Formation of Excited States in the Nanosecond-Pulse Radiolysis of Solutions of Benzene and Toluene

Abstract

Short-lived species have been observed via absorption in the 5000-Å region and fluorescence below 4000 Å in the nanosecond-pulse radiolysis of benzene and toluene. The decay of the absorption and fluorescence is first order with t1/2 = 18.2 ± 1.5 nsec in benzene and t1/2 = 21.6 ± 1.5 nsec in toluene. In benzene we assign the fluorescence and absorption at 5150 Å to the B2u1> state. Solutions of biphenyl in benzene and toluene show only a low yield of negative and positive ions (∼0.1), while solutions of naphthalene, anthracene, and biacetyl show large yields (G = 3–4) of triplet states. Naphthalene and anthracene quench the B2u1 of benzene, while at the same time, the characteristic fluorescence of naphthalene and anthracene are observed. The triplet states of these molecules are derived from the B2u1 and triplet 3B state of benzene, the latter state having a t1/2 ∼ 20 nsec. By using piperylene to preferentially scavenge triplet states in naphthalene–benzene solutions, the following G values are measured: G(1B2u) = 1.62 ± 0.28 and G(3B) = 1.85 ± 0.3. Ionic scavengers such as methanol, nitrous oxide, and cyclohexene reduce the yield of the B2u1 state, while the t1/2 remains unaffected. It is concluded that this state is derived from ion recombination, along with the triplet state. The yield of anthracene triplet is larger than naphthalene triplet, and from the effect of ionic scavengers and xenon on these solutions, it is concluded that the difference is due to a higher singlet state of benzene. This higher singlet, which may be the E1u1 state formed directly by the secondary electrons, is scavenged by anthracene but not by naphthalene.