formation is examined in isooctane through positron lifetime spectroscopy experiments, in the framework of a series of studies on AOT-based reverse micellar systems. This solute appears as a rather strong Ps formation inhibitor, as expected in view of its well-known electron scavenging properties. The reciprocal of the triplet Ps intensity (I/I-3) varies linearly with CCl4 concentration (C), up to about 0.08M, allowing to derive a high inhibition constant, k = 36.6M(-1). Above 0.08M, however, the inhibition slackens out, which is attributed to an anti-inhibition effect. On the basis of previous reports on halocompounds, a general scheme of reactions is proposed to occur in the positron spur, including, besides the primary Ps formation through the e(+)/e(-) reaction and the recombination reactions, electron scavenging by CCl4 as the direct inhibiting reaction, followed by the reaction of the thus formed anion with the positron. The latter reaction may lead either to a [CCl(4)Ps] bound-state or, if the positron picks off the electron from the anion, to some additional Ps formation. By using a simple quantitative approach to this scheme, an equation descriptive of the variation of I-3 with C is derived which agrees well with the experimental data.