Photoionization of Indole, N‐Methylindole and Tryptophan In Aqueous Solution upon Excitation at 193 nm

The 193 nm photoionization of aqueous indole, A'‐meth‐ylindole and tryptophan–as a function of pH and under several saturating gas conditions–has been studied by laser photolysis using optical and conductometric detection methods. Monophotonic ionization leads to production of the cation radicals and hydrated electrons, the quantum yield of electron ejection is 0.3–0.4. The cation radicals have pK a values of 4.5, <5 and 4.5 for indole, N ‐methylindole and tryptophan, respectively. Above these pH values, the cation radical deprotonate rapidly, having lifetimes of 1.0, ≅6 and 1.1 μs, respectively. Under N 2 O, neutral indolyl radical production is accompanied by formation of an OH adduct radical (<1 μs). The conductivity results in Ar‐ and N 2 O‐saturated solution support the deprotonation mechanism and indicate that in the acidic pH range, the cation radical decays by release of protons with kinetics on the millisecond time scale.