Emission and Decay of Liquid-Benzene and Naphthalene Derivatives Excited by Electron Impact

Our earlier work on the emission from organic liquids excited by electron impact has been extended to a large number of liquid‐benzene and naphthalene derivatives. In all cases the emission under intense electron impact is characteristic of excimers. A detailed comparison of the spectral characteristics of the electron‐impact‐ and ultraviolet‐induced emissions suggests that the emitting species for the organic liquids studied under electron impact are singlet‐state excimers, ${}^1{M}_2*$ . In six benzene derivatives, in addition to the dominant emission attributed to ${}^1{M}_2*$ , a weaker emission band appeared at ∼500 nm which may be due to emission from triplet‐state excimers, ${}^3{M}_2*$ . Measurements have been made also of the lifetimes of the emitting species under electron impact. The lifetimes (in nanoseconds) are 12.25 ± 0.2, 15.65 ± 0.2, 8.56 ± 0.1, 12.56 ± 0.2, 8.73 ± 0.1, 11.71 ± 0.14, 11.13 ± 0.24, 11.9 ± 0.3, and 22.86 ± 0.3 for benzene, toluene, ethylbenzene, n‐propylbenzene, iso‐propylbenzene, o‐xylene, m‐xylene, p‐xylene, and mesitylene, respectively, and 68.67 ± 0.2, 25.39 ± 0.1, 48.45 ± 0.36, and 35.4 ± 1 for 1‐methylnaphthalene, 2‐ethylnaphthalene, 1,2‐dimethylnaphthalene, and 1,6‐dimethylnaphthalene, respectively. Our values for the lifetimes of benzene, toluene, and p‐xylene are almost identical with those of ${}^1{M}_2*$ deduced from ultraviolet studies on these systems. This supports the spectroscopic result on the origin of the emission under electron impact and indicates that ${}^1{M}_2*$ is immune to ionization quenching, in contrast to the strong quenching of excited monomers ${}^1{M}_1*$ .