The behaviour of solvated electrons, ei-, produced by the pulse radiolysis of n-hexane at 20°C has been followed using optical absorption techniques. Rate constants for the reaction of eswith naphthalene, biphenyl, galvinoxyl, chloroform, carbon tetrachloride, perhoromethylcyclohexane, carbon dioxide, nitrous oxide and oxygen have been determined and are in the range 0 . 1 -1 . 8 ~ loL2 dm3 moI-' s-l.

Measurements of the ion current following an electron pulse have given values of k r / p (where k, is the ion recombination rate constant and p the electron mobility) and G p (extrapolated to zero field) of 1 .OO( & 0.08) x cm2 V-' respectively for n-hexane, and 0.91( k 0.07) x loe6 V cm and 5.0( k 0.3) x lo-* cm2 V-' for cyclohexane. For n-hexane these and results obtained optically are consistent with kr = 6( +_ 1) x loz3 dm3 mol-I s-l, G = 0.13 and p = 0.09 cm2 V-' s-'. Using p = 0.35 cm2 V-' for cyclohexane, kr = 1.93(+0.14) x 1014 dm3 mol-I S-l.

A study is made of the quenching by halide ions of the fluorescence of fluoranthene solubilized in CI'AB micelles, using both steady-state and lifetime measurements. Stern-Volmer plots or plots of fluorescence decay rate against quencher concentration are non-linear, even under conditions of constant ionic strength. Only halide ions present in the Stern layer are effective in quenching and a relationship is derived between the concentration of ions present in the Stern layer and the total ion concentration using a multiple equilibrium model. Good agreement between theory and experiment is observed for the fluorescence decay rate and intensity as a function of total quencher ion concentration. The model shows that the quenching process is controlled by the quenching rate in the micelles and the association constant of the ions to the micelles. An increase in ionic strength leads to a decrease in the association constant and, owing to changes in micelle structure, it leads to an increase in the quenching rate in the micelle. The overall quenching effect is qualitatively dominated by the variations of the association equilibrium. The product of the micellar quenching rate constant and the number of active sites per micelle at an ionic strength of 0.3 mol dm-3 is numerically identical to the quenching rate constant in water. This may indicate there being only one active site for quenching per micelle. The fluoranthene fluorescence lifetime is found to increase upon addition of chloride or fluoride ions, indicating that these ions are replacing the more strongly quenching bromide ion in the Stern layer. Iodide quenches fluoranthene fluorescence more strongly than bromide ion. Association equilibrium constants for halide ions with CTAB micelles are estimated. * A value of 8.23 x mol dnr3 is given for the c.m.c. of CTAB in water at 30°C in ref. (17). t The system is as described in ref. (19), except that we looked at transient emission rather than absorption.* In calculating the ionic strength we have ignored any contribution from the CTAB to the ionic strength. However, assuming an aggregation number of 60, and a charge of + 12 for the micelle, CTAB wouId only contribute 0.01 4 mol drn-3 to the ionic strength at the concentrations employed here.

The 100 eV yield of the excited singlet state of toluene following a pulse of 10 MeV electrons is found to be 1.35, and the rate constant for its reaction with naphthalene to give the excited naphthalene singlet is 5 . 4 ~ 1O'O M-l s-l. The corresponding yield of the triplet state of toluene is 2.8 and assuming it reacts with naphthalene at a normal diffusion controlled rate, its half life is calculated to be 17 ns. Recent pulse radiolysis workon excited states in solutions of aromatics has reconciled previously conflicting estimates of the yields of excited states of benzene produced on the absorption of ionising radiation. We now present a parallel study on toluene for which the corresponding yields are in some doubt.3* EXPERIMENTAL MATERIALS Analytical reagent benzene was recrystallised three times before use. Scintillation grade toluene was fractionated from lithium aluminium hydride and stored over sodium wire in an argon atmosphere. Purity was checked by vapour phase chromatography using a flame ionization detector. Naphthalene was scintillation grade. All solutions were deoxygenated by prolonged bubbling with pure argon.

M13 9PL 6oCo y-Radiation has been used to initiate the free-radical reduction reaction (i) and the mechanism is shown to be as in reactions (ii)-(iv), where reactions (ii) and (iii) are the propagating steps and reaction (iv) the terminating