Physical electron trapping in boric acid glass in competition with the H+ chemical trapping. A study by recombination delayed fluorescence

Tryptophan and naphthalene in boric acid glass at 77 K give rise to a delayed fluorescence under U.V. excitation and to a photostimulated delayed fluorescence under visible light excitation of an irradiated sample. It is shown that these emissions are due to electron-cation recombination and that, beside the chemical trapping of electrons by protons which is the main process, a physical electron trapping occurs in this strongly acidic medium. This unexpected property is related to the inhomogeneous structure of boric acid glasses which plays an important role in the unique properties of this medium for photoionization studies of aromatic compounds.