Intensity-Dependent Ultraviolet Laser Flash Excitation of Diphenylamine in Methanol: A Two-Photon Ionization Mechanism Involving the Triplet State

The UV laser flash excitation of diphenylamine in methanol at 308 nm leads to similar triplet decay routes as those found previously for N-methyldiphenylamine and triphenylamine. The temperature dependence of the diphenylamine triplet decay shows that both ring closure to form triplet dihydrocarbazole and intersystem crossing back to the ground state take place. The Arrhenius parameters for the two processes are given. The triplet-triplet extinction coefficient at 530 nm and the triplet quantum yield of diphenylamine (0.86) are determined. The intensity dependence of the transient absorption spectrum provides direct evidence for a two-photon ionization mechanism involving the diphenylamine triplet as the intermediate state. The relevant cross sections and the ion quantum yield are determined by model calculations. An ionization quantum yield of 0.36 is obtained for the diphenylamine triplet state.