Viscosity-Dependent Ion Recombination Luminescence in Organic Liquids and Solids from Electron and Gamma Irradiation

Abstract

Recombination luminescence of triphenylamine (TPA), both fluorescence and phosphorescence, has been used to investigate ionic processes in (a) γ-irradiated 3-methylpentane (3MP) at 77°K, (b) thermoluminescence (TL) in γ-irradiated 3MP, and (c) in 1-MeV electron-pulse-irradiated paraffin oil and squalane from 215°–298°K. The shape of the time-dependent decay of luminescent intensity, I(t), in (a) was independent of a 500-fold change in irradiation dose, providing evidence for substantially complete correlation of geminate charge pairs. Photoionization of TPA under otherwise similar conditions led to the same I(t), which provides evidence that the distribution, P(R), of charge separation produced by γ irradiation is determined by very low-energy electrons. TL curves in part (b) showed two well-defined maxima of intensity I′ attributed to recombination of TPA+− e− at 83°K and I″ attributed to TPA+− TPA− at ∼95°K. The ratio I″ / I′ increased with the concentration of TPA, I′ decreased with ir bleaching of solvent-trapped e− or addition of CO2 while both I′ and I″ decreased with addition of methyltetrahydrofuran, a hole trap. In part (c) intensity I was measured from 10−6–10−3 sec as appropriate at various temperatures and solvent viscosities which ranged from 1– ∼ 103 P. Excepting Cerenkov radiation, the luminescence spectrum coincides with TPA fluorescence, the phosphorescence intensity being negligible since its half-life is ∼ 2 sec. Linear plots of lnI vs t can be resolved into two exponential decays, with t1/2 of the slow component proportional to (viscosity)1/2, and attributed to recombination of TPA+ with TPA−. The faster component of I is attributed to recombination of TPA+ with e−, but t1/2 was too short for reliable measurement over most of the range. The dependence t1/2 ∝ η1/2 demonstrates the inapplicability of Stokes' law (which requires t1/2 ∝ R3) to recombination of isolated ion pairs, the high field reducing the randomness of relative ion displacements R. If the probability per ion jump mean free path λ against the central field at R is P = exp(− e2λ / εR2kT), the net displacement for dN jumps is dR = [1 − exp(− e2λ / εR2kT)]λdN. This leads to t1/2 ∝ Rm with 1 ≤ m ≤ 3. This relation combined with dn± / dt = (dn± / dR) (dR / dt) gives the ion pair distribution dn± / dR. From this expression and an assumed dependence Rn ∝ N for free charge one obtains the free-charge distribution function.