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 electronimpact- and ultraviolet-induced emissions suggests that the emitting species for the organic liquids studied under electron impact are singlet-state excimers, ¹m₂*. In six benzene derivatives, in addition to the dominant emission attributed to ¹M₂*, a weaker emission band appeared at ∼500 nm which may be due to emission from triplet-state excimers, ³M 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, w-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, 1,2dimethymaphthalene, and 1,6-dimethylnaphthalene, respectively. Our values for the lifetimes of benzene, toluene, and p-xylene are almost identical with those of ¹M 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 ¹M₂* is immune to ionization quenching, in contrast to the strong quenching of excited monomers ¹M₁*.