Indole photodissociation in the gas phase

Kinetic characteristics of the growth in delayed luminescence intensity of indole in the gas phase have been measured with excitation of the molecules in the spectral region 250–287 nm. The luminescence build-up kinetics show two components with characteristic lifetimes of about 1 and 10 μs that are associated with the formation of free radicals due to N–H bond dissociation. Two basic mechanisms for photodissociation of indole in the gas phase have been identified based on an analysis of the dependence of the kinetic characteristics for these components on the excitation radiation wavelength/intensity and the vapor pressures of indole and foreign gases. The fast component results from the dissociation of an N–H bond of vibrationally excited molecules in the ground state that is populated through internal conversion. The slow component is associated with the generation of free radicals because of annihilation of two triplet indole molecules. A dependence of the quantum yield of intersystem crossing on the excitation wavelength in the spectral region 260–287 nm has been found. It is shown that the fluorescence quantum yield drop with a decrease of the excitation radiation wavelength is caused by an increase in the internal conversion quantum yield to the electronic ground state.